Interaction of S-100b protein with cardiolipin vesicles as monitored by electron spin resonance, pyrene fluorescence and circular dichroism

Abstract

The interaction of S-100b protein with cardiolipin (CL) vesicles has been studied by electron spin resonance, pyrene fluorescence, and circular dichroism. Electron spin resonance and pyrene fluorescence data indicate that S-100b binds to the polar surface of vesicles Ca 2+-independently. In the presence of Ca 2+, S-100b potentiates the Ca 2+-induced clustering of the polar headgroups of CL molecules and causes a further reduction in the Ca 2+-dependent decrease in the lateral mobility of the pyrene inserted into the lipid bilayer, which points to an effect of the protein on the hydrophobic core of the lipid bilayer through a larger perturbation of its polar surface. Circular dichroism analyses indicate that CL vesicles cause a decrease in the α-helical content of S-100b, analogous to that produced by Ca 2+ and that the effects of CL vesicles and of Ca 2+ on the secondary structure of the protein are supra-additive. By this technique, we found that the affinity of Ca 2+ for S-100b increases substantially in the presence of CL vesicles, even in the presence of physiologic concentrations of KCl, suggesting that once S-100b had interacted with CL vesicles it assumes a new conformation in which its Ca 2+-binding properties are greatly enhanced. These results are discussed in relation to binding of S-100b proteins to natural membranes, and to a possible involvement of S-100b in the regulation of membrane structural organization.