Apolipoprotein (apo)A-I, the major protein constituent of high density lipoproteins (HDL), acts as a conformationally dynamic scaffold on the surface of the particle. Evidence suggests apoA-I conformation dictates HDL functionality and capacity to interface with other HDL proteins; however, the details of apoA-I structure and HDL formation are not completely understood. X-ray crystal structures of truncated forms of apoA-I and its cousin, apoA-IV, show reciprocal domain swapping structures in the lipid-free state. Recently, we showed the truncated mutant of apoA-I is also highly dynamic in solution with a flexible N-terminus. The purpose of this study was to a) derive a structural model of wild-type (WT) apoA-I using the new in-solution truncation model as a template and b) determine the relationship between monomeric, and oligomeric forms of lipid-free WT apoA-I. Using a novel isotope labeling strategy and chemical cross-linking, we defined spatial relationships within and between different molecules of WT apoA-I. The cross-linking patterns revealed a high degree of structural similarity between WT apoA-I oligomers. We found that monomeric and dimeric WT apoA-I structures were similar, but not identical, to those predicted by the crystal and solution structures of truncated apoA-I. Using cross-linking data, we derived two interconverting structures each for monomeric WT apoA-I, both containing dynamic N- and C-terminal regions. Similar structures were generated for WT dimeric and trimeric species. Small angle X-ray scattering was used to derive molecular envelopes of the molecules and refine the arrangements to propose the most detailed molecular models of lipid-free WT apoA-I to date. The models were used in conjunction with the reported double-belt and trefoil structures reconstituted HDL to reveal evidence of a parsimonious relationship between lipid-free and lipid-bound WT apoA-I. We propose a structural framework of WT apoA-I self-association and lipidation, centric to the C-terminal region of the molecule, that forms a basis for understanding HDL particle formation at the cell surface.
Molecular Model of a Soluble Guanylyl Cyclase Fragment Determined by Small-Angle X-ray Scattering and Chemical Cross-Linking
