Effects of calcium-binding proteins (S100ao, S100a, S100b) on desmin assembly in vitro.

S-100a(o), the alpha alpha isoform of a subfamily of Ca(2+)-binding proteins of the EF-hand type expressed in cardiac and skeletal muscle cells, is reported to inhibit the assembly of the intermediate filament subunit desmin and to stimulate the disassembly of desmin intermediate filaments in the presence of micromolar levels of free Ca(2+). These effects are dose-dependent with respect to the S-100a(o) concentration and maximal at a desmin/S-100a(o) (dimer) molar ratio of approximately 2. Other members of the S-100 subfamily [S-100a (alpha beta) and S-100b (beta beta) and the unfractionated mixture of S-100a plus S-100b produce qualitatively similar effects on desmin assembly, with a potency that depends on the fraction of S-100alpha subunit (the most potent) or S-100beta subunit (the least potent) present in the S-100 isoforms tested. A binding stoichiometry of 2 mol of desmin/mol of S-100a(o) (dimer) and an affinity in the submicromolar range are calculated. The S-100beta subunit also interacts with desmin, but with a lower affinity compared with S-100alpha. By contrast, the S-100-like proteins calcyclin and p11 neither interact with desmin nor affect desmin assembly. The present data suggest that S-100a(o) might play a role in the regulation of the state of assembly of desmin intermediate filaments.