The size of S-100a and S-100b proteins in solution have been examined by gel filtration and ultracentrifugation in the presence and absence of Ca2+. S-100a and S-100b proteins, in the absence of Ca2+, have intrinsinc sedimentation coefficient, s o 20,w of 2.20 and 2.15 S, respectively and in 1 mM Ca2+ their s o 20,w values decreased to 2.05 and 1.95 S, respectively, indicating an unfolding of the protein molecules. The Stokes radii of S-100a and S-100b (—Ca2+) were 23.4 Å and 24.0 Å and they decreased to 22.2 Å and 22.3 Å in the presence of Ca2+. The Ca2+ effect on S-100b > S-100a was in agreement with our earlier CD observations. Among the monovalent cations tested (K+, Na+ and Li+) K+ had the maximum effect on the Stokes radii and s o 20,w values of S-100 proteins. Since certain functions of the nervous system are accompanied by local changes in ionic concentrations of Ca2+, Na+ and K+, it is conceivable that these respective conformational changes induced in S-100 proteins by these metals may be related to their function in the brain.
