Regulates the GDP/GTP exchange reaction of the Rho proteins by inhibiting the dissociation of GDP from them, and the subsequent binding of GTP to them (By similarity). Regulates the Rac-dependent signaling pathways controlling cytokinesis, actin reorganization and the contractile vacuole. Required for efficient accumulation of cap at the cell cortex. Interacts with rac1A, rac1B, rac1C, racB, raCC and RacE. Down-regulated in the absence of cap. Belongs to the Rho GDI family.