Interaction of divalent cations and proteins with phospholipid vesicles

The broadening of spin-label absorption lines resulting from spin-exchange reactions that occur during collision with paramagnetic Ni2+ is diminished when Ni2+ binds to phospholipid vesicles. Subsequent addition of non-paramagnetic ions that compete for binding sites releases Ni2+ into solution and restores the line-broadening. The concentrations of various ions required to achieve this effect was used to order the ions with respect to their binding to vesicles containing phosphatidylethanolamine and phosphatidylglycerol. The relative strengths of binding for those ions studied were: Ca2+ > Mg2+ > Zn2+ > Sr2+ > Ba2+. The spin-broadening assay was also used to study the effects of two proteins on the availability of Ni2+-binding sites on the vesicles. Ribonuclease, which is thought to associate electrostatically as an extrinsic protein on the surface of vesicles, completely blocked the Ni2+-binding sites at comparatively low protein concentrations. Quantitative considerations of these data suggest the possibility that Ni2+ may bind preferenetially to phosphatidylglycerol, and that these binding sites are aggregated in the ribonuclease-containing vesicles. In contract to ribonuclease, cytochrome c does not block Ni2+-bindings sites on the phospholipid vesicles, but rather contains sites of its own that bind Ni2+, both when the protein is in solution and when it is associated with the vesicles. These results are consistent with other studies which suggest that cytochrome c becomes partially embedded in membrane bilayers and associates with phospholipid molecules through hydrophobic interactions.