Structural studies of apolipoprotein A-I and ATP-binding cassette A1 and their roles in nascent high density lipoprotein biogenesis
Apolipoprotein A-I (apoA-I) and ATP-Binding Cassette A1 (ABCA1) transporter play important roles in nascent high density lipoprotein (nHDL) biogenesis – the first step in the reverse cholesterol transport pathway. Based on the crystal structure of a C-terminally truncated form of apoA-I (apoA-I(1-184)) determined in the laboratory, structurally designed and naturally occurring mutants of apoA-I were conformationally characterized in solution. The function of these mutants in nHDL formation was assessed in ABCA1-transfected HEK293 cells. An apoA-I mutant designed to destabilize the N-terminal helical bundle at the first hinge region, 38/40G, exhibited a locally reduced α-helical content, destabilized overall structure, and increased lipid binding ability in solution, indicating a destabilized N-terminal helical bundle. In the cellular system, 38/40G showed significantly enhanced nHDL forming ability, suggesting that a destabilized N-terminal bundle will facilitate nHDL formation. Other designed N-terminal mutants (Q41A, P66G, G65A, V67P, T68P, 65/67/68P) and the naturally occurring mutants (R153P, L178P, and insertion mutant apoA-INashua) all showed either unchanged or destabilized overall structure, unchanged lipid binding abilities in solution and unchanged nHDL formation and cholesterol efflux promotion from the cells. Mutants designed to progressively extend the C-terminus (1-184, 1-198, 1-209, 1-220, 1-231) yielded progressively increased nHDL formation and cholesterol efflux, suggesting that the C-terminus of apoA-I is critical for these two activities. Central Helix 5 triple glycine mutation (H5 3xG) designed to lock the monomer conformation of apoA-I resulted in reduced nHDL formation but unaffected cholesterol efflux, suggesting that hindering apoA-I monomer to dimer conversion could retard nHDL formation. Remarkably, studies of cholesterol efflux and nHDL particle formation indicated that the two processes might be two uncoupled events. Analysis of the nHDL particles revealed the presence of ganglioside (GM1) in the complexes. Cross-linking data demonstrated binding of apoA-I to ABCA1-expressing cells. The binding level of apoA-I mutants to ABCA1-expressing cells was positively correlated with nHDL forming ability of these mutants. ABCA1 was isolated from FreeStyle™ HEK293-F cells in suspension by detergent solubilization and was shown to have ATPase activity. A direct interaction between apoA-I and amphipol solubilized- ABCA1 in solution was detected for the first time. Furthermore, the successful purification of ABCA1 has laid the foundation of structure determination of this protein in the future.