| Ras p21 can exist in either a physiologically quiescent GDP-binding state or a GTP-binding signal-emitting state. Interaction of Ras p21 with GTPase activating protein (GAP) can increase the rate of hydrolysis of Ras p21-bound GTP by as much as 1000-fold. In mitogenically activated and tyrosine kinase-transformed cells, Ras GAP forms a complex with a protein designated p190. At its N-terminus, p190 contains sequence motifs characteristic of all known GTPases, whereas the C-terminus contains sequences similar to those found in the Bcr gene product, n-chimerin and Rho GAP, all of which exhibit intrinsic GAP activity. Additional proteins with GTPase activating activity include GAP-associated protein p190 and Ral BP-1 (also designated RLIP 76), which displays a GAP activity specific for Rac 1 and Cdc42, Gap1m and oligophrenin-1. Oligophrenin-1 is a RhoGAP protein that stimulates GTP hydrolysis of Rho subfamily members and is involved in cell migration, morphogenesis and axon outgrowth. |
