Control of Spontaneous Lipid and Protein Transport

Abstract

Lipids in plasma are solubilized by a specialized group of molecules known as the apolipoproteins. In addition to carrying plasma lipids, these proteins have other regulatory activities related to lipid metabolism. Apolipoproteins A-I and C-I are activators of plasma lecithin:cholesterol acyltransferase, apolipoprotein C-II stimulates lipoprotein activity, and apolipoproteins B and E contain ligands for the receptormediated endocytosis of lipoproteins. Specific transfer proteins can also move lipid molecules among lipoprotein classes. In addition, apoproteins and sparingly soluble lipids transfer among cell and lipoprotein surfaces by a mechanism that involves rate-limiting desorption from the surface of the lipoprotein followed by diffusion-controlled association with acceptor cells or lipoproteins. Following association with cells, lipids may be internalized by a flip-flop mechanism. Thus, plasma lipids and proteins can undergo a large number of physical transport processes that are independent of receptors and enzyme activities. Below I will summarize the current understanding of how these spontaneous transfer processes are regulated by specific functional groups in lipids and apolipoproteins.