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¹ Department of Cell Biology, National Institute for Basic Biology, Okazaki, Aichi 444-8585, Japan
² Department of Life Science, Gakushuin University, Mejiro 1-5-1, Toshima-ku, Tokyo 171-8588, Japan

Cleavage furrows (CFs) have been isolated from dividing sea urchin eggs and the protein constituents have been analyzed by two-dimensional gel electrophoresis (Fujimoto & Mabuchi, J. Biochem. 122, 518–524, 1997). Two proteins of 51 and 32 kDa, respectively, have been found to be enriched in the CF preparation. Here, we show that these proteins are identical to the protein elongation factor 1 (EF-1) and 1β (EF-1β), respectively. Furthermore, the CF 51-kDa protein is identical to the 51-kDa protein which had been isolated as a component of the microtubule organizing granules of mitotic sea urchin eggs. The 51-kDa protein bundles F-actin in vitro. This activity is suppressed by Ca²⁺/calmodulin or GTPS. The 32-kDa protein binds EF-1 both in vitro and in cell extract, and is shown to suppress the F-actin-bundling activity of the 51-kDa protein. Microinjection of a monoclonal antibody against the 51-kDa protein or that of His-tagged 32-kDa protein into dividing sea urchin eggs at the onset of cleavage leads to failure of cytokinesis. These results strongly suggest that EF-1 is involved in maintenance of the structure of the contractile ring and EF-1β regulates the F-actin-bundling activity of EF-1.

^aPresent address: Life Science Research Center, Shimadzu Corporation, Kandanishiki-cho 1-3, Chiyoda-ku, Tokyo 101-8448, Japan.

^* issei.mabuchi{at}gakushuin.ac.jp