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Protein Science, Vol 7, Issue 2 336-341, Copyright © 1998 by Cold Spring Harbor Laboratory Press

ARTICLE

Interhelical contacts are required for the helix bundle fold of apolipophorin III and its ability to interact with lipoproteins

J. WANG, V. NARAYANASWAMI, B. D. SYKES and R. O. RYAN
Lipid and Lipoprotein Research Group, Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2S2 Protein Engineering Networks of Centers of Excellence, Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2S2

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 (1)H 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 a 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.
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P. M. M. Weers, V. Narayanaswami, C. M. Kay, and R. O. Ryan
Interaction of an Exchangeable Apolipoprotein with Phospholipid Vesicles and Lipoprotein Particles. ROLE OF LEUCINES 32, 34, AND 95 IN LOCUSTA MIGRATORIA APOLIPOPHORIN III
J. Biol. Chem., July 30, 1999; 274(31): 21804 - 21810.
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