Crystal Structure of Δ(185-243)ApoA-I Suggests a Mechanistic Framework for the Protein Adaptation to the Changing Lipid Load in Good Cholesterol: From Flatland to Sphere land via Double Belt, Belt Buckle, Double Hairpin and Trefoil/Tetrafoil

Apolipoprotein A-I (apoA-I) is the major protein of plasma high-density lipoproteins (HDLs), nnacromolecular assemblies of proteins and lipids that remove cell cholesterol and protect against atherosclerosis. HDL heterogeneity, large size (7.7-12 nm), and ability to exchange proteins have prevented high-resolution structural analysis. Low-resolution studies showed that two apoA-I molecules form an antiparallel alpha-helical double belt around an HDL particle. The atomic-resolution structure of the C-terminal truncated lipid-free Delta(185-243)apoA-I, determined recently by Mei and Atkinson, provides unprecedented new insights into HDL structure-function. It allows us to propose a molecular mechanism for the adaptation of the full-length protein to increasing lipid load during cholesterol transport. ApoA-I conformations on small, midsize, and large HDLs are proposed based on the tandem alpha-helical repeats and the crystal structure of Delta(185-243)apoA-I and are validated by comparison with extensive biophysical data reported by many groups. In our models, the central half of the double belt (constant segment 66-184) is structurally conserved while the N- and C-terminal half (variable segments 1-65 and 185-243) rearranges upon HDL growth. This includes incremental unhinging of the N-terminal bundle around two flexible regions containing G39 and 065 to elongate the belt, along with concerted swing motion of the double belt around G65-P66 and G185-G186 hinges that are aligned on various-size particles, to confer two-dimensional surface curvature to spherical HDLs. The proposed conformational ensemble integrates and improves several existing HDL models. It helps provide a structural framework necessary to understand functional interactions with over 60 other HDL-associated proteins and, ultimately, improve the cardioprotective function of HDL. (C) 2012 Elsevier Ltd. All rights reserved.