STRUCTURAL INFLUENCE OF CATION-BINDING TO RECOMBINANT HUMAN BRAIN S100B - EVIDENCE FOR CALCIUM-INDUCED EXPOSURE OF A HYDROPHOBIC SURFACE

The dimeric calcium-binding protein S100b is proposed to undergo a cal cium-induced structural change allowing it to interact, via a hydropho bic surface, with other proteins. Previously it has been suggested tha t calcium binding to S100b leads to the exposure of at least one pheny lalanine residue (Mani et al., 1982, 1983). This effect appears to be ''reversed'' at higher ionic strength, leading to a possible reburying of phenylalanine residues (Mani et al., 1982, 1983). To study these e ffects, we monitored calcium binding to recombinant human S100b by NMR spectroscopy under different salt (KCl) conditions. N-15-Labeled glyc ine residues in S100b showed calcium-induced chemical shift changes si milar to those reported for the related monomeric protein calbindin D- 9k, suggesting similar conformational changes are occurring in the cal cium-binding loops of these two proteins. Calcium binding to S100b als o resulted in a shifting and broadening of several H-1 resonances from the Ca-S100b form only including those from the side chains of residu es F14, F70, and F73 but not those of residue Y17. This broadening was enhanced with increased ionic strength (KCl). However, small addition s (<15% v/v) of the hydrophobic solvent trifluoroethanol relieved this phenomenon, leading to narrower line widths. These observations are c onsistent with the calcium-induced exposure of at least one of these h ydrophobic residues, resulting in self-association of the S100b dimer. Trifluoroethanol serves to dissociate these complexes back to the dim eric calcium species. We propose that this cluster of hydrophobic resi dues which include F14, F73, and F88 may be important for interactions with a target protein.