Functions as a GTPase-activating protein (GAP) for RHOA and CDC42. Downstream partner of ARF1 which may control Golgi apparatus structure and function. Also required for CTNNA1 recruitment to adherens junctions. Interacts with GTP-bound ARF1 and ARF6. Interacts with CTNNA1. Localization to the Golgi is dependent on interaction with GTP-bound ARF1. Widely expressed with higher expression in brain, heart, skeletal muscle and placenta. Up-regulated upon cell differentiation. Sumoylated with SUMO2 and SUMO3 in proliferating lymphocytes. Depletion of ARHGAP21 induces cell spreading and accumulation of F-actin stress fibers. Required for In1A-dependent entry of Listeria monocytogenes into cells. It is uncertain whether Met-1 or Met-2 is the initiator. Truncated N-terminus.