The group started with the furrow-localized protein Pebble (PBL), a guanine nucleotide exchange factor (GEF) and thus activator for Rho1 and actin remodeling. Two-hybrid and coimmunoprecipitation experiments with PBL turned up RacGAP50C. The worm homologue of RacGAP50C, CYK-4, is essential for cytokinesis completion and binds a kinesin-like protein that bundles microtubules in the central spindle.
Consistent with these interactions, RacGAP50C was found in an inner ring near central spindle microtubules, abutting an outer ring of PBL. How these rings affect each other is unclear. PBL and RacGAP50C, despite being an activating GEF and an inhibitory GTPase-activating protein (GAP), interact synergistically rather than antagonistically. Somers and Saint find that the GAP activity is not directed at Rho1 but is required for cytokinesis. Perhaps the logic behind the activity will be tied up in the need to regulate the timing of PBL activation. ▪