Interfacial properties of the apolipoprotein Cs: implications for the regulation of lipoprotein catabolism and atherosclerosis
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The risk of cardiovascular disease increases with elevated plasma levels of very-low density lipoproteins (VLDL) and chylomicrons. The human apolipoprotein Cs (apo C1, C2, C3) are small secretory proteins that circulate in plasma and play unique roles in the metabolism of VLDL and chylomicrons. ApoC2 is the required cofactor for lipoprotein lipase (LPL) which hydrolyzes plasma triacylglycerol. ApoC3 promotes VLDL synthesis in hepatocytes and both apoC1 and apoC3 inhibit LPL. The molecular details of these processes are largely unknown, but we hypothesized that apoC functions depend on protein structure, protein:lipid interactions, and surface pressure. Each apoC contains amphipathic N- and C-terminal helices that bind to and remodel lipid surfaces. Surface pressure—or the density of amphipathic molecules—increases significantly as LPL hydrolyzes triacylglycerol in VLDL. To probe the effects of protein structure and surface pressure on protein:lipid interactions, we used wild-type and point mutant variants of the apoCs, which differed in helical content and hydrophobicity. We used Oil-Drop Tensiometry to characterize the adsorption, conformational rearrangement, and desorption of each protein at lipid/water interfaces that mimic the core and surface of VLDL. This technique measured the effect of protein adsorption on surface pressure, and the surface area and pressure response of protein/lipid/water interfaces to volume changes that mimic lipogenic and lipolytic processes. We showed that the degree of protein amphipathic α-helical structure correlated with lipid affinity and provide a model for phenotypes in subjects with point mutations in apoC2 and apoC3. Each apoC exhibited multiple, pressure-dependent conformations at lipid surfaces, which indicates that the C-terminus of apoC2 likely desorbs from lipid at higher pressures to interact with LPL. ApoC3 exhibited a marked preference for lipid in the VLDL core, which provides novel insight into its role in VLDL assembly and secretion.