Dictyostelium IQGAP-related Protein Specifically Involved in the Completion of Cytokinesis

Abstract

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− 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.

Myosin II-Independent Cytokinesis in Dictyostelium. Its Mechanism and Implications.

“…Cells lacking both of the two genes for profilin, a ubiquitous G-actin binding protein, are similar to cortexillin null cells, in that both cytokinesis A and B, but not cytokinesis C, are severely impaired (Haugwitz et al, 1994). Both cytokinesis A and B, but not cytokinesis C, are partially affected by null mutations of RasG (Tuxworth et al, 1997), GAPA (Adachi et al, 1997), dynamin (Wienke et al, 1999) and DAip1 (Konzok et al, 1999) as well, because cytokinesis in suspension and on substrates are both partially inhibited in these mutants. If we consider the differences between cytokinesis A and B, the former absolutely requires myosin II, whereas the latter does not.…”

Regulation of blastocyst formation

“…They lack an arginine finger residue in their Ras GAP domain, and may not have significant GAP activity (109,110). Both IQGAPs interact with Cdc42 and Rac1 to inhibit their GTPase activity; thus maintaining them in an active, GTP-bound state (108)(109)(110)(111)(116)(117)(118)(119)(120)(121)(122). The interaction of Cdc42 or Rac1 with IQGAP1 inhibits it from binding to beta-catenin.…”

The Mechanism for Regulation of the F-actin Binding Activity of IQGAP1 by Calcium/Calmodulin

“…The most similar proteins in terms of primary sequence and domain organization are IQGAP2, a liver-enriched protein in mammals (3,10), and a Hydra protein that has been implicated in tentacle formation (11). Additional IQGAP1-related proteins have been found in Dictyostelium (12)(13)(14), and Saccharomyces cerevisiae (15)(16)(17). Although some of these proteins do not contain every major structural motif of IQGAP1, they are sufficiently similar to IQGAP1 to be classified as members of the IQGAP protein family.…”