Regulation of cytokinesis by Rho GTPase flux

Abstract

In animal cells, cytokinesis is powered by a contractile ring of actin filaments (F-actin) and myosin-2. Formation of the contractile ring is dependent on the small GTPase, RhoA 1, 2, which is activated in a precise zone at the cell equator 3. It has long been assumed that cytokinesis and other Rho-dependent processes are controlled in a sequential manner, whereby Rho activation via guanine nucleotide exchange factors (GEFs) initiates a particular event, and Rho inactivation via GTPase activating proteins (GAPs) terminates that event. MgcRacGAP is a conserved cytokinesis regulator thought to be required only at the end of cytokinesis 4, 5. Here we show that MgcRacGAP’s GAP activity is necessary early during cytokinesis for the formation and maintenance of the Rho activity zone. Disruption of GAP activity by point mutation results in poorly focused Rho activity zones, while complete removal of the GAP domain results in unfocused zones that display lateral instability and/or rapid side-to-side oscillations. We propose that the GAP domain of MgcRacGAP plays two unexpected roles throughout cytokinesis: first, it transiently anchors active Rho, and second it promotes local Rho inactivation resulting in the constant flux of Rho through the GTPase cycle.