High-density lipoprotein (HDL) particles, generated in the process of removing excess cellular cholesterol, play crucial roles in maintaining cholesterol homeostasis in arterial cells and in protecting the cardiovascular system from the development of atherosclerosis. Cholesterol-loaded cells increase their binding capacity to the HDL scaffolding protein, apolipoprotein A-I (ApoA-I), however, cell surface factors necessary for ApoA-I binding remains to be elucidated. To characterize cell membrane microdomains interacting with ApoA-I, primary human skin fibroblasts were incubated with ApoA-I for 1h at 4°C. After linking protein-protein interactions with a membrane-impermeable crosslinker, DTSSP, cells were subjected to homogenization. The cell homogenate was separated by a discontinuous sucrose gradient centrifugation and ten fractions were collected. ApoA-I-associated cell membrane fraction was located by immunoblotting for ApoA-I and organelle markers. Membrane-containing fractions were fragmented using sonication prior to immunoprecipitation of ApoA-I-associated microdomains using an anti-ApoA-I antibody. Major lipid classes present in the microdomains are phosphatidylcholine, phosphatidylserine, sphingomyelin and cholesterol. Two cell membrane proteins, caveolin and ABCA1, were excluded from the microdomains. These data suggest that ApoA-I bind to cholesterol-rich cell surface microdomains that are different from ABCA1 and caveolae domains. LC-MS/MS analysis identified the presence of 26 proteins in the microdomains. Among these, several desmosomal proteins, lipid binding proteins and protease inhibitors were identified. Overall, our results suggest that the initial binding of ApoA-I to cell surface occurs on the lateral sides of cell membranes where desmosomal proteins provide a binding site for ApoA-I, and that lipid binding proteins facilitate lipidation of ApoA-I while protease inhibitors protect ApoA-I and related proteins from degradation. In conclusion, we established a new method to isolate cell membrane microdomains interacting with ApoA-I. Using this method, we found that ApoA-I associates with desmosomal proteins for the formation of HDL.
Characterization of two structurally related Xenopus laevis protein tyrosine phosphatases with homology to lipid-binding proteins.
