1-(1,2-diacyl-glycerol 3-phospho)-inositol 4-phosphate, G-protein alpha-i family, cAMP-GEFII, cAMP-GEFI, Krit1, RASA3, PDZ-GEF1, 22.214.171.124, Tuberin, NEDD4, CD3, SPA1, B-Raf, p120GAP, inositol 1,4-bisphosphate, TCR alpha/beta, M-Ras, CalDAG-GEFIII, MAGI-1(BAIAP1), Adenylate cyclase type I, RAP-1A, CrkL, CALDAG-GEFI, ZAP70, G-protein alpha-s, KAP3, C3G, GGTase-I, cAMP, PLC-delta 1, ATP cytosol, Rap1GDS1, MR-GEF, 126.96.36.199, Ca('2+) cytosol, PKA-cat (cAMP-dependent), Rap1GAP1, DAG, RalGDS, c-Raf-1
RAP1A, member of RAS oncogene family ( RAP-1A) belongs to a family of small GTP-binding proteins (G-proteins) called monomeric G-proteins. The RAP subfamily consists of four members, RAP-1A, RAP-1B, RAP-2A and RAP-2B proteins , .
RAP-1A is a target of posttranslational modification via attachment of lipid moieties, such as geranyl, catalyzed by Geranylgeranyltransferase type I ( GGTase-I ). These posttranslational modifications affect localization and biological activity of RAP-1A , .
Like other G-proteins, RAP-1A is found in two interconvertible forms, GDP-bound inactive and GTP-bound active forms . Conversion from GDP-bound form to GTP-bound is catalyzed by Guanine nucleotide exchange factors (GEFs). Rap GEF 1 ( C3G ), Calcium and DAG-regulated GEFs (e.g., RAS guanyl releasing protein 1 ( CALDAG-GEFI ) and RAS guanyl releasing protein 3 ( CalDAG-GEFIII )), Cyclic AMP ( cAMP )-dependent GEFs (e.g., Rap GEF 3 ( cAMP-GEFI ) and Rap GEF 4 ( cAMP-GEFII )), Rap GEF 5 ( MR-GEF ), and RAP1 GTP-GDP dissociation stimulator 1 ( Rap1GDS ) are known GEFs for RAP-1A. GEF first interacts with the GDP-bound form and releases bound GDP. As a result, a binary complex of the small G protein and GEF is formed. Then GEF in this complex is replaced by GTP resulting in formation of the GTP-bound small G protein.
GEF activity is regulated by an upstream signal. C3G is activated in response to T cell receptor ( TCR alpha/beta/ CD3 complex) stimulation via Zeta-chain (TCR) associated protein kinase 70kDa ( ZAP70 )/ v-Crk sarcoma virus CT10 oncogene homolog (avian)-like ( CrkL ) interaction. C3G interacts with the SH3 domain of CrkL and enhances GEF-activity of C3G .
Activity of CALDAG-GEFs and cAMP-GEFs is modulated by the second messengers. Binding of Calcium ( Ca(2+) ) and Diacylglycerol ( DAG ) to CALDAG-GEFs activate these GEFs. Cyclic AMP ( cAMP) activates cAMP-GEFs. Several signaling pathways, which regulate activity of Adenylate cyclase and Phospholipase C (e.g., PLC-delta ), are involved in modulating GEF-activity of CALDAG-GEFs and cAMP-GEFs , .
Phosphorylation of RAP-1A by cAMP-dependent protein kinase ( PKA ) leads to activation of RAP-1A .
Conversion from GTP-bound active form to GDP-bound inactive form is a result of intrinsic GTPase activity of RAP-1A. This activity is slow, and proteins called GTPase activated proteins (GAPs), e.g., Signal-induced proliferation-associated 1 ( SPA1 ), RAS p21 protein activator 3 ( RASA3 ), RAP1 GTPase activating protein ( Rap1GAP ), and Tuberous sclerosis 2 ( Tuberin ) , are known to stimulate it , , , .
GAP-activity is also regulated by several stimuli. Rap1GAP1 binds specifically to the alpha-subunits of the G(i) family (G-protein alpha-i family) of heterotrimeric G-proteins. Stimulation of the G(i)-coupled receptors translocates Rap1GAP1 from the cytosol to the membrane and decreases the amount of GTP-bound RAP-1A. This decrease in GTP-bound RAP-1A abolishes v-Raf-1 murine leukemia viral oncogene homolog 1 ( c-Raf-1 ) inactivation and activates Mitogen-activated protein kinase (ERK1/2) cascade .
RAP-1A binds to c-Raf-1 and competes with RAS proteins for binding to c-Raf-1, thereby antagonizing RAS-dependent activation of c-Raf-1 . On the other hand, v-Raf murine sarcoma viral oncogene homolog B1 ( B-Raf ), another Raf family member, is activated by RAP-1A , .
Small G-proteins are also known to cross-talk with each other. RAP-1A modulates activity of RAL and RAS proteins via their respective effectors Ral guanine nucleotide dissociation stimulator ( RalGDS ) and RAS p21 protein activator 1 ( p120GAP ) , . At the same time, another member of the RAS family, Muscle RAS oncogene homolog ( M-Ras ), regulates activity of RAP-1A exchange factor MR-GEF .