Mitosis-specific phosphorylation of myosin light chain kinase.

Cell cytosol preparations from mitotic HeLa cells exhibit a kinase activity that phosphorylates myosin light chain kinase (MLCK). This MLCK kinase activity is apparently distinct from the known MLCK kinases, including cAMP-dependent protein kinase, cGMP-dependent protein kinase, Ca(2+)-activated phospholipid-dependent protein kinase, or Ca(2+)-calmodulin-dependent protein kinase II, based on the following criteria. First, the MLCK kinase activity of mitotic cells does not respond to a variety of characteristic activators or inhibitors of these known kinases. Second, one- and two-dimensional peptide maps have revealed that the site of phosphorylation by the MLCK kinase of mitotic cells differs from those by these known kinases. The mitotic MLCK kinase phosphorylates MLCK at a threonine residue at a ratio of up to 1 mol of phosphate/mol of chicken gizzard MLCK. The MLCK kinase is mitosis-specific because mitotic cell extracts show much higher phosphorylation activity than nonmitotic cell extracts.