Signaling to build a contractile ring
To establish the contractile ring the small GTPase RhoA, the master regulator of contractility, must be localized and activated at the site of division. RhoA cycles between and active form facilitated by guanine nucleotide exchange factors (GEFs) and an inactive form facilitated by GTPase activating proteins (GAPs). When in the active GTP-bound conformation, RhoA associates with the membrane and activate specific effector proteins, resulting in localized effects on the cytoskeleton. For example, active RhoA promotes formation of actomyosin contractile arrays via its key effector proteins: formin, which nucleates unbranched actin filaments, and Rho-associated coiled-coil kinase (ROCK), which phosphorylates the regulatory light chain of Myosin II to increase contractility (fig. qq). RhoA activity is properly positioned through the delivery of GEFs and GAPs along microtubules to the division site. In brief, MLKP1, a motor protein, transports MgcRacGAP which can bind the Rho GEF Ect2. The co-accumulation and MgcRacGAP and Ect2 results in an activation and inactivation flux of RhoA at the division site maintaining properly localized RhoA activity \cite{Miller_2008}