Phosphorylation of the myosin-II light chain does not regulate the timing of cytokinesis in fission yeast

Proper coordination of cytokinesis with chromosome separation during mitosi s is crucial to ensure that each daughter cell inherits an equivalent set o f chromosomes. It has been proposed that one mechanism by which this is ach ieved is through temporally regulated myosin regulatory light chain (RLC) p hosphorylation (Satterwhite, L. L., and Pollard, T. D. (1992) Curr. Opin. C ell Biol, 4, 43-52), A variety of evidence is consistent with this model. A direct test of the importance of RLC phosphorylation in vivo has been done only in Dictyostelium and Drosophila; phosphorylation of the RLC is essent ial in Drosophila (Jordan, P., and Karess, R. (1997) J. Cell Biol, 139, 180 5-1819) but not essential in Dictyostelium (Ostrow, B, D,, Chen, P,, and Ch isholm, R, L, (1994) J, Cell Biol, 127, 1945-1955), The Schizosaccharomyces pombe myosin light chain Cdc4p is essential for cytokinesis, but it was un known whether phosphorylation played a role in its regulation. Here we show that the S, pombe myosin light chain Cdc4p is phosphorylated in vivo on ei ther serine 2 or 6 but not both. Mutation of either or both of these sites to alanine did not effect the ability of Cdc4p to bind the type II myosin M yo2p, and cells expressing only these mutated versions of Cdc4p grew and di vided normally. Similarly, mutation of Ser-2, Ser-6, or both residues to as partic acid did not affect growth or division of cells. Thus we conclude th at phosphorylation of Cdc4p is not essential in vivo for the function of th e protein.