Interhelical contacts are required for the helix bundle fold of apolipophorin III and its ability to interact with lipoproteins.
Apolipophorin-III (apoLp-III) from the insect, Manduca sexta, is a 166-residue exchangeable apolipoprotein that plays a critical role in the dynamics of plasma lipoprotein interconversions. Our previous work indicated that a 36-residue C-terminal peptide fragment, generated by cyanogen bromide digestion of apoLp-III, was unable to bind to lipid surfaces (Narayanaswami V, Kay CM, Oikawa K, Ryan RO, 1994, Biochemistry 33:13312-13320), and showed no secondary structure in aqueous solution. In this paper, we have performed structural studies of this peptide (E131-Q166) complexed with SDS detergent micelles, or in the presence of the helix-inducing solvent trifluoroethanol (TFE), by two-dimensional 1H NMR spectroscopy. The peptide adopts an alpha-helical structure in the presence of both SDS and 50% TFE. The lipid-bound structure of the peptide, generated from the NMR NOE data, showed an elongated, slightly curved alpha-helix. Despite its high alpha-helix forming propensity, the peptide requires alpha helix-promoting environment to adopt an alpha-helical structure. This indicates the importance of the surrounding chemical environment and implies that, in the absence of lipid, tertiary contacts in the folded protein play a role in maintaining its structural integrity. Furthermore, the data suggest that the amphipathic helix bundle organization serves as a prerequisite structural motif for the reversible lipoprotein-binding activity of M. sexta apoLp-III.
Wang
J
J
Lipid and Lipoprotein Research Group, Department of Biochemistry, University of Alberta, Edmonton, Canada.
Narayanaswami
V
V
Sykes
B D
BD
Ryan
R O
RO
D000595:Amino Acid Sequence; D000818:Animals; D001053:Apolipoproteins / Q000737:chemistry*; D008074:Lipoproteins / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy; D018402:Manduca; D008969:Molecular Sequence Data; D017433:Protein Structure, Secondary
D001053:Apolipoproteins; D008074:Lipoproteins; C042470:apolipophorin III