Effects of Inhibitors of Myosin Light Chain Kinase and Other Protein Kinases on the First Cell Division of Sea Urchin Eggs

We investigated effects of protein kinase inhibitors on the first cell division in sea urchin eggs on the assumption that phosphorylation of myosin is requisite for the formation and/or the contraction of the contractile ring. ML‐7 or ML‐9, which inhibits myosin light chain kinase (MLCK), inhibited cytokinesis with a half maximal inhibition at 0.1–0.2 mM. The nuclear division was accomplished normally at 0.2–0.25 mM where the cytokinesis was completely blocked. Fluorescent staining of actin filaments with rhodamine‐labeled phalloidin revealed that the contractile ring was not formed in the cleavage‐inhibited eggs. H‐7 which inhibits cAMP‐dependent protein kinase, cGMP‐dependent protein kinase and protein kinase C arrested the process of the division at mid‐cleavage at 0.25–0.3 mM and at metaphase or anaphase at 0.5 mM. H‐8 and HA1004, which inhibit cAMP‐dependent and cGMP‐dependent protein kinases did not show significant effect at millimolar order. In the presence of micromolar concentrations of staurosporine which preferentially inhibits protein kinase C and MLCK small mitotic apparatuses were formed, in which chromosomes did not form the metaphase plate. The role of phosphorylation in the cell division is discussed.