THIRD SUPPLEMENTAL INDENTURE

Exhibit 4.3

THIRD SUPPLEMENTAL INDENTURE

THIRD SUPPLEMENTAL INDENTURE (this “Third Supplemental Indenture”), dated as of June 24, 2013, between Southern Union Company, a Delaware (the “Company”) and The Bank of New York Mellon Trust Company, N.A. (formerly known as The Bank of New York Trust Company, N.A.) , a national banking association, as trustee (the “Trustee”).

WITNESSETH

WHEREAS, the Company has heretofore executed and delivered to the Trustee a Base Indenture, dated as of May 10, 1995 (the “Base Indenture”), and the Second Supplemental Indenture, dated October 23, 2006 ( together with the Base Indenture, as heretofore amended or supplemented, the “Indenture”) providing for the issuance of the Company’s 7.20% 2006 Series A Junior Subordinated Notes due November 1, 2066 (the “Notes”);

WHEREAS, Section 902 of the Indenture provides that, subject to certain conditions, with the consent of the Holders of not less than a majority in principal amount of all Outstanding Securities of any series, by Act of said Holders delivered to the Company and the Trustee, the Company, when authorized by or pursuant to a Board Resolution, and the Trustee may enter into supplemental indentures for the purpose of changing in any manner or eliminating any of the provisions of the Indenture which affect such series of Securities or of modifying in any manner the rights of Holders of Securities under the Indenture;

WHEREAS, Holders of a majority in aggregate principal amount of Notes outstanding have consented to the amendments to the Indenture set forth below in this Third Supplemental Indenture;

WHEREAS, the Company has delivered to the Trustee an Officers’ Certificate as well as an Opinion of Counsel to the effect that the execution and delivery of this Third Supplemental Indenture by the Company is authorized or permitted under the Indenture;

WHEREAS, the Company is undertaking to execute and deliver this Third Supplemental Indenture to amend or eliminate certain terms and covenants in the Indenture in connection with the Offering Memorandum and Consent Solicitation Statement of the Company dated as of May 16, 2013, and any amendments, modifications or supplements thereto (the “Exchange Offer and Consent Solicitation”);

WHEREAS, the Company has been authorized by Board Resolutions to enter into this Third Supplemental Indenture;

WHEREAS, all conditions necessary to authorize the execution and delivery of this Third Supplemental Indenture by the Company and to make this Third Supplemental Indenture valid and binding on the Company have been complied with or have been done or performed; and

WHEREAS, pursuant to Section 902 of the Indenture, the Trustee is authorized to execute and deliver this Third Supplemental Indenture.

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

ARTICLE ONE

Section 1.01. CAPITALIZED TERMS. Capitalized terms used herein without definition shall have the meanings assigned to them in the Indenture.

ARTICLE TWO

Section 2.01. EFFECTIVENESS OF THIRD SUPPLEMENTAL INDENTURE. This Third Supplemental Indenture shall become effective as of the date hereof provided that the amendments to the Indenture set forth in Section 2.02 below shall not become operative unless and until Energy Transfer Partners, L.P., the indirect parent of the Company, consummates the Exchange Offer (as defined in the Exchange Offer and Consent Solicitation) with respect to the Notes pursuant to the terms and in the manner described in the Exchange Offer and Solicitation.

Section 2.02. AMENDMENTS TO THE INDENTURE.

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ARTICLE THREE

Section 3.01. RATIFICATION OF INDENTURE; THIRD SUPPLEMENTAL INDENTURE. Except as expressly amended hereby, the Indenture, as amended or supplemented by this Third Supplemental Indenture, is in all respects ratified and confirmed and all the terms, conditions and provisions thereof shall remain in full force and effect. Upon the execution and delivery of this Third Supplemental Indenture by the Company and the Trustee, this Third Supplemental Indenture shall form a part of the Indenture for all purposes, and the Company, the Trustee and every Holder of Notes heretofore or hereafter authenticated and delivered shall be bound hereby. Any and all references to the Indenture, whether within the Indenture or in any notice, certificate or other instrument or document, shall be deemed to include a reference to this Third Supplemental Indenture (whether or not made), unless the context shall otherwise require.

Section 3.02. GOVERNING LAW. THIS THIRD SUPPLEMENTAL INDENTURE WILL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF NEW YORK.

Section 3.03. THE TRUSTEE. The Trustee shall not be responsible in any manner for or in respect of the validity or sufficiency of this Third Supplemental Indenture or for or in respect of the recitals or statements contained herein, all of which are made by the Company and the Trustee assumes no responsibility for their correctness.

Section 3.04. SUCCESSORS. All covenants and agreements of the Trustee in this Third Supplemental Indenture shall bind its successors and assigns. All covenants and agreements of the Company in this Third Supplemental Indenture shall bind its successors and assigns.

Section 3.05. COUNTERPARTS. The parties may sign any number of copies of this Third Supplemental Indenture. Each signed copy shall be an original, but all of them together represent the same agreement. The exchange of copies of this Third Supplemental Indenture and of signature pages by facsimile or electronic format (i.e., “pdf” or “tif”) transmission shall constitute effective execution and delivery of this Third Supplemental Indenture as to the parties hereto and may be used in lieu of the original Third Supplemental Indenture for all purposes. Signatures of the parties hereto transmitted by facsimile or electronic format (i.e., “pdf” or “tif”) shall be deemed to be their original signatures for all purposes.

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Section 3.06. EFFECT OF HEADINGS. The Section headings herein are for convenience only and shall not affect the construction hereof.

Section 3.07. SEVERABILITY. If any provision in this Third Supplemental Indenture shall be invalid, illegal or unenforceable, the validity, legality or enforceability of the remaining provisions of this Third Supplemental Indenture or the Indenture shall not in any way be affected or impaired thereby. This Third Supplemental Indenture is subject to the provisions of the Trust Indenture Act that are required to be part of the Indenture and shall, to the extent applicable, be governed by such provisions. If any provision of this Third Supplemental Indenture limits, qualifies or conflicts with another provision hereof which is required to be included herein by any provisions of the Trust Indenture Act, such required provision shall control.

[Remainder of Page Left Intentionally Blank]

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

[Signature Page to the Third Supplemental Indenture]

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