Binding of insect apolipophorin III to dimyristoylphosphatidylcholine vesicles. Evidence for a conformational change.

摘要: Apolipophorin-III (apoLp-III), a hemolymph protein of Manduca sexta, can reversibly associate with the surface lipoprotein particles. In order to examine lipid-associated form apoLp-III, present studies investigate structure and properties apoLp-III complexes dimyristoylphosphatidylcholine (DMPC). Association DMPC vesicles results in formation uniform discs an average diameter width 18.5 +/- 2.0 nm 4.8 0.8 nm, respectively, as determined by electron microscopy. ApoLp-III.DMPC analyzed pore-limiting native gradient PAGE demonstrated that single major species complex was formed within wide range lipid molar ratios (DMPC:apoLp-III; 13:1 360:1). Flotation equilibrium experiments, conducted analytical ultracentrifuge, confirmed only one apoLp-III.DMPC at initial ratio 67:1, apparent molecular mass 642,000. Complexes cross-linked dimethyl suberimidate indicate there are maximum 6 molecules per disc. Circular dichroism experiments revealed becomes essentially completely alpha-helical on complexes. Compared lipid-free state, were relatively resistant denaturation guanidine HCl, displaying transitions midpoints 2.2 3.7 M respectively. The fluorescence excitation emission spectra demonstrate large enhancement tyrosine compared suggesting conformational change occurs when associates surface. Denaturation HCl resulted level similar presence HCl. tyrosine-induced quenched both Cs+ (Kq = 0.573 M-1) KI 0.376 M-1). presented this study conformation is stabilized complexed phospholipids suggest provides sensitive method detect M. sexta interaction surfaces.(ABSTRACT TRUNCATED AT 400 WORDS)