Previous studies suggested that heavy chain phosphorylation regulates non-muscle myosin-II assembly in an isoform-specific manner, affecting the assembly of myosin-IIB, but not myosin-IIA. We re-examined the effects of heavy chain phosphorylation on myosin-IIA filament formation and also examined mts1 binding. We demonstrated that heavy chain phosphorylation by either protein kinase C (PKC) or casein kinase 2 (CK2) inhibits the assembly of myosin-IIA into filaments. PKC phosphorylation had no affect on mts1 binding, but CK2 phosphorylation decreased the affinity of mts1 for the myosin-IIA rod by approximately 6.5-fold. Mts1 destabilized PKC-phosphorylated myosin-IIA filaments and inhibited the assembly of myosin-IIA monomers with maximal inhibition of assembly and promotion of disassembly occurring at a molar ratio of one mts1 dimer per myosin-IIA rod. At this molar ratio, mts1 only weakly disassembled CK2-phosphorylated myosin-IIA filaments and weakly inhibited the assembly of CK2-phosphorylated myosin-IIA monomers. These observations demonstrate that CK2 phosphorylation of the myosin-IIA heavy chain protects against mts1-induced filament disassembly and inhibition of assembly, and suggest that heavy chain phosphorylation provides an additional level of regulation for the mts1-myosin-IIA interaction.
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