Myosin II ATPase activity is enhanced by the phosphorylation of MRLC (myosin II regulatory light chain) in non-muscle cells. It is well known that pMRLC (phosphorylated MRLC) co-localizes with F-actin (filamentous actin) in the CR (contractile ring) of dividing cells. Recently, we reported that HeLa cells expressing non-phosphorylatable MRLC show a delay in the speed of furrow ingression, suggesting that pMRLC plays an important role in the control of furrow ingression. However, it is still unclear how pMRLC regulates myosin II and F-actin at the CR to control furrow ingression during cytokinesis. In the present study, to clarify the roles of pMRLC, we measured the turnover of myosin II and actin at the CR in dividing HeLa cells expressing fluorescent-tagged MRLCs and actin by FRAP (fluorescence recovery after photobleaching). A myosin II inhibitor, blebbistatin, caused an enhancement of the turnover of MRLC and actin at the CR, which induced a delay in furrow ingression. Furthermore, only non-phosphorylatable MRLC and a Rho-kinase inhibitor, Y-27632, accelerated the turnover of MRLC and actin at the CR. Interestingly, the effect of Y-27632 was cancelled in the cell expressing phosphomimic MRLCs. Taken together, these results reveal that pMRLC reduces the turnover of myosin II and also actin at the CR. In conclusion, we show that the enhancement of myosin II and actin turnover at the CR induced slower furrowing in dividing HeLa cells.

Myosin II is activated by the monophosphorylation of its regulatory light chain (MRLC) at Ser19 (1P-MRLC). Its ATPase activity is further enhanced by MRLC diphosphorylation at Thr18/Ser19 (2P-MRLC). As these phosphorylated MRLCs are colocalized with their heavy chains at the contractile ring in dividing cells, we believe that the phosphorylated MRLC acts as a subunit of the activated myosin II during cytokinesis. However, the distinct role(s) of 1P- and 2P-MRLC during cytokinesis has not been elucidated. In this study, a monoclonal antibody (4F12) specific for 2P-MRLC was raised and used to examine the roles of 2P-MRLC in cultured mammalian cells. Our confocal microscopic observations using 4F12 revealed that 2P-MRLC localized to the contractile ring, and, unexpectedly, to the midzone also. Interestingly, 2P-MRLC did not colocalize with 1P-MRLC, myosin II heavy chain, and F-actin at the midzone. These results suggest that 2P-MRLC has a role different from that of 1P-MRLC at the midzone, and is not a subunit of myosin II.

The anaphase-promoting complex (APC) is a major regulator of chromosome segregation and is implicated in centriole engagement, whose de-regulation causes abnormal number of centrosomes. The emb-27 gene in C. elegans encodes a subunit of APC. The paternal emb-27 mutant was reported to show cell division with multiple furrows, suggesting the presence of excess centrosomes. In this study, we examined the number of centrosomes and the mechanism underlying de-regulation of centrosome number in emb-27 mutants. Our observations indicated excess centrosomes in emb-27 sperms, which resulted in zygotes with excess centrosomes. Further, the secondary spermatocyte of emb-27 produced reduced number of spermatids, which is likely the direct cause of the excess number of centrosomes per sperm. We propose that a chromosome segregation defect in emb-27 induced centrosome separation defect, resulting in reduced number of buds. Additionally, treatment with cytoskeletal inhibiting drugs indicated presence of three kinds of forces working in combination to move the centrosomes into the spermatids. The present study suggested a novel role of microtubule in the budding cytokinesis of spermatocytes. Azimzadeh, J. and Bornens, M. (2007). Structure and duplication of the centrosome. J. Cell Sci. 120, 2139-2142. Bobinnec, Y., Fukuda, M. and Nishida, E. (2000). Identification and characterization of Caenorhabditis elegans gamma-tubulin in dividing cells and differentiated tissues.

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