Calcium and DAG regulated guanine nucleotide exchange factors (CalDAG-GEFs, also called RasGRPs) contain a regulatory C1 diacylglycerol (DAG) -binding domain analogous to the C1 domain found in Protein Kinase C, and a pair of calcium-binding EF-hand domains. All forms show enhanced activity in response to DAG and bind calcium, but the effect of Ca2+ seems to differ between isoforms. CalDAG-GEFI exhibited additive enhancement of Rap1 activation in response to Ca2+ ionophore and phorbol ester (Kawasaki et al. 1998). RasGRP2, an isofom of CalDAG-GEFI with an alternatively spliced N-terminal extension, reported to target it to the plasma membrane, was stimulated by diacylglycerol but inhibited by calcium (Clyde-Smith et al. 2000). CalDAG-GEF II/RasGRP1 was additively stimulated by Ca2+ ionophore and phorbol ester.
CalDAG-GEFI was found to primarily target Rap1A and inhibit Ras-dependent activation of the Erk/MAP kinase cascade (Kawasaki et al. 1998). RasGRP2 selectively activated N- and Ki-Ras, but not Ha-Ras. It also had Rap1A stimulating activity, but less than CalDAG-GEFI. The difference in substrate specificity seen for these isoforms may be due to their different cellular locations, as prolonged exposure to phorbol esters, or growth in serum, resulted in localization of CalDAG-GEFI to the cell membrane and restoration of Ras exchange activity (Clyde-Smith et al. 2000). CalDAG-GEF II/RasGRP1 targeted Ras proteins rather than Rap (Kawasaki et al. 1998, Ebinu et al. 1998).
Mouse platelets that lack CalDAG-GEFI are severely compromised in integrin-dependent aggregation as a consequence of their inability to signal through CalDAG-GEFI to its target, the small GTPase Rap1 (Crittenden et al. 2004)