Parameters that specify the timing of cytokinesis

One model for the timing of cytokinesis is based on findings that p34cdc2 c an phosphorylate myosin regulatory light chain (LC20) on inhibitory sites ( serines 1 and 2) in vitro (Satterwhite, L.L., M.H. Lohka, K.L. Wilson, T.Y. Scherson, L.J. Cisek, J.L. Corden, and T.D. Pollard. 1992. J. Cell Biol. 1 18:595-605), and this inhibition is proposed:to delay cytokinesis until p34 cdc2 activity falls at anaphase,We have characterized previously several ki nase activities associated with the isolated cortical cytoskeleton of divid ing sea urchin embryos (Walker, G.R., C.B. Shuster, and D.R. Burgess. 1997, J. Cell Sci. 110:1373-1386). Among these kinases and substrates is p34cdc2 and LC20, In comparison with whole cell activity, cortical H1 kinase activ ity is delayed, with maximum levels in cortices prepared from late anaphase /telophase embryos,To determine whether cortical-associated p34cdc2 influen ces cortical myosin II activity during cytokinesis, we labeled eggs in vivo with [32P]orthophosphate, prepared cortices, and mapped LC20 phosphorylati on through the first cell division. We found no evidence of serine 1,2 phos phorylation at any time during mitosis on LC20 from cortically associated m yosin. Instead, we observed a sharp rise in serine 19 phosphorylation durin g anaphase and telophase, consistent with an activating phosphorylation by myosin light chain kinase, However, serine 1,2 phosphorylation was detected on light chains from detergent-soluble myosin II. Furthermore, cells arres ted in mitosis by microinjection of nondegradable cyclin B could be induced to form cleavage furrows if the spindle poles were physically placed in cl ose proximity to the cortex. These results suggest that factors independent of myosin II inactivation, such as the delivery of the cleavage stimulus t o the cortex, determine the timing of cytokinesis.