Effects of calcium‐binding proteins (S‐100a o , S‐100a, S‐100b) on desmin assembly in vitro

S‐100a o , the α 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 (dinier) molar ratio of ~2. Other members of the S‐100 subfamily [S‐100a (αβ) and S‐100h ββ)] and the unfractionated mixture of S‐100a plus S‐100b pro‐duce qualitatively similar effects on desmin assembly, with a potency that depends on the fraction of S‐100a subunit (the most potent) or S‐100β subunit (the least potent) present in the S‐100 isoforms tested. A binding stoichiometry of 2 mol of des‐min/mol of S‐100a o (dimer) and an affinity in the submicromolar range are calculated. The S‐100 β subunit also interacts with desmin, but with a lower affinity compared with S‐100α, By contrast, the S‐100‐like proteins calcyclin and pll 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.—Garbuglia, M., Verzini, M., Giambanco, I., Spreca, A., Donato, R. Effects of calcium‐binding proteins (S‐100a o , S‐100a, S‐100b) on desmin assembly in vitro. FASEB J. 10, 317‐324 (1996)