GTPase-activating protein for members of the Rho subfamily including Rac1, RhoA and cdc42 and other Ras-related subfamilies including let-60 (PubMed:18583611, PubMed:8288633). Mediates radial (inner-outer) polarity and gastrulation by excluding par-6 from contacted cell surfaces; acts by inactivating cdc42 at inner cell surfaces which limits active cdc42 to outer cell surfaces devoid of cell-cell contacts, where cdc42 can bind and recruit par-6 (PubMed:18583611, PubMed:8288633, PubMed:25938815). Required for blastomere polarization (PubMed:25938815). Associated with the catenin-cadherin complex consisting of hmr-1, hmp-1 and hmp-2; this is mediated by interaction with picc-1. Locates to inner but not outer surfaces of early embryonic somatic (EES) cells (PubMed:18583611). Specifically localizes to cell contacts in hmr-1-dependent and -independent manner in embryos (PubMed:25938815). Expressed in four-cell stage embryos. RNAi-mediated knockdown of isoform a results in mis-localization of par-6 in embryos.