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 ind icated that a 36-residue C-terminal peptide fragment, generated by cya nogen bromide digestion of apoLp-III, was unable to bind to lipid surf aces (Narayanaswami V, Kay CM, Oikara K, Ryan RO, 1994, Biochemistry 3 3:13312-13320), 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 o f the helix-inducing solvent trifluoroethanol (TFE), by two-dimensiona l H-1 NMR spectroscopy. The peptide adapts an alpha-helical structure in the presence of both SDS and 50% TFE. The lipid-hound structure of the peptide, generated from the NMR NOE data, showed an elongated, sli ghtly curved alpha-helix. Despite its high alpha-helix forming propens ity the peptide requires a helix-promoting environment to adopt an alp ha-helieal structure, This indicates the importance of the surrounding chemical environment and implies that, in the absence of lipid, terti ary contacts in the folded protein play a role in maintaining its stru ctural integrity, Furthermore, the data suggest that the amphipathic h elix bundle organization serves as a prerequisite structural motif for the reversible lipoprotein-binding activity of M. sexta apoLp-IlI.