Two distinct cytoplasmic regions of the β2 integrin chain regulate RhoA function during phagocytosis

alpha(M)beta(2) integrins mediate phagocytosis of opsonized particles in a process controlled by RhoA, Rho kinase, myosin II, Arp2/3, and actin polymerization. alpha(M)beta(2), Rho, Arp2/3, and F-actin accumulate underneath bound particles; however, the mechanism regulating Rho function during alpha(M)beta(2)-mediated phagocytosis is poorly understood. We report that the binding of C3bi-opsonized sheep red blood cells (RBCs) to alpha(M)beta(2) increases Rho-GTP, but not Rac-GTP, levels. Deletion of the cytoplasmic domain of beta(2), but not of alpha(M), abolished Rho recruitment and activation, as well as phagocytic uptake. Interestingly, a 16-amino acid (aa) region in the membrane-proximal half of the beta(2) cytoplasmic domain was necessary for activating Rho. Three COOH-terminal residues (aa 758-760) were essential for beta(2)-induced accumulation of Rho at complement receptor 3 (CR3) phagosomes. Activation of Rho was necessary, but not sufficient, for its stable recruitment underneath bound particles or for uptake. However, recruitment of active Rho was sufficient for phagocytosis. Our data shed light on the mechanism of outside-in signaling, from ligated integrins to the activation of Rho GTPase signaling.