CaLB: a 43 amino acid calcium-dependent membrane/phospholipid binding domain in p120 Ras GTPase-activating protein.

CaLB was originally observed as a conserved sequence motif in various calcium-responsive signalling proteins and also in p120 Ras GTPase activating protein (p120GAP) (Clark et al. Cell 65: 1043-1051, 1991). Here we show the 43 residue CaLB motif in p120GAP is a functional protein domain that when expressed as a fusion protein in vitro confers Ca(2+)-dependent interactions with cellular membranes and phosphatidylserine and phosphatidylinositol vesicles. p120GAP, but not a mutant lacking the CaLB domain, associates with the particulate fraction of cells in response to elevated intracellular Ca2+ suggesting that p120GAP may be regulated in part by calcium signals. Addition of the p120GAP CaLB domain was able to restore transforming activity and particulate localization to an otherwise transformation-defective and cytosolic mutant v-Sre tyrosine kinase. The CaLB domain appears to be a prevalent protein module that may affect the molecular interactions and subcellular localization of signalling proteins.