Spectroscopic characterization of the conformational adaptability of Bombyx mori apolipophorin III

Abstract

Apolipophorin III (apoLp-III) from the silkmoth, Bombyx mori, has been over-expressed in Escherichia coli, purified and characterized. Far-UV CD spectroscopic analysis revealed 65% α-helix secondary structure. NearUV CD spectra obtained in buffer or complexed with dimyristoylglycerophosphocholine (DMPC), provided evidence that apoLp-III α-helices reorient upon interaction with lipid, indicative of a protein conformational change. In guanidine hydrochloride (GdnHCl) denaturation studies, a transition midpoint of 0.33 M was observed, corresponding to a ΔG(D)/(H2O) = 2.46 kcal·mol-1. Fluorescence studies of the sole tryptophan residue (Trp40) in apoLp-III revealed an emission λ(max) = 327 nm. Compared to free tryptophan, Stern- Volmer constants (K(SV)) for acrylamide and KI quenching of Trp40 fluorescence were decreased by 20-fold and sevenfold, respectively. In studies of apoLp-III-DMPC disc complexes, far-UV CD spectroscopy revealed an increase in α-helix content to ≃ 85% and a ninefold increase in the GdnHCl- induced denaturation transition midpoint to 3 m. In studies of lipid interaction, apoLp-III was shown to disrupt both negatively charged and zwitterionic phospholipid bilayer vesicles, transforming them into discoidal complexes. Characterization of apoLp-III-DMPC discs, using 5-doxyl or 12- doxyl stearic acid as lipid-based quenching agents, revealed that Trp40 localizes near the phospholipid polar head groups. K(SV) values for acrylamide and KI quenching of intrinsic fluorescence of apoLp-III-DMPC discs indicate that Trp40 is embedded in the lipid milieu, with little or no accessibility to the aqueous quenchers. Given the large amount of α-helix in apoLp-III, the data presented support a model in which amphipathic α-helical segments are stabilized by helix-helix interactions and lipid association induces a protein conformational change which results in substitution of helix-helix interactions for helix-lipid contacts.