Influence of C-terminal α-helix hydrophobicity and aromatic amino acid content on apolipoprotein A-I functionality

• Published in: Biochimica et biophysica acta Vol. 1821; no. 3; pp. 456 - 463
• Main Authors: Lyssenko, Nicholas N., Hata, Mami, Dhanasekaran, Padmaja, Nickel, Margaret, Nguyen, David, Chetty, Palaniappan Sevugan, Saito, Hiroyuki, Lund-Katz, Sissel, Phillips, Michael C.
• Format: Journal Article
• Language: English
• Published: Netherlands Elsevier B.V 01.03.2012
• Subjects: amino acid composition; Amino Acid Substitution; Amino Acids, Aromatic - chemistry; Amphipathic α-helix; Animals; Apolipoprotein A-I; Apolipoprotein A-I - chemistry; Apolipoprotein A-I - genetics; Apolipoprotein A-I - metabolism; ATP Binding Cassette Transporter 1; ATP binding cassette transporter A1 (ABCA1); ATP-Binding Cassette Transporters - metabolism; Cells, Cultured; Cellular cholesterol efflux; cholesterol; Cholesterol - metabolism; Cricetinae; high density lipoprotein; High density lipoprotein (HDL); Humans; Hydrophobic and Hydrophilic Interactions; hydrophobicity; leucine; Lipid solubilization; Lipoproteins, HDL - metabolism; metabolism; mice; Mutagenesis, Site-Directed; phospholipids; Protein Denaturation; Protein Stability; Protein Structure, Secondary; Protein Structure, Tertiary; solubilization; Transition Temperature; ABCA1; Lipid solubilization; apo; BHK; ANS; High density lipoprotein (HDL); Amphipathic α-helix; ATP binding cassette transporter A1 (ABCA1); MEM; Cellular cholesterol efflux; FBS; MLV; Apolipoprotein A-I; DMPC; GdnHCl; PL
• ISSN: 1388-1981; 0006-3002; 1879-2618; 0006-3002
• DOI: 10.1016/j.bbalip.2011.07.020

The apoA-I molecule adopts a two-domain tertiary structure and the properties of these domains modulate the ability to form HDL particles. Thus, human apoA-I differs from mouse apoA-I in that it can form smaller HDL particles; the C-terminal α-helix is important in this process and human apoA-I is unusual in containing aromatic amino acids in the non-polar face of this amphipathic α-helix. To understand the influence of these aromatic amino acids and the associated high hydrophobicity, apoA-I variants were engineered in which aliphatic amino acids were substituted with or without causing a decrease in overall hydrophobicity. The variants human apoA-I (F225L/F229A/Y236A) and apoA-I (F225L/F229L/A232L/Y236L) were compared to wild-type (WT) apoA-I for their abilities to (1) solubilize phospholipid vesicles and form HDL particles of different sizes, and (2) mediate cellular cholesterol efflux and create nascent HDL particles via ABCA1. The loss of aromatic residues and concomitant decrease in hydrophobicity in apoA-I (F225L/F229A/Y236A) has no effect on protein stability, but reduces by a factor of about three the catalytic efficiencies (Vmax/Km) of vesicle solubilization and cholesterol efflux; also, relatively large HDL particles are formed. With apoA-I (F225L/F229L/A232L/Y236L) where the hydrophobicity is restored by the presence of only leucine residues in the helix non-polar face, the catalytic efficiencies of vesicle solubilization and cholesterol efflux are similar to those of WT apoA-I; this variant forms smaller HDL particles. Overall, the results show that the hydrophobicity of the non-polar face of the C-terminal amphipathic α-helix plays a critical role in determining apoA-I functionality but aromatic amino acids are not required. This article is part of a Special Issue entitled Advances in High Density Lipoprotein Formation and Metabolism: A Tribute to John F. Oram (1945–2010). ► Apolipoprotein (apo) A-I structure and function. ► ApoA-I interacts with ABCA1 to mediate efflux of cellular cholesterol. ► ApoA-I solubilizes lipids to form HDL particles. ► Increased apoA-I α-helix hydrophobicity enhances formation of smaller HDL particles.