title: CYK-4 regulates Rac, but not Rho, during cytokinesis.
abstract: Cytokinesis is driven by constriction of an actomyosin contractile ring that is controlled by Rho-family small GTPases. Rho, activated by the guanine-nucleotide exchange factor ECT-2, is upstream of both myosin-II activation and diaphanous formin-mediated filamentous actin (f-actin) assembly, which drive ring constriction. The role for Rac and its regulators is more controversial, but, based on the finding that Rac inactivation can rescue cytokinesis failure when the GTPase-activating protein (GAP) CYK-4 is disrupted, Rac activity was proposed to be inhibitory to contractile ring constriction and thus specifically inactivated by CYK-4 at the division plane. An alternative model proposes that Rac inactivation generally rescues cytokinesis failure by reducing cortical tension, thus making it easier for the cell to divide when ring constriction is compromised. In this alternative model, CYK-4 was instead proposed to activate Rho by binding ECT-2. Using a combination of time-lapse in vivo single-cell analysis andCaenorhabditis elegansgenetics, our evidence does not support this alternative model. First, we found that Rac disruption does not generally rescue cytokinesis failure: inhibition of Rac specifically rescues cytokinesis failure due to disruption of CYK-4 or ECT-2 but does not rescue cytokinesis failure due to disruption of two other contractile ring components, the Rho effectors diaphanous formin and myosin-II. Second, if CYK-4 regulates cytokinesis through Rho rather than Rac, then CYK-4 inhibition should decrease levels of downstream targets of Rho. Inconsistent with this, we found no change in the levels of f-actin or myosin-II at the division plane when CYK-4 GAP activity was reduced, suggesting that CYK-4 is not upstream of ECT-2/Rho activation. Instead, we found that the rescue of cytokinesis in CYK-4 mutants by Rac inactivation was Cdc42 dependent. Together our data suggest that CYK-4 GAP activity opposes Rac (and perhaps Cdc42) during cytokinesis.
authors.affiliation: Department of Genetics and Development, Columbia University Medical Center, New York, NY 10032.
authors.forename: Yelena
authors.initials: Y
authors.lastname: Zhuravlev
authors.affiliation: Department of Genetics and Development, Columbia University Medical Center, New York, NY 10032.
authors.forename: Sophia M
authors.initials: SM
authors.lastname: Hirsch
authors.affiliation: Department of Pathology and Cell Biology, Columbia University Medical Center, New York, NY 10032.
authors.forename: Shawn N
authors.initials: SN
authors.lastname: Jordan
authors.affiliation: Institut Jacques Monod, CNRS, UMR 7592, Université Paris Diderot, Sorbonne Paris Cité, F-75205 Paris, France.
authors.forename: Julien
authors.initials: J
authors.lastname: Dumont
authors.affiliation: Department of Genetics and Development, Columbia University Medical Center, New York, NY 10032.
authors.forename: Mimi
authors.initials: M
authors.lastname: Shirasu-Hiza
authors.affiliation: Department of Pathology and Cell Biology, Columbia University Medical Center, New York, NY 10032 jcc2210@cumc.columbia.edu.
authors.forename: Julie C
authors.initials: JC
authors.lastname: Canman
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000199:Actins / Q000378:metabolism; D000818:Animals; D017173:Caenorhabditis elegans / Q000235:genetics / Q000378:metabolism; D029742:Caenorhabditis elegans Proteins / Q000235:genetics / Q000378:metabolism*; D048749:Cytokinesis / Q000502:physiology*; D020690:GTPase-Activating Proteins / Q000378:metabolism; D020662:Guanine Nucleotide Exchange Factors / Q000378:metabolism; D024462:Myosin Type II / Q000378:metabolism; D059010:Single-Cell Analysis; D020744:rac GTP-Binding Proteins / Q000378:metabolism; D054460:rho-Associated Kinases / Q000378:metabolism; D020742:rhoA GTP-Binding Protein / Q000378:metabolism
chemical_list: D000199:Actins; C513388:CYK-4 protein, C elegans; D029742:Caenorhabditis elegans Proteins; D020690:GTPase-Activating Proteins; D020662:Guanine Nucleotide Exchange Factors; D054460:rho-Associated Kinases; D024462:Myosin Type II; D020744:rac GTP-Binding Proteins; D020742:rhoA GTP-Binding Protein