S-100b protein regulates aggregation and fusion of cardiolipin vesicles

We have recently shown that S-100b protein interacts with the polar surface of cardiolipin vesicles [6]. This interaction produces changes in the secondary structure of S-100b as well as changes in the structural organization of cardiolipin vesicles. We report here on the effects of S-100b on cardiolipin vesicles as investigated by turbidity, terbium-dipicolinate fluorescence and freeze-fracture. Experiments were carried out in the absence and in the presence of Ca2+. In the absence of Ca2+ (0.1 mM EDTA), S-100b favors the aggregation and fusion of vesicles to some extent. Under these conditions, electron microscope analyses reveal the presence of fused vesicles along with particles similar to those observed in protein reconstituted systems or to lipid particles observed during fusional processes. In the presence of Ca2+, S-100b counteracts the Ca2(+)-dependent tendency of vesicles to aggregate and fuse. Under these conditions, bilayer phases along with hexagonal phases can be observed by electron microscopy. The latter effects of S-100b are not due to chelation of Ca2+ because of the relative concentrations of S-100b and Ca2+ under our experimental conditions and since much larger concentrations of EDTA are required to produce the S-100b effects. We propose that the dimeric nature of S-100b plays a major role in these events. In the absence of Ca2+, the S-100b molecules probably cross-link adjacent vesicles, one subunit contacting one vesicle and the other subunit contacting another vesicle through electrostatic bonds. In the presence of Ca2+, due to the large changes occurring in the conformation of the protein (which loses about 52% of its alpha-helical content), S-100b associates strongly with the polar surface of individual vesicles, thus generating some kind of physical barrier to aggregation and fusion of vesicles.