Rap1 regulation and activation of effector proteins. Rap1 is activated (Rap1-GTP) by specific guanine nucleotide exchange factors (GEFs) and inhibited by specific GTPase activating proteins to regulated GTP-dependent binding to potential effector proteins. A diverse family of GEFs activates Rap1 by exchanging GDP for GTP (selected GEFs are shown). C3G, the first Rap1 GEF identified, is activated through its association with CrkL, a member of the Crk family of adaptor proteins, via a proline-rich (PPP) region on C3G and an SH3 region on CrkL. The inducible association of Crk/C3G to sites of tyrosine phosphorylation on receptor-associated adaptor/scaffold proteins occurs following ligand activation of a variety of receptors. CalDAG-GEFs respond to calcium (Ca²⁺) and diacylglycerol (DAG), making them targets of phospholipase C (PLC) action. cAMP-GEFs or EPACs (exchange proteins directly activated by cAMP; EPAC 1 shown here) are activated by directly binding cAMP to cyclic nucleotide binding domains (CNDB). Two Rap1GAPs are shown here: Rap1GAP1 and SPA-1. Rap1GAP1 is expressed widely and contains a GOLoCo motif that can associate with heterotrimeric G-protein alpha subunits.¹²⁴  SPA-1 is highly expressed in hematopoietic cells and contains a PDZ (domain found in PSD-95, Discs large, and ZO-1) domain that can mediate interactions with membrane-associated proteins. Following GTP binding, Rap1-GTP associates with a number of potential effectors, some of which are shown here. One of the best-studied effector pathways of Rap1 is the stimulation of cell adhesion. RapL and RIAM (Rap1-GTP–interacting adaptor molecule) have been shown to mediate integrin-mediated adhesion in hematopoietic cells. Both contain a Ras association (RA) domain, and RIAM also contains a Pleckstrin-homology (PH) domain. B-Raf and Raf-1 are related kinases that contribute to the MAP kinase pathway upstream of ERK. Both are recruited to Rap1-GTP via their Ras binding domain (RBD). In cells expressing B-Raf, Rap1 activates B-Raf–dependent signals to ERKs. In B-Raf–negative cells, such as T cells, Rap1 cannot couple to ERK, but can antagonize Ras-dependent signals to ERK, in part by binding to Raf-1. Rap1 can also sequester other effectors of Ras, such as the upstream activators of the p38 MAP kinase, to antagonize the activation of p38 by IL-1. In addition, Rap1 has been proposed to inhibit the phosphatidylinositol 3-kinase–dependent phosphorylation of the survival factor Akt, in B cells.²⁵  DEP indicates domain found in Disheveled, Eg-10, and pleckstrin; IP3, inositol 1,4,5 triphosphate.