Activation of myosin phosphatase targeting subunit by mitosis-specific phosphorylation

It has been demonstrated previously that during mitosis the sites of myosin phosphorylation are switched between the inhibitory sites, Ser 1/2, and th e activation sites, Ser 19/Thr 18 (Yamakita, Y., S. Yamashiro, and F. Matsu mura. 1994. J. Cell Biol. 124:129-137; Satterwhite, L.L., M.J. Lohka, K.L. Wilson, T.Y. Scherson, L.J. Cisek, J.L. Corden, and T.D. Pollard. 1992. J. Cell Biol. 118:595-605), suggesting a regulatory role of myosin phosphoryla tion in cell division. To explore the function of myosin phosphatase in cel l division, the possibility that myosin phosphatase activity may be altered during cell division was examined. We have found that the myosin phosphata se targeting subunit (MYPT) undergoes mitosis-specific phosphorylation and that the phosphorylation is reversed during cytokinesis. MYPT phosphorylate d either in vivo or in vitro in the mitosis-specific way showed higher bind ing to myosin II (two- to threefold) compared to MYPT from cells in interph ase. Furthermore, the activity of myosin phosphatase was increased more tha n twice and it is suggested this reflected the increased affinity of myosin binding. These results indicate the presence of a unique positive regulato ry mechanism for myosin phosphatase in cell division. The activation of myo sin phosphatase during mitosis would enhance dephosphorylation of the myosi n regulatory light chain, thereby leading to the disassembly of stress fibe rs during prophase. The mitosis-specific effect of phosphorylation is lost on exit from mitosis, and the resultant increase in myosin phosphorylation may act as a signal to activate cytokinesis.