The Role of Apolipoprotein A-I Helix 10 in Apolipoprotein-mediated Cholesterol Efflux via the ATP-binding Cassette Transporter ABCA1

Stacey E Panagotopulos,Scott R Witting,Erica M Horace,David Y Hui,J Nicholas Maiorano,W Sean Davidson

doi:10.1074/jbc.m207005200

Stacey E Panagotopulos, Scott R Witting + Show 4 more

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https://doi.org/10.1074/jbc.m207005200

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Abstract

Recent studies of Tangier disease have shown that the ATP-binding cassette transporter A1 (ABCA1)/apolipoprotein A-I (apoA-I) interaction is critical for high density lipoprotein particle formation, apoA-I integrity, and proper reverse cholesterol transport. However, the specifics of this interaction are unknown. It has been suggested that amphipathic helices of apoA-I bind to a lipid domain created by the ABCA1 transporter. Alternatively, apoA-I may bind directly to ABCA1 itself. To better understand this interaction, we created several truncation mutants of apoA-I and then followed up with more specific point mutants and helix translocation mutants to identify and characterize the locations of apoA-I required for ABCA1-mediated cholesterol efflux. We found that deletion of residues 221-243 (helix 10) abolished ABCA1-mediated cholesterol efflux from cultured RAW mouse macrophages treated with 8-bromo-cAMP. Point mutations in helix 10 that affected the helical charge distribution reduced ABCA1-mediated cholesterol efflux versus the wild type. We noted a strong positive correlation between cholesterol efflux and the lipid binding characteristics of apoA-I when mutations were made in helix 10. However, there was no such correlation for helix translocations in other areas of the protein as long as helix 10 remained intact at the C terminus. From these observations, we propose an alternative model for apolipoprotein-mediated efflux.

Highlights

Numerous studies have shown that high levels of high density lipoprotein (HDL)1 and its most abundant protein constituent apolipoprotein A-I can reduce the risk of atherosclerosis, the leading cause of death in industrialized countries [1]
Recent studies of Tangier disease have shown that the ATP-binding cassette transporter A1 (ABCA1)/apolipoprotein A-I interaction is critical for high density lipoprotein particle formation, apoA-I integrity, and proper reverse cholesterol transport
It is likely that lipid-free apoA-I is rapidly cleared from plasma unless it becomes associated with lipid through interaction with the ABCA1 transporter, making the ABCA1/apoA-I interaction critical for the initial formation of HDL particles [11]

Summary

Introduction

Numerous studies have shown that high levels of high density lipoprotein (HDL) and its most abundant protein constituent apolipoprotein A-I (apoA-I) can reduce the risk of atherosclerosis, the leading cause of death in industrialized countries [1]. It has been shown that lipid-poor forms of apoA-I can actively remove cholesterol by directly interacting with the cell membrane to form a nascent HDL particle in a process called apolipoproteinmediated lipid efflux [3,4,5,6]. The lack of an obvious common binding sequence among these apolipoproteins has cast doubt on the idea of a highly specific interaction with ABCA1 In light of these studies, a second hypothesis has been proposed suggesting an interaction between apoA-I and lipid domains in the cell membrane formed by ABCA1, a suspected membrane lipid. Because the ultimate product of apolipoprotein-mediated cholesterol efflux is a lipidated form of apoA-I, it is perhaps not surprising that a protein lacking its best lipid-binding region did not perform well in the assay

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