The DEPDC1 protein LET-99 is required for cortical stability and antagonizes branched actin formation to promote robust cytokinesis

During cytokinesis, signals from the central spindle stimulate the accumulation of active RhoA-GTPase and thus contractile ring components at the equator, while the astral microtubules inhibit such components at the polar cortex. The DEPDC1 family protein LET-99 is required for furrow ingression in the absence of the central spindle signal, and for timely onset of furrowing even in the presence of the central spindle signal. Here we show that LET-99 works downstream or independently of RhoA-GTP and antagonizes branched F-actin and the Rac protein CED-10 to promote furrow initiation. This interaction with CED-10 is separable from LET-99s function in spindle positioning. We also characterize a new role for LET-99 in regulating cortical stability, where LET-99 acts in parallel with the actomyosin scaffolding protein anillin, but LET-99 does not antagonize CED-10 in this case. We propose that LET-99 acts in a pathway that inhibits the Rac CED-10 to promote the proper balance of branched versus linear F-actin for cytokinesis, and that LET-99 also regulates another factor that contributes to cortical stability.