A GAP that Divides

Abstract

Cytokinesis in metazoan cells is mediated by an actomyosin-based contractile ring that assembles in response to activation of the small GTPase RhoA. The guanine nucleotide exchange factor that activates RhoA during cytokinesis, ECT-2, is highly regulated. In most metazoan cells, with the notable exception of the early Caenorhabditis elegans embryo, RhoA activation and furrow ingression require the centralspindlin complex. This exception is due to the existence of a parallel pathway for RhoA activation in C. elegans. Centralspindlin contains CYK-4 which contains a predicted Rho family GTPase-activating protein (GAP) domain. The function of this domain has been the subject of considerable debate. Some publications suggest that the GAP domain promotes RhoA activation (for example, Zhang and Glotzer, 2015; Loria, Longhini and Glotzer, 2012), whereas others suggest that it functions to inactivate the GTPase Rac1 (for example, Zhuravlev et al., 2017). Here, we review the mechanisms underlying RhoA activation during cytokinesis, primarily focusing on data in C. elegans. We highlight the importance of considering the parallel pathway for RhoA activation and detailed analyses of cyk-4 mutant phenotypes when evaluating the role of the GAP domain of CYK-4.

Spatiotemporal Regulation of RhoA during Cytokinesis

“…Another model proposes that its GAP activity is required during cytokinesis to turn off the GTPase Rac1. We recently elaborated on the merits of these models in a separate review [69]. A major concern with the latter model is that the experiments to support it have been performed in the presence of a parallel RhoA activation pathway peculiar to C. elegans (see NOP-1 section).…”

Animal Cell Cytokinesis: The Rho-Dependent Actomyosin-Anilloseptin Contractile Ring as a Membrane Microdomain Gathering, Compressing, and Sorting Machine

“…However, RacGAP exhibits preferential GAP activity toward Rac and Cdc42 rather than Rho in vitro ( Toure et al, 1998 ; Bastos et al, 2012 ; Jantsch-Plunger et al, 2000 ). Genetic experiments in C. elegans embryos have led to conflicting models (discussed in Basant and Glotzer, 2017 ) in which the CYK4 GAP domain either targets Rac ( Canman et al, 2008 ; Zhuravlev et al, 2017 ), or acts non-canonically to activate Rho ( Loria et al, 2012 ; Zhang and Glotzer, 2015 ). Another candidate GAP for Rho inactivation at the CR is p190RhoGAP-A/ARHGAP35 ( Mikawa et al, 2008 ; Su et al, 2009 ; Manukyan et al, 2015 ).…”

Mitotic Cell Division in Caenorhabditis elegans

“…Consistent with a role in activating RhoA, CYK-4 orthologs have been shown to bind ECT-2 via a region in their N-terminal half, positioned between the coiled-coil and C1 domains, and this binding is proposed to relieve autoinhibition of ECT-2 to activate RhoA for contractile ring assembly (Somers and Saint 2003;Yüce et al 2005;Burkard et al 2009;Wolfe et al 2009;Loria et al 2012;Zhang and Glotzer 2015;Basant and Glotzer 2017). It is surprising that a GAP domain, expected to inactivate small GTPases by stimulating their GTP hydrolysis activity, acts to promote RhoA signaling (Basant and Glotzer 2017). Recent work has shown that mutations that specifically disrupt the Rho GTPase binding interface of the GAP domain compromise RhoA activation during cytokinesis (Zhang and Glotzer 2015).…”

“…Across systems, including C. elegans, there is good evidence that CYK-4 homologs play a role in the local activation of the small GTPase RhoA to promote contractile ring assembly (Motegi et al 2006;Loria et al 2012;Zhang and Glotzer 2015;Basant and Glotzer 2018). Consistent with a role in activating RhoA, CYK-4 orthologs have been shown to bind ECT-2 via a region in their N-terminal half, positioned between the coiled-coil and C1 domains, and this binding is proposed to relieve autoinhibition of ECT-2 to activate RhoA for contractile ring assembly (Somers and Saint 2003;Yüce et al 2005;Burkard et al 2009;Wolfe et al 2009;Loria et al 2012;Zhang and Glotzer 2015;Basant and Glotzer 2017). It is surprising that a GAP domain, expected to inactivate small GTPases by stimulating their GTP hydrolysis activity, acts to promote RhoA signaling (Basant and Glotzer 2017).…”

“…Signaling by the anaphase spindle activates RhoA at the cell equator, and active RhoA is in turn required for recruitment of all of the other components of the contractile ring Dechant and Glotzer 2003;Motegi et al 2006;Loria et al 2012). RhoA is activated by the guanine nucleotide exchange factor (GEF) ECT-2 (Somers and Saint 2003;Yüce et al 2005;Burkard et al 2009;Wolfe et al 2009;Loria et al 2012;Zhang and Glotzer 2015;Basant and Glotzer 2017), and is inactivated by the RhoA GAP RGA-3/4 (Schmutz et al 2007;Schonegg et al 2007). Active RHO-1/ RhoA localizes to the furrow, while ECT-2 appears uniformly distributed over the cortex but is presumably activated only at the furrow (Motegi et al 2006).…”