Probably involved in the organization of the actin cytoskeleton by acting downstream of CDC42, inducing actin filament assembly. Alters CDC42-induced cell shape changes. In activated T-cells, may play a role in CDC42-mediated F-actin accumulation at the immunological synapse. May play a role in early contractile events in phagocytosis in macrophages (By similarity). Interacts with CDC42 (in GTP-bound form). Interacts weakly with RAC1 and not at all with RHOA (By similarity). Recruited to the activated TCR prior actin polymerization. Localizes at the phagocytic cup of macrophages. The CRIB domain mediates interaction with CDC42. Belongs to the CDC42SE/SPEC family.