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Dictyostelium IQGAP-related protein specifically involved in the completion of cytokinesis.

Adachi H, Takahashi Y, Hasebe T, Shirouzu M, Yokoyama S, Sutoh K - J. Cell Biol. (1997)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153. f37771@m-unix.cc.u-tokyo.ac.jp

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.

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Schematic drawing of the similarity among the members of the IQGAP family. Identical residues (%) were calculated. CH, calponin homology domain; IR, IQGAP repeats; WW,  WW domain; IQ, four repeats of the calmodulin-binding motif;  GRD, RasGAP-related domain; Hs, Homo sapiens; Dd, Dictyostelium discoideum; Sp, Schizosaccharomyces pombe.
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Figure 5: Schematic drawing of the similarity among the members of the IQGAP family. Identical residues (%) were calculated. CH, calponin homology domain; IR, IQGAP repeats; WW, WW domain; IQ, four repeats of the calmodulin-binding motif; GRD, RasGAP-related domain; Hs, Homo sapiens; Dd, Dictyostelium discoideum; Sp, Schizosaccharomyces pombe.

Mentions: The deduced amino acid sequence of this protein was compared with the NBRF-PIR and SWISS PLOT databases, which revealed that it is very closely related to Gap1/ Sar1 from fission yeast (Imai et al., 1991; Wang et al., 1991) and the COOH-terminal half of IQGAP1 from human (Weissbach et al., 1994) (Fig. 5). In the middle of the homologous regions of these three proteins lies a conserved sequence for RasGTPase-activating proteins (RasGAPs) called the GAP-related domain (GRD; Figs. 4 and 5). Thus, we named this protein GAPA, and the gene gapA, from its sequence similarity to RasGAPs. The homology of the Dictyostelium GAPA to the members of the RasGAP family other than IQGAP1, IQGAP2 (recently identified homologue of IQGAP1 [Brill et al., 1996]), and Gap1/Sar1 is restricted to the GAP-related domain. Fig. 6 shows amino acid sequence alignment of the most conserved region of the GRDs of typical RasGAP-related proteins. The most prominent feature is that the Phe-Leu residues in the invariant FLRXXXPAXXXP (X: any amino acid) motif are replaced by Tyr-Tyr residues only in GAPA and IQGAP1 (in IQGAP2 also Tyr-Tyr). It was reported that substitution of the invariant Leu with an Ile residue resulted in the loss of the activity of p120GAP (Brownbridge et al., 1993), and that RasGAP activity was not detected for IQGAPs (Hart et al., 1996; Brill et al., 1996; McCallum et al., 1996). In contrast with IQGAPs, both biochemical (Hart et al., 1996) and genetic (Imai et al., 1991; Wang et al., 1991) analyses indicated that yeast Gap1/Sar1 activates the GTPase of yeast RAS2, and the invariant Phe-Leu residues are conserved in this protein. These suggest that GAPA might be much more related functionally to IQGAPs than to Gap1/Sar1. On the basis of this finding, we looked for the sequence motifs that were found in the NH2-terminal half of IQGAPs (Weissbach et al., 1994; Hart et al., 1996) (Fig. 6) in the NH2-terminal region of GAPA. One putative IQ motif (I145AEIQELKRNMVAE158; the conserved residues are underlined) was found in the region where the homology between IQGAP1 and GAPA starts, although the conserved arginine was replaced by glutamic acid158. We found neither a calponin homology domain, an IQGAP repeat, nor a WW domain (Sudol et al., 1995).


Dictyostelium IQGAP-related protein specifically involved in the completion of cytokinesis.

Adachi H, Takahashi Y, Hasebe T, Shirouzu M, Yokoyama S, Sutoh K - J. Cell Biol. (1997)

Schematic drawing of the similarity among the members of the IQGAP family. Identical residues (%) were calculated. CH, calponin homology domain; IR, IQGAP repeats; WW,  WW domain; IQ, four repeats of the calmodulin-binding motif;  GRD, RasGAP-related domain; Hs, Homo sapiens; Dd, Dictyostelium discoideum; Sp, Schizosaccharomyces pombe.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2139833&req=5

Figure 5: Schematic drawing of the similarity among the members of the IQGAP family. Identical residues (%) were calculated. CH, calponin homology domain; IR, IQGAP repeats; WW, WW domain; IQ, four repeats of the calmodulin-binding motif; GRD, RasGAP-related domain; Hs, Homo sapiens; Dd, Dictyostelium discoideum; Sp, Schizosaccharomyces pombe.
Mentions: The deduced amino acid sequence of this protein was compared with the NBRF-PIR and SWISS PLOT databases, which revealed that it is very closely related to Gap1/ Sar1 from fission yeast (Imai et al., 1991; Wang et al., 1991) and the COOH-terminal half of IQGAP1 from human (Weissbach et al., 1994) (Fig. 5). In the middle of the homologous regions of these three proteins lies a conserved sequence for RasGTPase-activating proteins (RasGAPs) called the GAP-related domain (GRD; Figs. 4 and 5). Thus, we named this protein GAPA, and the gene gapA, from its sequence similarity to RasGAPs. The homology of the Dictyostelium GAPA to the members of the RasGAP family other than IQGAP1, IQGAP2 (recently identified homologue of IQGAP1 [Brill et al., 1996]), and Gap1/Sar1 is restricted to the GAP-related domain. Fig. 6 shows amino acid sequence alignment of the most conserved region of the GRDs of typical RasGAP-related proteins. The most prominent feature is that the Phe-Leu residues in the invariant FLRXXXPAXXXP (X: any amino acid) motif are replaced by Tyr-Tyr residues only in GAPA and IQGAP1 (in IQGAP2 also Tyr-Tyr). It was reported that substitution of the invariant Leu with an Ile residue resulted in the loss of the activity of p120GAP (Brownbridge et al., 1993), and that RasGAP activity was not detected for IQGAPs (Hart et al., 1996; Brill et al., 1996; McCallum et al., 1996). In contrast with IQGAPs, both biochemical (Hart et al., 1996) and genetic (Imai et al., 1991; Wang et al., 1991) analyses indicated that yeast Gap1/Sar1 activates the GTPase of yeast RAS2, and the invariant Phe-Leu residues are conserved in this protein. These suggest that GAPA might be much more related functionally to IQGAPs than to Gap1/Sar1. On the basis of this finding, we looked for the sequence motifs that were found in the NH2-terminal half of IQGAPs (Weissbach et al., 1994; Hart et al., 1996) (Fig. 6) in the NH2-terminal region of GAPA. One putative IQ motif (I145AEIQELKRNMVAE158; the conserved residues are underlined) was found in the region where the homology between IQGAP1 and GAPA starts, although the conserved arginine was replaced by glutamic acid158. We found neither a calponin homology domain, an IQGAP repeat, nor a WW domain (Sudol et al., 1995).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Life Sciences, Graduate School of Arts and Sciences, University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153. f37771@m-unix.cc.u-tokyo.ac.jp

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.

Show MeSH
Related in: MedlinePlus