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Published 8 July 2002. doi:10.1083/jcb.200203148
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© The Rockefeller University Press, 0021-9525/2002/7/127 $5.00
The Journal of Cell Biology, Volume 158, Number 1, July 8, 2002 127-137

Article

Cortical recruitment of nonmuscle myosin II in early syncytial Drosophila embryos

: its role in nuclear axial expansion and its regulation by Cdc2 activity



Anne Royou1, William Sullivan2 and Roger Karess1

1 Centre National de la Recherche Scientifique, Centre de Génétique Moléculaire, 91198 Gif-sur-Yvette, France
2 Department of Molecular, Cell, and Developmental Biology, Sinsheimer Labs, University of California, Santa Cruz, CA 95064

Address correspondence to Roger E. Karess, Centre de Génétique Moléculaire, Centre National de la Recherche Scientifique, Ave de la Terrasse, 91198 Gif sur Yvette, France. Tel.: 33-1-69-82-32-25. Fax: 33-1-69-82-31-50. E-mail: karess{at}cgm.cnrs-gif.fr

The nuclei of early syncytial Drosophila embryos migrate dramatically toward the poles. The cellular mechanisms driving this process, called axial expansion, are unclear, but myosin II activity is required. By following regulatory myosin light chain (RLC)–green fluorescent protein dynamics in living embryos, we observed cycles of myosin recruitment to the cortex synchronized with mitotic cycles. Cortical myosin is first seen in a patch at the anterocentral part of the embryo at cycle 4. With each succeeding cycle, the patch expands poleward, dispersing at the beginning of each mitosis and reassembling at the end of telophase. Each cycle of actin and myosin recruitment is accompanied by a cortical contraction. The cortical myosin cycle does not require microtubules but correlates inversely with Cdc2/cyclinB (mitosis-promoting factor) activity. A mutant RLC lacking inhibitory phosphorylation sites was fully functional with no effect on the cortical myosin cycle, indicating that Cdc2 must be modulating myosin activity by some other mechanism. An inhibitor of Rho kinase blocks the cortical myosin recruitment cycles and provokes a concomitant failure of axial expansion. These studies suggest a model in which cycles of myosin-mediated contraction and relaxation, tightly linked to Cdc2 and Rho kinase activity, are directly responsible for the axial expansion of the syncytial nuclei.

Key Words: regulatory myosin light chain; nuclear axial expansion; Rho kinase; phosphorylation; cytokinesis


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