FIRST SUPPLEMENTAL INDENTURE

Exhibit 4.5

FIRST SUPPLEMENTAL INDENTURE

This FIRST SUPPLEMENTAL INDENTURE (this “Supplemental Indenture”), dated as of February 2, 2015, between WILLIAMS PARTNERS L.P., a Delaware limited partnership, formerly known as Access Midstream Partners, L.P. (the “Company”), and THE BANK OF NEW YORK MELLON TRUST COMPANY, N.A., a national banking association, duly organized and validly existing under the laws of the United States of America, as trustee (the “Trustee”).

WHEREAS, Williams Partners L.P., a Delaware limited partnership and predecessor by merger to the Company (the “Predecessor”), has heretofore executed and delivered to the Trustee an Indenture, dated as of February 9, 2010, between the Predecessor and the Trustee (the “Indenture”), pursuant to which the Predecessor’s 3.800% Senior Notes due 2015, 5.250% Senior Notes due 2020 and 6.300% Senior Notes due 2040 (together, the “Securities”) have been issued;

WHEREAS, on the date hereof, immediately prior with the effectiveness of this Supplemental Indenture, the Predecessor merged with and into the Company, with the Company surviving and continuing its existence under the laws of the State of Delaware, and the merger became effective under the laws of the State of Delaware;

WHEREAS, Section 801(2) of the Indenture provides, among other things, that the Predecessor shall not merge with or into any Person unless the Person formed by or surviving any such merger expressly assumes, by an indenture supplemental thereto, in form reasonably satisfactory to the Trustee, the due and punctual payment of the principal of, and any premium and interest on, all the Securities and the performance of every obligation of the Predecessor in the Indenture and the Securities of each series;

WHEREAS, Section 802 of the Indenture provides that, upon any merger in accordance with Section 801 of the Indenture, the successor Person formed by such merger shall succeed to, and be substituted for, and may exercise every right and power of, the Predecessor under the Indenture with the same effect as if such successor Person had been named as the Predecessor in the Indenture; and thereafter, except in the case of a lease, the Predecessor shall be released from all obligations and covenants under the Indenture and the Securities;

WHEREAS, Section 901(1) of the Indenture provides that, without the consent of any Holders of Securities of any series, the Company (when authorized by or pursuant to a Board Resolution) and the Trustee, at any time and from time to time, may enter into one or more supplemental indentures with respect to the Securities of such series, in form satisfactory to the Trustee, to evidence the succession of the Company to the Predecessor, and the assumption by the Company of the covenants of the Predecessor contained in the Indenture and in the Securities of such series;

WHEREAS, the Company desires and has requested that the Trustee join with it in the execution and delivery of this Supplemental Indenture for the purpose of evidencing such succession and assumption;

WHEREAS, the Company has been duly authorized to enter into this Supplemental Indenture;

WHEREAS, all conditions necessary to authorize the execution and delivery of this Supplemental Indenture and to make this Supplemental Indenture a valid and binding instrument enforceable in accordance with its terms have been complied with or have been done or performed; and

WHEREAS, pursuant to Section 901 of the Indenture, the Trustee is authorized to execute and deliver this Supplemental Indenture.

NOW THEREFORE, in consideration of the foregoing and for other good and valuable consideration, the receipt of which is hereby acknowledged, the parties mutually covenant and agree for the equal and ratable benefit of the Holders of the Securities as follows:

ARTICLE ONE

Except as otherwise expressly provided in or pursuant to this Supplemental Indenture or unless the context otherwise requires, for all purposes of this Supplemental Indenture the terms used herein without definition shall have the meanings assigned to them in the Indenture.

The terms and provisions contained in the Indenture shall constitute, and are hereby expressly made, a part of this Supplemental Indenture and the Company and the Trustee, by their execution and delivery of this Supplemental Indenture, expressly agree to such terms and provisions and to be bound thereby. However, to the extent any provision of the Indenture conflicts with the express provisions of this Supplemental Indenture, the provisions of this Supplemental Indenture shall govern and be controlling.

The Trustee accepts the amendment of the Indenture effected by this Supplemental Indenture and agrees to execute the trust created by the Indenture as hereby amended, but only upon the terms and conditions set forth in the Indenture, including the terms and provisions defining and limiting the liabilities and responsibilities of the Trustee in the performance of the trust created by the Indenture, and without limiting the generality of the foregoing, the Trustee shall not be responsible in any manner whatsoever for or with respect to any of the recitals or statements contained herein, all of which recitals or statements are made solely by the Company, or for or with respect to (1) the validity or sufficiency of this Supplemental Indenture or any of the terms or provisions hereof, (2) the proper authorization hereof by the Company, (3) the due execution hereof by the Company or (4) the consequences (direct or indirect and whether deliberate or inadvertent) of any amendment herein provided for, and the Trustee makes no representation with respect to any such matters.

The Article and Section headings in this Supplemental Indenture are for convenience only and shall not affect the construction hereof.

All covenants and agreements in this Supplemental Indenture by the Company shall bind its successors and assigns, whether so expressed or not.

In case any provision in this Supplemental Indenture or the Securities shall be invalid, illegal or unenforceable, the validity, legality and enforceability of the remaining provisions shall not in any way be affected or impaired thereby.

This Supplemental Indenture shall be governed by and construed in accordance with the laws of the State of New York applicable to agreements made or instruments entered into and, in each case, performed in said state. Each of the Company and the Trustee hereby irrevocably waives, to the fullest extent permitted by applicable law, any and all right to trial by jury in any legal proceeding arising out of or relating to this Supplemental Indenture, the Securities or the transactions contemplated hereby.

This Supplemental Indenture may be executed in several counterparts, each of which shall be an original and all of which shall constitute but one and the same instrument.

ARTICLE TWO

(a) In accordance with Section 801 of the Indenture, the Company hereby expressly assumes the due and punctual payment of the principal of, and any premium and interest on, all the Securities and the performance of every obligation of the Predecessor in the Indenture and the Securities of each series.

(b) In accordance with Section 802 of the Indenture, the Company shall succeed to, and be substituted for, and may exercise every right and power of, the Predecessor under the Indenture with the same effect as if the Company had been named as the Predecessor in the Indenture.

Signature page follows.

IN WITNESS WHEREOF, the parties hereto have caused this Supplemental Indenture to be duly executed as of the day and year first above written.

FIRST SUPPLEMENTAL INDENTURE