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* Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo
153; The gapA gene encoding a novel RasGTPase-activating protein (RasGAP)-related protein was
found to be disrupted in a cytokinesis mutant of Dictyostelium that grows as giant and multinucleate cells in
a dish culture. The predicted sequence of the GAPA
protein showed considerable homology to those of
Gap1/Sar1 from fission yeast and the COOH-terminal
half of mammalian IQGAPs, the similarity extending
beyond the RasGAP-related domain. In suspension
culture, gapA Department of Applied Chemistry, Kogakuin University, Shinjuku-ku, Tokyo 163-91; § Cellular Signaling Laboratory,
Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako-shi, Saitama 351-01; and Department of Biophysics
and Biochemistry, School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
cells showed normal growth in terms of
the increase in cell mass, but cytokinesis inefficiently
occurred to produce spherical giant cells. Time-lapse
recording of the dynamics of cell division in a dish culture revealed that, in the case of gapA cells, cytokinesis was very frequently reversed at the step in which the
midbody connecting the daughter cells should be severed. Earlier steps of cytokinesis in the gapA cells
seemed to be normal, since myosin II was accumulated
at the cleavage furrow. Upon starvation, gapA cells
developed and formed fruiting bodies with viable
spores, like the wild-type cells. These results indicate
that the GAPA protein is specifically involved in the
completion of cytokinesis. Recently, it was reported
that IQGAPs are putative effectors for Rac and
CDC42, members of the Rho family of GTPases, and
participate in reorganization of the actin cytoskeleton.
Thus, it is possible that Dictyostelium GAPA participates in the severing of the midbody by regulating the
actin cytoskeleton through an interaction with a member of small GTPases.
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