STRUCTURAL STUDIES OF FARNESYLATED RAS ASSOCIATED W/ MEMBRANE BILAYERS

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M. sexta apolipophorin-III is a 166 residue exchangeable apolipoprotein from the insect Manduca Sexta involved in the lipid transport processes within the circulatory system. It is a lipid-associated protein, however it is not a transmembrane protein. In its water-soluble form, this protein has been crystallized, and the structure was found to adopt a five-helix bundle conformation. Lipid association is believed to involve a drastic conformational change wherein the helix bundle unravels. The resulting lipoprotein particle is believed to adopt a sphere-like shape with an outer core of protein surrounding an inner core of hydrophobic lipid. The protein presumably orients with hydrophylic regions directed towards the aqueous environment, and its hydrophobic regions toward the lipid core. Solid-state NMR spectra were recorded of the uniformly 15N labeled apolipophorin - III in lipid bilayers on glass plates. The resulting 15N chemical shift spectrum indicated a dispersion of intensities across the 15N powder pattern range with an increase of intensity at the ?33 discontinuity compared to the typical powder pattern distribution. This result indicates a portion of the sites in the protein orient with a helical axis aligned along the bilayer normal.