Mts1 regulates the assembly of nonmuscle myosin-IIA.

The formation of myosin-II filaments is fundamental to contractile and motile processes in nonmuscle cells, and elucidating the mechanisms controlling filament assembly is essential for understanding how myosin-II rapidly responds to changing conditions within the cell. Several proteins including KRP and a novel 38 kDa protein (1, 2) have been shown to modulate filament assembly through the stabilization of myosin-II assemblies. In contrast, we demonstrate that mts1, a member of the Ca(2+)-regulated S100 family of proteins, may regulate the monomeric, unassembled state in an isoform-specific manner. Biochemical analyses demonstrate that mts1 has a 9-fold higher affinity for myosin-IIA filaments than for myosin-IIB filaments. At stoichiometric levels, mts1 inhibits the assembly of myosin-IIA monomers into filaments and promotes the disassembly of myosin-IIA filaments into monomers; however, mts1 has little effect on the assembly properties of myosin-IIB. Using a solution based-assay, we have demonstrated that mts1 binds to residues 1909-1924 of the myosin-IIA heavy chain, which is near the C-terminal tip of the alpha-helical coiled-coil. The observation that mts1 binds a linear sequence of approximately 16 amino acids is consistent with other S100 family members, which bind linear sequences of 13-22 residues in their protein targets. In addition, mts1 increases the critical monomer concentration for myosin-IIA filament assembly by approximately 11-fold. Kinetic assembly assays indicate that the elongation rate and the extent of polymerization depend on the initial myosin-IIA concentration; however, mts1 had only a small affect on the half-time for assembly and predominately affected the extent of myosin IIA polymerization. Altogether, these observations are consistent with mts1 regulating myosin IIA assembly by monomer sequestration and suggest that mts1 regulates cell shape and motility through the modulation of myosin-IIA function.