The ATPase activity of ABCA1 is increased by cholesterol in the presence of anionic lipids.

Abstract

High-density lipoprotein (HDL) transports excess cholesterol from peripheral tissues back to the liver, and plasma HDL levels are inversely related to cardiovascular disease incidence. ATP-binding cassette A1 (ABCA1) is a member of the ABC protein superfamily, and generates nascent HDL, which consists of several hundreds of phospholipids and cholesterol wrapped by apolipoprotein A-I (apoA-I). However, it remains unclear whether cholesterol is a transport substrate of ABCA1. Since ATP hydrolysis of ABC proteins is typically increased by their transport substrates, we characterized the effects of cholesterol on the ATPase activity of purified ABCA1 using liposomes of various lipid compositions. ABCA1 showed substantial ATPase activity (20-30 nmol$\cdot$min-1$\cdot$mg-1) only in liposomes containing anionic lipids, including phosphatidylserine. Cholesterol increased the ATPase activity by 1.6- to 3-fold in the presence of anionic lipids. Moreover, phosphatidylserine addition to BHK/ABCA1 cells increased phosphatidylcholine and cholesterol efflux to apoA-I. Next, we investigated the sterol specificity of ABCA1. The ATPase activity of ABCA1 was strongly enhanced by desmosterol and zymosterol, similar to cholesterol. In contrast, 7-dehydrocholesterol and lathosterol weakly increased the ATPase activity, and no increase was observed with stigmasterol or brassicasterol. These findings suggest that ABCA1 transports cholesterol and prefers cholesterol over plant sterols as a transport substrate.