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
RAS guanyl releasing protein 3
(CalDAG-GEFIII)), Cyclic AMP
(cAMP)-dependent GEFs (e.g., Rap GEF 3
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
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
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
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