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In metazoans, cytokinesis is triggered by activation of the GTPase RhoA at the equatorial plasma membrane. ECT-2, the guanine nucleotide exchange factor (GEF) required for RhoA activation, is activated by the centralspindlin complex that concentrates on spindle midzone microtubules. However, these microtubules and the plasma membrane are not generally in apposition, and thus the mechanism by which RhoA is activated at the cell equator remains unknown. Our data provide a molecular mechanism for RhoA activation by the centralspindlin complex at the equatorial cortex. We demonstrate that centralspindlin localizes to this sub-cellular site by virtue of its ability to oligomerize and to directly interact with the plasma membrane. The Chromosome Passenger Complex subunit Aurora B promotes cortical RhoA activation by antagonizing the inhibitor of centralspindlin oligomerization, PAR-5/14-3-3. Our study places the CPC as a direct activator of centralspindlin in the pathway leading to contractile ring assembly and find that this function is conserved in both C. elegans and human cells. We have designed optogenetic probes to spatiotemporally control Aurora B kinase and test whether its activity is sufficient to generate RhoA-directed contractility via centralspindlin oligomers. We also find that the localization of centralspindlin on the membrane depends on active RhoA and myosin levels on the cortex. The former is governed by a direct interaction between CYK-4 and RhoA. The nature of the interaction between myosin and centralspindlin is yet to be determined.


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