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J. Biol. Chem., Vol. 283, Issue 39, 26528-26537, September 26, 2008

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The Epidermal Growth Factor Homology Domain of the LDL Receptor Drives Lipoprotein Release through an Allosteric Mechanism Involving H190, H562, and H586^*

From the Department of Cell Biology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390-9039

The low density lipoprotein (LDL) receptor (LDLR) mediates efficient endocytosis of VLDL, VLDL remnants, and LDL. As part of the endocytic process, the LDLR releases lipoproteins in endosomes. The release process correlates with an acid-dependent conformational change in the receptor from an extended, "open" state to a compact, "closed" state. The closed state has an intramolecular contact involving H190, H562, and H586. The current model for lipoprotein release holds that protonation of these histidines drives the conformational change that is associated with release. We tested the roles of H190, H562, and H586 on LDLR conformation and on lipoprotein binding, uptake, and release using variants in which the three histidines were replaced with alanine (AAA variant) or in which the histidines were replaced with charged residues that can form ionic contacts at neutral pH (DRK variant). Contrary to expectation, both the AAA and the DRK variants exhibited normal acid-dependent transitions from open to closed conformations. Despite this similarity, both the AAA and DRK mutations modulated lipoprotein release, indicating that H190, H562, and H586 act subsequent to the conformational transition. These observations also suggest that the intramolecular contact does not drive release through a competitive mechanism. In support of this possibility, mutagenesis experiments showed that β-VLDL binding was inhibited by mutations at D203 and E208, which are exposed in the closed conformation of the LDLR. We propose that H190, H562, and H586 are part of an allosteric mechanism that drives lipoprotein release.

Received for publication, June 17, 2008 , and in revised form, July 24, 2008.

^* This work was supported, in whole or in part, by National Institutes of Health Grant HL085218. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Cell Biology, The University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9039. Tel.: 214-648-3238; Fax: 214-648-8694; E-mail: Peter.Michaely{at}utsouthwestern.edu.

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