Description of Registrants Securities

The following description of our ordinary shares and Euro Deferred Shares is a summary. This summary does not purport to be complete and is qualified in its entirety by reference to the Irish Companies Act 2014 (the “Companies Act”), and the complete text of our Constitution. You should read those laws and documents carefully.

For the avoidance of any doubt, the ordinary shares are the subject of this registration statement. The Euro Deferred Shares are not listed on any stock exchange and are not the subject of any registration.

As of December 31, 2022, our issued share capital was 52,103,608 ordinary shares. We have no Euro Deferred Shares in issue. Our ordinary shares are listed on the Nasdaq Global Select Market (“Nasdaq”), under the symbol “PRTA.”

The authorized share capital of the Company is $1,000,000 and €220,000 consisting of 100,000,000 ordinary shares with a par value of $0.01 per share and 10,000 Euro Deferred Shares with a par value of €22 per share. The authorized share capital may be increased or reduced by a resolution approved by a simple majority of the votes cast at a general meeting of our shareholders at which a quorum is present (referred to under Irish law as an “ordinary resolution”). The shares comprising our authorized share capital may be divided into shares of such nominal value as the resolution shall prescribe. As a matter of Irish law, our board of directors (our “Board”), may issue new ordinary shares or Euro Deferred Shares without shareholder approval once authorized to do so by our Constitution or by an ordinary resolution adopted by the shareholders at a general meeting. The authorization may be granted for a maximum period of five years, at which point it must be renewed by the shareholders by an ordinary resolution.

Our Board is authorized pursuant to an ordinary resolution passed by shareholders at our annual general meeting held on May 17, 2022, to issue new ordinary shares for cash without shareholder approval up to an aggregate nominal amount equal to the authorized but unissued share capital of the Company as at May 17, 2022, for a period of five years from the date of the passing of the resolution. As a result, our shareholders must renew this authorization by an ordinary resolution no later than May 17, 2027.

The rights and restrictions to which our ordinary shares and Euro Deferred Shares are subject are prescribed in our Constitution. We may, by ordinary resolution and without obtaining any vote or consent of the holders of any class or series of shares, unless expressly provided by the terms of that class or series of shares, provide from time to time for the issuance of other classes or series of shares and to establish the characteristics of each class or series, including the number of shares, designations, relative voting rights, dividend rights, liquidation and other rights, redemption, repurchase or exchange rights and any other preferences and relative, participating, optional or other rights and limitations not inconsistent with applicable law.

Irish law does not recognize fractional shares held of record. Accordingly, our Constitution does not provide for the issuance of fractional shares of the Company, and the official Irish share register of the Company will not reflect any fractional shares. Whenever as a result of an issuance, alteration, reorganization, consolidation, division, or subdivision of the share capital of the Company would result in any shareholder becoming entitled to fractions of a share, no such fractions shall be issued or delivered to any shareholder. All such fractions of a share will be aggregated into whole shares and sold in the open market at prevailing market prices and the aggregate cash

proceeds from such sale (net of tax, commissions, costs and other expenses) shall be distributed on a pro rata basis, rounding down to the nearest cent, to each shareholder who would otherwise have been entitled to receive fractions of a share.

Under Irish law, certain statutory preemption rights apply automatically in favor of shareholders where shares are to be issued for cash. However, as permitted by Irish law, we have opted out of these preemption rights by way of special resolution (a “special resolution” requires the approval of not less than 75% of the votes of our shareholders cast at a general meeting at which a quorum is present) passed at our annual general meeting on May 17, 2022. Irish law requires this opt-out to be renewed every five years by a special resolution. As a result, our shareholders must renew this opt-out authorization by a special resolution no later than May 17, 2027.

If the opt-out is not renewed, shares issued for cash must be offered to existing shareholders of the Company on a pro rata basis to their existing shareholding before the shares may be issued to any new shareholders. The statutory preemption rights do not apply (i) where shares are issued for non-cash consideration (such as in a share-for-share acquisition), (ii) to the issue of non-equity shares (that is, shares that have the right to participate only up to a specified amount in any income or capital distribution) or (iii) where shares are issued pursuant to an employee share option or similar equity plan.

Our Constitution provides that, subject to any shareholder approval requirement under any laws, regulations or the rules of any stock exchange to which we are subject, our Board is authorized, from time to time, in its discretion, to grant such persons, for such periods and upon such terms as it deems advisable, options to purchase such number of shares of any class or classes or of any series of any class as our Board may deem advisable, and to cause warrants or other appropriate instruments evidencing such options to be issued. The Companies Act provides that directors may issue share warrants or options without shareholder approval once authorized to do so by a company’s constitution or an ordinary resolution of shareholders. We are subject to the rules of Nasdaq and the U.S. Internal Revenue Code of 1986, as amended, which require shareholder approval of certain equity plans and share issuances. Our Board may issue shares upon exercise of warrants or options without shareholder approval or authorization (up to the relevant authorized share capital limit).

Under Irish law, dividends and distributions may only be made from distributable reserves. Distributable reserves generally means accumulated, realized profits, so far as not previously utilized by distribution or capitalization, less accumulated, realized losses, so far as not previously written off in a reduction or re-organization of capital duly made. In addition, no distribution or dividend may be made unless our net assets are equal to, or in excess of, the aggregate of our called up share capital plus undistributable reserves and the distribution does not reduce our net assets below such aggregate. Undistributable reserves include the undenominated capital (effectively the share premium and capital redemption reserve) and the amount by which our accumulated unrealized profits, so far as not previously utilized by any capitalization, exceed the Company’s accumulated unrealized losses, so far as not previously written off in a reduction or reorganization of capital.

The determination as to whether or not we have sufficient distributable reserves to fund a dividend must be made by reference to the “relevant financial statements” of the Company. The relevant financial statements are either the last set of unconsolidated annual audited financial statements or other financial statements properly prepared in accordance with the Companies Act, which give a “true and fair view” of our unconsolidated financial position and accord with accepted accounting practice. The relevant financial statements must be filed in the Companies Registration Office (the official public registry for companies in Ireland).

Our Constitution authorizes our Board to declare dividends without shareholder approval to the extent they appear justified by profits lawfully available for distribution. Our Board may also recommend a dividend to be approved and declared by the shareholders at a general meeting. Our Board may direct that the payment be made by distribution of assets, shares or cash, and no dividend issued may exceed the amount recommended by the directors.

Dividends may be declared and paid in the form of cash or non-cash assets and may be paid in dollars or any other currency.

Our Board may deduct from any dividend payable to any shareholder any amounts payable by such shareholder to the Company in relation to the shares of the Company.

The Board may also authorize the Company to issue shares with preferred rights to participate in dividends declared by the Company from time to time, as determined by ordinary resolution. The holders of preferred shares may, depending on their terms, rank senior to our ordinary shares in terms of dividend rights and or be entitled to claim arrears of a declared dividend out of subsequently declared dividends in priority to ordinary shareholders.

Under our Constitution, our Board may resolve to capitalize any amount credited to any reserve available for distribution or the share premium account or other of our undistributable reserves for issuance and distribution to shareholders as fully paid up bonus shares on the same basis of entitlement as would apply in respect of a dividend distribution.

Our Constitution provides that any ordinary share that we have agreed to acquire shall be deemed to be a redeemable share. Accordingly, for Irish law purposes, the repurchase of ordinary shares by us may technically be effected as a redemption of those shares as described below under “Description of Share Capital—Repurchases and Redemptions by Prothena.” If our Constitution did not contain such provision, repurchases by us would be subject to many of the same rules that apply to purchases of our ordinary shares by subsidiaries described below under “Description of Share Capital—Purchases by Subsidiaries of Prothena,” including the shareholder approval requirements described below, and the requirement that any overseas market purchases be effected on a recognized stock exchange, which, for purposes of the Companies Act, includes Nasdaq. Neither Irish law nor any of our constituent documents places limitations on the right of non-resident or foreign owners to vote or hold our ordinary shares. Except where otherwise noted, references in this Prospectus Supplement to repurchasing or buying back our ordinary shares refer to the redemption of ordinary shares by us or the purchase of our ordinary shares by one of our subsidiaries, in each case in accordance with our Constitution and Irish company law as described below.

Under Irish law, a company may issue redeemable shares and redeem them out of distributable reserves or the proceeds of a new issue of shares for that purpose. Please see also “Description of Share Capital—Dividends.” We may only issue redeemable shares if the nominal value of the issued share capital that is not redeemable is not less than 10% of the nominal value of our total issued share capital. All redeemable shares must also be fully-paid and the terms of redemption of the shares must provide for payment on redemption. Redeemable shares may, upon redemption, be cancelled or held in treasury. Based on the provisions of our Constitution, shareholder approval will not be required to redeem our shares.

We may also be given an additional general authority for overseas market purchases of our ordinary shares by way of ordinary resolution, which would take effect on the same terms and be subject to the same conditions as applicable to purchases by our subsidiaries as described below.

Repurchased and redeemed shares may be cancelled or held as treasury shares. The nominal value of treasury shares held by us at any time must not exceed 10% of the nominal value of our issued share capital. We may not exercise any voting rights in respect of any shares held as treasury shares. Treasury shares may be cancelled by us or re-issued subject to certain conditions.

Under Irish law, an Irish or non-Irish subsidiary of the Company may purchase our shares by way of an: (i) overseas market purchase; or (ii) off-market purchase. For one of our subsidiaries to make overseas market purchases of our ordinary shares, our shareholders must provide general authorization for such purchase by way of ordinary resolution. However, as long as this general authority has been granted, no specific shareholder authority for a particular overseas market purchase by a subsidiary of our ordinary shares is required. For a purchase by one of our subsidiaries off-market, the proposed purchase contract must be authorized by special resolution of our shareholders before the contract is entered into. The person whose ordinary shares are to be bought back cannot vote in favor of the special resolution and from the date of the notice of the meeting at which the resolution approving the contract is proposed, the purchase contract must be on display or must be available for inspection by our shareholders at our registered office.

In order for one of our subsidiaries to make overseas market purchases of our shares, such shares must be purchased on a recognized stock exchange. Nasdaq, on which our ordinary shares are listed, is specified as a recognized stock exchange for this purpose in accordance with Irish law.

The number of shares held by our subsidiaries at any time will count as treasury shares and will be included in any calculation of the permitted treasury share threshold of 10% of the nominal value of our issued share capital. While a subsidiary holds our shares, it cannot exercise any voting rights in respect of those shares. The acquisition of our ordinary shares by a subsidiary must be funded out of distributable reserves of the subsidiary.

Our Constitution provides that we have a first and paramount lien on every share that is not a fully paid up share for all amounts payable at a fixed time or called in respect of that share. Subject to the terms of their allotment, directors may call for any unpaid amounts in respect of any shares to be paid, and if payment is not made, the shares may be forfeited. These provisions are standard inclusions in the constitution of an Irish public company limited by shares such as Prothena and are only applicable to our shares that have not been fully paid up. Irish stamp duty may be payable in respect of transfers of our ordinary shares at the rate of 1%.

Under our Constitution, we may, by ordinary resolution, consolidate and divide all or any of our share capital into shares of larger nominal value than our existing shares or subdivide our shares into smaller amounts than are fixed by our Constitution.

We may, by ordinary resolution, reduce our authorized share capital in any way. We also may, by special resolution and subject to confirmation by the Irish High Court, reduce or cancel our issued share capital in any manner permitted by the Companies Act.

Under Irish company law, we are required to hold annual general meetings at intervals of no more than 15 months from the previous annual general meeting, provided that an annual general meeting is held in each calendar year following the first annual general meeting and no more than nine months after our fiscal year-end. Subject to compliance with the Companies Act, any of our annual general meetings may be held outside Ireland. Notice of an annual general meeting must be given to all of our shareholders and to our auditors. Our Constitution provides for a minimum notice period of 21 days’ notice, which is the minimum permitted by the Companies Act.

The only matters which must, as a matter of Irish company law, be transacted at the Company’s annual general meeting are the consideration of the statutory financial statements, report of the directors and the report of the auditors on those statements and that report, the review by the shareholders of the Company’s affairs, the election and re-election of directors in accordance with our Constitution, the declaration of a dividend (if any), the

appointment or reappointment of the auditors and the fixing of the auditor’s remuneration (or delegation of same). If no resolution is made in respect of the reappointment of an existing auditor at an annual general meeting, the existing auditor will be deemed to have continued in office.

Extraordinary general meetings of the Company may be convened by (i) our Board, (ii) on requisition of our shareholders holding not less than 10% of the paid up share capital of our carrying voting rights, (iii) on requisition of our auditors, or (iv) in exceptional cases, by order of the Irish High Court. Extraordinary general meetings are generally held for the purpose of approving shareholder resolutions as may be required from time to time. At any extraordinary general meeting only such business shall be conducted as is set forth in the notice thereof.

Notice of an extraordinary general meeting must be given to all of our shareholders and to our auditors. Under Irish law and our Constitution, the minimum notice periods are 21 days’ notice in writing for an extraordinary general meeting to approve a special resolution and 14 days’ notice in writing for any other extraordinary general meeting.

In the case of an extraordinary general meeting convened by the requisition of our shareholders under part (ii) above, the proposed purpose of the meeting must be set out in the requisition notice. Upon receipt of any such valid requisition notice, our Board has 21 days to convene a meeting of our shareholders to vote on the matters set out in the requisition notice. This meeting must be held within two months of the receipt of the requisition notice. If our Board does not convene the meeting within such 21-day period, the requisitioning shareholders, or any of them representing more than one half of the total voting rights of all of them, may themselves convene a meeting, which meeting must be held within three months of our receipt of the requisition notice.

If our Board becomes aware that our net assets are not greater than half of the amount of our called-up share capital, it must convene an extraordinary general meeting of our shareholders not later than 28 days from the date that they learn of this fact to consider how to address the situation.

Our Constitution provides that no business shall be transacted at any general meeting unless a quorum is present. One or more of our shareholders present in person or by proxy holding not less than one-half of our issued and outstanding shares entitled to vote at the meeting in question constitute a quorum.

Our Constitution provides that our Board or chairman may determine the manner in which the poll is to be taken and the manner in which the votes are to be counted.

Each Company shareholder is entitled to one vote for each ordinary share that he or she holds as of the record date for the meeting. Voting rights may be exercised by shareholders registered in our share register as of the record date for the meeting or by a duly appointed proxy, which proxy need not be a Company shareholder. Where interests in shares are held by a nominee trust company, such company may exercise the rights of the beneficial holders on their behalf as their proxy. All proxies must be appointed in the manner prescribed by our Constitution, which permit shareholders to notify us of their proxy appointments electronically in such manner as may be approved by our Board.

In accordance with our Constitution, we may from time to time be authorized by ordinary resolution to issue preferred shares. These preferred shares may have such voting rights as may be specified in the terms of such preferred shares (e.g., they may carry more votes per share than ordinary shares or may entitle their holders to a class vote on such matters as may be specified in the terms of the preferred shares). Treasury shares or our shares that are held by our subsidiaries are not entitled to be voted at general meetings of shareholders.

Irish law requires special resolutions of our shareholders at a general meeting to approve certain matters. Examples of matters requiring special resolutions include:

•amending our Constitution;

•approving a change of name of Prothena;

•authorizing the entering into of a guarantee or provision of security in connection with a loan, quasi loan or credit transaction to a director or connected person;

•opting out of preemption rights on the issuance of new shares;

•re-registration of Prothena from a public limited company to a private company;

•variation of class rights attaching to classes of shares (where the Constitution does not provide otherwise);

•purchase of our shares off-market;

•reduction of issued share capital;

•sanctioning a compromise/scheme of arrangement with creditors or shareholders;

•resolving that we be wound up by the Irish courts;

•resolving in favor of a shareholders’ voluntary winding-up; and

•setting the re-issue price of treasury shares.

Under our Constitution and the Companies Act, any variation of class rights attaching to our issued shares must be approved by a special resolution of our shareholders of the affected class or with the consent in writing of the holders of three-quarters of all the votes of that class of shares.

The provisions of our Constitution relating to general meetings apply to general meetings of the holders of any class of our shares except that the necessary quorum is determined in reference to the shares of the holders of the class. Accordingly, for general meetings of holders of a particular class of our shares, a quorum consists of the holders present in person or by proxy representing at least one-half of the issued shares of the class.

Our Board may from time to time fix a record date for the purposes of determining the rights of shareholders to notice of and/or to vote at any general meeting of the Company. The record date shall not precede the date upon which the resolution fixing the record date is adopted and may not be more than 90 days nor less than 10 days before the date of such meeting. If no record date is fixed by the Board, the record date for determining shareholders entitled to notice of, or to vote at, a meeting shall be the date immediately preceding the date on which notice of the meeting is deemed given under the Constitution.

The Board may also set a record date to determine the identity of the shareholders entitled to receive payment of any dividend or for any other proper purpose. The record date shall not precede the date upon which the resolution fixing the record date is adopted and the record date shall not be more than ninety days prior to such action. If no record date is fixed, the record date for determining shareholders for such purpose shall be the date on which the Board adopts the resolution relating to the payment of any dividend.

Under Irish law, there is no general right for a shareholder to put items on the agenda of an annual general meeting of the Company, other than as set out in the Constitution. Our Constitution permits shareholders to nominate persons for election to the Board at general meetings called for the purpose of electing directors once they

comply with certain requirements set out in the Constitution. Under our Constitution, in addition to any other applicable requirements, for director nominations to be properly brought before a general meeting by a shareholder, such shareholder must have given timely notice thereof in writing to our corporate secretary.

To be timely for an annual general meeting, a shareholder’s notice to our secretary as to the nominations to be brought before the meeting must be delivered to our registered office not less than 90 days nor more than 150 days prior to the first anniversary of the notice convening our annual general meeting for the prior year. In the event that the date of the annual general meeting is changed by more than 30 days from the first anniversary date of the prior year’s annual general meeting, notice by the member must be delivered not earlier than 150 days prior to such annual general meeting and not later than the later of (a) 90 days prior to the day of the contemplated annual general meeting or (b) ten days after the day on which public announcement of the date of the contemplated annual general meeting is first made.

To be timely for nominations of a director at an extraordinary general meeting, notice must be delivered not more than 150 days prior to the date of such extraordinary general meeting and not later than 90 days prior to such extraordinary general meeting or 10 days after the day on which public announcement is first made of the date of the general meeting and of the nominees proposed by the Board to be elected at such meeting.

For nominations to the Board, the notice must include all information about the director nominee that is required to be disclosed by SEC rules regarding the solicitation of proxies for the election of directors pursuant to Regulation 14A under the Exchange Act. The notice also must include information about the shareholder and the shareholder’s holdings of our shares. The chairman of the meeting shall have the power and duty to determine whether any proposed nomination was made or proposed in accordance with these provisions (as set out in our Constitution), and if any proposed nomination is not in compliance with these provisions, to declare that such nomination is defective and shall be disregarded.

In Ireland, the decision to institute proceedings on behalf of a company is generally taken by the company’s board of directors. In certain limited circumstances, a shareholder may be entitled to bring a derivative action on our behalf. The central question at issue in deciding whether a minority shareholder may be permitted to bring a derivative action is whether, unless the action is brought, a wrong committed against us would otherwise go unredressed. The cause of action may be against a director, another person or both.

A shareholder may also bring proceedings against us in his or her own name where the shareholder’s rights as such have been infringed or where our affairs are being conducted, or the powers of the board of directors are being exercised, in a manner oppressive to any shareholder or shareholders or in disregard of their interests as shareholders. Oppression connotes conduct that is burdensome, harsh or wrong. This is an Irish statutory remedy under Section 212 of the Companies Act and the court can grant any order it sees fit, including providing for the purchase or transfer of the shares of any shareholder.

Under Irish law, shareholders have the right to: (i) receive a copy of our Constitution; (ii) inspect and obtain copies of the minutes of our general meetings and resolutions; (iii) inspect and receive a copy of the register of shareholders, register of directors and secretaries, register of directors’ interests; (iv) inspect copies of directors’ service contracts; (v) inspect copies of instruments creating charge; (vi) receive copies of the statutory financial statements and directors’ and auditors’ reports which have previously been sent to shareholders prior to an annual general meeting; and (vii) receive copies of the statutory financial statements and directors’ and auditors’ reports of any of our subsidiaries which have previously been sent to the shareholders of the subsidiaries prior to an annual general meeting for the preceding ten years. Our auditors also have the right of access, at all reasonable times, to the accounting records of the Company. The auditors’ report must be circulated to the shareholders with our financial statements prepared in accordance with Irish law 21 days (not including the day of mailing or the day of the meeting) before the annual general meeting and must be laid before the shareholders at our annual general meeting.

The Company may be acquired in a number of ways, including:

•a court-approved scheme of arrangement under the Companies Act. A scheme of arrangement with shareholders requires a court order from the Irish High Court and the approval of a majority in number representing 75% in value of each class of shareholders present and voting in person or by proxy at a meeting called to approve the scheme;

•through a tender or takeover offer by a third party for all of our shares. Where the holders of 80% or more of our shares have accepted an offer for their shares in Prothena, the remaining shareholders may also be statutorily required to transfer their shares. If the bidder does not exercise its “squeeze out” right, then the non-accepting shareholders also have a statutory right to require the bidder to acquire their shares on the same terms; and

•by way of a merger with a company incorporated in the European Economic Area (“EEA”) under the EU Cross-Border Mergers Directive (EU) 2017/1132 or with another Irish company under the Companies Act. Such a merger must be approved by a special resolution. Shareholders also may be entitled to have their shares acquired for cash. See the section entitled “Description of Share Capital – Appraisal Rights.”

Irish law does not generally require shareholder approval for a sale, lease or exchange of all or substantially all of a company’s property and assets.

Generally, under Irish law, shareholders of an Irish company do not have statutory appraisal rights. If we are being merged as the transferor company with another EEA company under the EU Cross-Border Mergers Directive (EU) 2017/1132 as implemented in Ireland by the European Communities (“Cross-Border Mergers”) Regulations 2008 (as amended) or if we are being merged with another Irish company under the Irish Companies Act, (i) any of our shareholders who voted against the special resolution approving the merger or (ii) if 90% of our shares are held by the successor company, any other of our shareholders, may be entitled to require that the successor company acquire its shares for cash.

Under the Companies Act, our shareholders must notify us if, as a result of a transaction, the shareholder will become interested in three percent or more of the Prothena voting shares, or if as a result of a transaction a shareholder who was interested in more than three percent of Prothena voting shares ceases to be so interested. Where a shareholder is interested in more than three percent of Prothena voting shares, the shareholder must notify us of any alteration of his or her interest that brings his or her total holding through the nearest whole percentage number, whether an increase or a reduction. The relevant percentage figure is calculated by reference to the aggregate nominal value of the voting shares in which the shareholder is interested as a proportion of the entire nominal value of our issued share capital (or any such class of share capital in issue). Where the percentage level of the shareholder’s interest does not amount to a whole percentage, this figure may be rounded down to the next whole number. We must be notified within five business days of the transaction or alteration of the shareholder’s interests that gave rise to the notification requirement. If a shareholder fails to comply with these notification requirements, the shareholder’s rights in respect of any our shares it holds will not be enforceable, either directly or indirectly. However, such person may apply to the court to have the rights attaching to such shares reinstated.

In addition to these disclosure requirements, we, under the Companies Act, may, by notice in writing, require a person whom we know or have reasonable cause to believe to be, or at any time during the three years immediately preceding the date on which such notice is issued to have been, interested in shares comprised in our relevant share capital: (i) to indicate whether or not it is the case; and (ii) where such person holds or has during that time held an interest in our shares, to provide additional information, including the person’s own past or present

interests in our shares. If the recipient of the notice fails to respond within the reasonable time period specified in the notice, we may apply to court for an order directing that the affected shares be subject to certain restrictions, as prescribed by the Companies Act, as follows:

•any transfer of those shares or, in the case of unissued shares, any transfer of the right to be issued with shares and any issue of shares, shall be void;

•no voting rights shall be exercisable in respect of those shares;

•no further shares shall be issued in right of those shares or in pursuance of any offer made to the holder of those shares; and

•no payment shall be made of any sums due from Prothena on those shares, whether in respect of capital or otherwise.

The court may also order that shares subject to any of these restrictions be sold with the restrictions terminating upon the completion of the sale.

In the event we are in an offer period pursuant to the Irish Takeover Rules (as defined below), accelerated disclosure provisions apply for persons holding an interest in our securities of one percent or more.

A transaction in which a third party seeks to acquire 30% or more of Prothena voting rights and any other acquisitions of our securities are governed by the Irish Takeover Panel Act 1997 and the Irish Takeover Rules made thereunder, which are referred to in this Prospectus Supplement as the “Irish Takeover Rules,” and are regulated by the Irish Takeover Panel. The “General Principles” of the Irish Takeover Rules and certain important aspects of the Irish Takeover Rules are described below.

The Irish Takeover Rules are built on the following General Principles which will apply to any transaction regulated by the Irish Takeover Panel:

•in the event of an offer, all holders of securities of the target company must be afforded equivalent treatment and, if a person acquires control of a company, the other holders of securities must be protected;

•the holders of securities in the target company must have sufficient time and information to enable them to reach a properly informed decision on the offer; where it advises the holders of securities, the board of directors of the target company must give its views on the effects of the implementation of the offer on employment, employment conditions and the locations of the target company’s place of business;

•a target company’s board of directors must act in the interests of the company as a whole and must not deny the holders of securities the opportunity to decide on the merits of the offer;

•false markets must not be created in the securities of the target company, the bidder or any other company concerned by the offer in such a way that the rise or fall of the prices of the securities becomes artificial and the normal functioning of the markets is distorted;

•a bidder can only announce an offer after ensuring that he or she can fulfill in full any cash consideration, if such is offered, and after taking all reasonable measures to secure the implementation of any other type of consideration;

•a target company may not be hindered in the conduct of its affairs for longer than is reasonable by an offer for its securities; and

•a “substantial acquisition” of securities (whether such acquisition is to be effected by one transaction or a series of transactions) shall take place only at an acceptable speed and shall be subject to adequate and timely disclosure.

Under certain circumstances, a person who acquires our shares, or other voting securities, may be required under the Irish Takeover Rules to make a mandatory cash offer for remaining outstanding Prothena voting securities at a price not less than the highest price paid for the securities by the acquiror, or any parties acting in concert with the acquiror, during the previous 12 months. This mandatory bid requirement is triggered if an acquisition of securities would increase the aggregate holding of an acquiror, including the holdings of any parties acting in concert with the acquiror, to securities representing 30% or more of Prothena voting rights, unless the Irish Takeover Panel otherwise consents. An acquisition of securities by a person holding, together with its concert parties, securities representing between 30% and 50% of Prothena voting rights would also trigger the mandatory bid requirement if, after giving effect to the acquisition, the percentage of the voting rights held by that person (together with its concert parties) would increase by 0.05% within a 12-month period. Any person (excluding any parties acting in concert with the holder) holding securities representing more than 50% of the voting rights of a company is not subject to these mandatory offer requirements in purchasing additional securities.

If a person makes a voluntary offer to acquire our outstanding ordinary shares, the offer price must not be less than the highest price paid for our ordinary shares by the bidder or its concert parties during the three-month period prior to the commencement of the offer period. The Irish Takeover Panel has the power to extend the “look back” period to 12 months if the Irish Takeover Panel, taking into account the General Principles, believes it is appropriate to do so.

If the bidder or any of its concert parties has acquired our ordinary shares (i) during the period of 12 months prior to the commencement of the offer period that represent more than 10% of our total ordinary shares or (ii) at any time after the commencement of the offer period, the offer must be in cash (or accompanied by a full cash alternative) and the price per our ordinary shares must not be less than the highest price paid by the bidder or its concert parties during, in the case of (i), the 12-month period prior to the commencement of the offer period or, in the case of (ii), the offer period. The Irish Takeover Panel may apply this rule to a bidder who, together with its concert parties, has acquired less than 10% of our total ordinary shares in the 12-month period prior to the commencement of the offer period if the Irish Takeover Panel, taking into account the General Principles, considers

it just and proper to do so. An offer period will generally commence from the date of the first announcement of the offer or proposed offer.

The Irish Takeover Rules also contain rules governing substantial acquisitions of shares and other voting securities which restrict the speed at which a person may increase his or her holding of shares and rights over shares to an aggregate of between 15% and 30% of the Prothena voting rights. Except in certain circumstances, an acquisition or series of acquisitions of shares or rights over shares representing 10% or more of the Prothena voting rights is prohibited, if such acquisition(s), when aggregated with shares or rights already held, would result in the acquirer holding 15% or more but less than 30% of the Prothena voting rights and such acquisitions are made within a period of seven days. These rules also require accelerated disclosure of acquisitions of shares or rights over shares relating to such holdings.

Under the Irish Takeover Rules, our Board is not permitted to take any action that might frustrate an offer for our shares once our Board has received an approach that may lead to an offer or has reason to believe that such an offer is or may be imminent, subject to certain exceptions. Potentially frustrating actions such as (i) the issue of shares, options or convertible securities, (ii) material acquisitions or disposals, (iii) entering into contracts other than in the ordinary course of business or (iv) any action, other than seeking alternative offers, which may result in frustration of an offer, are prohibited during the course of an offer or at any earlier time during which our Board has reason to believe an offer is or may be imminent. Exceptions to this prohibition are available where:

•the action is approved by our shareholders at a general meeting; or

•the Irish Takeover Panel has given its consent, where:

•it is satisfied the action would not constitute frustrating action;

•our shareholders holding more than 50% of the voting rights state in writing that they approve the proposed action and would vote in favor of it at a general meeting;

•the action is taken in accordance with a contract entered into prior to the announcement of the offer (or any earlier time at which our Board considered the offer to be imminent); or

•the decision to take such action was made before the announcement of the offer and either has been at least partially implemented or is in the ordinary course of business.

Certain other provisions of Irish law or our Constitution may be considered to have antitakeover effects, including advance notice requirements for director nominations, as well as those described under the following captions: “Description of Share Capital—Capital Structure—Authorized Share Capital” (regarding issuance of preferred shares), “Description of Share Capital—Preemption Rights, Share Warrants and Share Options,” “Description of Share Capital—Disclosure of Interests in Shares” and “Description of Share Capital—Corporate Governance.”

Our Constitution allocates authority over the day-to-day management of Prothena to our Board. Our Board may then delegate the management of Prothena to committees of the Board (consisting of one or more members of the Board) or executives, but regardless, our Board remains responsible, as a matter of Irish law, for the proper management of the affairs of Prothena. Committees may meet and adjourn as they determine proper. A vote at any committee meeting will be determined by a majority of votes of the members present.

The Board has a standing audit committee, a standing compensation committee and a standing nominating and corporate governance committee, with each committee comprised solely of independent directors, as prescribed

by the Nasdaq listing standards and SEC rules and regulations. We have adopted corporate governance guidelines, as well as a code of conduct and other compliance policies.

The Companies Act provides for a minimum of two directors. Our Constitution provides that the Board may determine the size of the Board from time to time.

Our Constitution provides that at least one-third of the directors serving on the Board shall come up for re-election at a given annual general meeting, and that directors must come up for re-election at the third annual general meeting subsequent to their appointment or reappointment to the Board. Except as otherwise provided by law, vacancies on the Board may be filled only by ordinary resolution or the affirmative vote of a majority of the remaining directors. A director elected by the Board to fill a vacancy shall serve until the subsequent annual general meeting. At each annual general meeting of shareholders, the successors to directors whose terms will then expire will be elected to serve from the time of election and qualification until the third subsequent annual general meeting of shareholders.

Under the Companies Act and notwithstanding anything contained in our Constitution or in any agreement between us and a director, the shareholders may, by an ordinary resolution, remove a director from office before the expiration of his or her term at a meeting held on no less than 28 days’ notice and at which the director is entitled to be heard. The power of removal is without prejudice to any claim for damages for breach of contract (e.g., employment contract) that the director may have against us in respect of his removal.

Our Constitution provides that the Board may fill any vacancy occurring on the Board. If the Board fills a vacancy, the director’s term expires at the next annual general meeting. A vacancy on the Board created by the removal of a director may be filled by the shareholders at the meeting at which such director is removed and, in the absence of such election or appointment, the remaining directors may fill the vacancy.

Prothena Corporation plc was formed under the laws of Ireland on September 26, 2012 as a private limited company, under the name “Neotope Corporation Limited” (registration number 518146), and reregistered as a public limited company and changed its name to “Neotope Corporation plc” on October 25, 2012. On November 1, 2012, our shareholders resolved, by way of special resolution, to change the name of the company to “Prothena Corporation plc,” and this was approved by the Irish Registrar of Companies on November 7, 2012. Our fiscal year ends on December 31 and our registered address is 77 Sir John Rogerson’s Quay, Block C, Grand Canal Docklands, Dublin 2, D02 VK60, Ireland.

Our duration is unlimited. We may be dissolved and wound up at any time by way of a shareholders’ voluntary winding up or a creditors’ winding up. In the case of a shareholders’ voluntary winding up, a special resolution of shareholders is required. We may also be dissolved by way of court order on the application of a creditor, or by the Companies Registration Office as an enforcement measure where we have failed to file certain returns.

If our Constitution contains no specific provisions in respect of a dissolution or winding up, then, subject to the priorities of any creditors, the assets will be distributed to our shareholders in proportion to the paid-up nominal value of the shares held. Our Constitution provides that our ordinary shareholders are entitled to participate pro rata in a winding up.

Holders of our ordinary shares that hold their ordinary shares electronically have the right to require us to issue certificates for their shares.

Our ordinary shares are listed on the Nasdaq Global Select Market under the symbol “PRTA.”

Our ordinary shares have no sinking fund provisions.

The transfer agent for our ordinary shares is Computershare Trust Company, N.A. Its address is 250 Royall Street, Canton, MA 02021. An Irish based affiliate of the transfer agent, Computershare Investor Services (Ireland) Limited, maintains our share register, registration in which is determinative of ownership of our ordinary shares. This affiliate provides an inspection facility in Ireland for inspection and copying of our register in accordance with the Companies Act. A shareholder who holds shares beneficially is not the holder of record of such shares. Instead, the depository (for example, Cede & Co. as nominee for DTC) or other nominee is the holder of record of those shares. Accordingly, a transfer of shares from a person who holds such shares beneficially to a person who also holds such shares beneficially through a depository or other nominee will not be registered in our official share register, as the depository or other nominee will remain the record holder of any such shares.

A written instrument of transfer is required under Irish law in order to register on our official share register any transfer of shares (i) from a person who holds such shares directly to any other person, (ii) from a person who holds such shares beneficially to a person who holds such shares directly, or (iii) from a person who holds such shares beneficially to another person who holds such shares beneficially where the transfer involves a change in the depository or other nominee that is the record owner of the transferred shares. An instrument of transfer is also required for a shareholder who directly holds shares to transfer those shares into his or her own broker account (or vice versa). Such instruments of transfer may give rise to Irish stamp duty, which must be paid prior to registration of the transfer on our official Irish share register. However, a shareholder who directly holds shares may transfer those shares into his or her own broker account (or vice versa) without giving rise to Irish stamp duty provided there is no change in the ultimate beneficial ownership of the shares as a result of the transfer and the transfer is not made in contemplation of a sale of the shares.

Any transfer of our ordinary shares that is subject to Irish stamp duty will not be registered in the name of the buyer unless an instrument of transfer is duly stamped and provided to the transfer agent. We, in our absolute discretion and insofar as the Companies Act or any other applicable law permit, may provide that one of our subsidiaries will, pay Irish stamp duty arising on a transfer of our ordinary shares on behalf of the transferee of such ordinary shares. If stamp duty resulting from the transfer of our ordinary shares which would otherwise be payable by the transferee is paid by us or any of our subsidiaries on behalf of the transferee, then in those circumstances, we will, on our behalf or on behalf of our subsidiary (as the case may be), be entitled to (i) seek reimbursement of the stamp duty from the transferee, (ii) set-off the stamp duty against any dividends payable to the transferee of those ordinary shares and (iii) claim a first and permanent lien on our ordinary shares on which stamp duty has been paid by us or our subsidiary for the amount of stamp duty paid. Our lien shall extend to all dividends paid on those ordinary shares. Parties to a share transfer may assume that any stamp duty arising in respect of a transaction in our ordinary shares has been paid unless one or both of such parties is otherwise notified by us or the transfer agent.

Our Constitution delegates to any director, the secretary or any of our assistant secretaries duly appointed (or such other person as may be appointed by the secretary for this purpose) the authority, on our behalf, to execute an instrument of transfer on behalf of a transferring party.

The directors may suspend registration of transfers from time to time, not exceeding 30 days in aggregate each year.