GTPase activating proteins (GAPs) stimulate the GTP-hydrolyzing activity of GTPases, such as p21ras and Rho. p190-A is a Ras-GAP associated protein that is tyrosine phosphorylated in transformed and growth factor-stimulated cells. Ras-GAP and p190-A are targets of oncoproteins and growth factor receptors. p190-B is another Ras-GAP in the p190 family. It has 51% identity with p190-A and contains several GTPase-related domains in the N-terminal region and a Rho-GAP domain in the C-terminal region. p190-B is expressed in kidney, brain, liver, and lung, as well as in human foreskin fibroblasts, RD muscle cells, and HT-1080 cells. In fibroblasts, p190-B is localized diffusely in the cytoplasm and co-localizes with the α5β1 integrin receptor for fibronectin. Adhesion of fibronectin-coated latex beads to cells leads to the recruitment of p190-B and Rho to the plasma membrane at sites of bead contact. In addition, the recombinant Rho-GAP domain of p190-B displays GAP activity for RhoA, Rac1, and G25K/CDC42Hs. Thus, p190-B is thought to act as a transmembrane link between integrins and Rho GTPases during fibronectin-induced changes in cell morphology and motility.