Structural basis for distinct functions of the naturally occurring Cys mutants of human apolipoprotein A-I

Olga Gursky,Martin K Jones,Xiaohu Mei,Jere P Segrest,David Atkinson

doi:10.1194/jlr.r037911

Olga Gursky, Martin K Jones + Show 3 more

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https://doi.org/10.1194/jlr.r037911

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Abstract

HDL removes cell cholesterol and protects against atherosclerosis. ApoA-I provides a flexible structural scaffold and an important functional ligand on the HDL surface. We propose structural models for apoA-I(Milano) (R173C) and apoA-I(Paris) (R151C) mutants that show high cardioprotection despite low HDL levels. Previous studies established that two apoA-I molecules encircle HDL in an antiparallel, helical double-belt conformation. Recently, we solved the atomic structure of lipid-free Δ(185-243)apoA-I and proposed a conformational ensemble for apoA-I(WT) on HDL. Here we modify this ensemble to understand how intermolecular disulfides involving C173 or C151 influence protein conformation. The double-belt conformations are modified by belt rotation, main-chain unhinging around Gly, and Pro-induced helical bending, and they are verified by comparison with previous experimental studies and by molecular dynamics simulations of apoA-I(Milano) homodimer. In our models, the molecular termini repack on various-sized HDL, while packing around helix-5 in apoA-I(WT), helix-6 in apoA-I(Paris), or helix-7 in apoA-I(Milano) homodimer is largely conserved. We propose how the disulfide-induced constraints alter the protein conformation and facilitate dissociation of the C-terminal segment from HDL to recruit additional lipid. Our models unify previous studies of apoA-I(Milano) and demonstrate how the mutational effects propagate to the molecular termini, altering their conformations, dynamics, and function.

Highlights

HDL removes cell cholesterol and protects against atherosclerosis
The nascent HDL particle is generally envisioned as a contorted disc composed of a cholesterolcontaining phospholipid bilayer that is encircled by two antiparallel apoA-I molecules in a highly ␣-helical dynamic “double-belt” conformation [3,4,5]
Despite the overall similarity of the double belts formed by apoA-IWT, apoA-IM, and apoA-IP homodimers, there are important structural and dynamic differences resulting from the Cys mutations and the intermolecular disulfides in the mutant proteins (Fig. 7 and supplementary Fig. V)

Summary

Introduction

HDL removes cell cholesterol and protects against atherosclerosis. ApoA-I provides a flexible structural scaffold and an important functional ligand on the HDL surface. This conformation was obtained as previously described [9] by using the modular protein sequence (supplementary Fig. I) together with the crystallographic dimer of lipid-free ⌬(185–243)apoA-I [53] (Fig. 2A) and assuming that the C-terminal segment 185– 243 forms a highly helical closed double belt on HDL, similar to that observed in the low-resolution structure of ⌬(1–43)apoA-I [3].

Results

Conclusion