A molecular trigger of lipid binding-induced opening of a helix bundle exchangeable apolipoprotein.

Apolipophorin III (apoLp-III) from the sphinx moth, Manduca sexta, is a helix bundle protein that interacts reversibly with lipoproteins. Its five elongated amphipathic alpha-helices are organized in an antiparallel fashion, with helices 3 and 4 connected by a short 6-residue (PDVEKE) linker helix, termed helix 3'. Upon interaction with lipoproteins, apoLp-III opens to expose a continuous hydrophobic interior. It was postulated that helix bundle opening is preceded by an initiation step wherein helix 3' serves to recognize available lipoprotein surface binding sites. To test this hypothesis, helix 3' was replaced by residues that have a propensity to form a type I beta-turn, NPNG. This mutant apoLp-III was defective in lipoprotein binding assays. To define a more precise mode of interaction, the relevance of the presence of the hydrophobic Val-97 flanked by Asp-96 and Glu-98 was investigated by site-directed mutagenesis. V97N and D96N/V97N/E98Q apoLp-III were unable to compete with wild-type apoLp-III to initiate an interaction with lipoproteins, whereas D96N/E98Q apoLp-III was as competent as wild-type apoLp-III. The results suggest that Val-97 is critical, whereas Asp-96 and Glu-98 are irrelevant for initiating binding to lipoproteins. A model of binding is presented wherein apoLp-III is oriented with the helix 3' end of the molecule juxtaposed to the lipoprotein surface. Recognition of lipoprotein surface hydrophobic defects by Val-97 triggers opening of the helix bundle and facilitates formation of a stable binding interaction.