Rap1/Krev1 is a member of the ras family of low
molecular weight GTP-binding proteins. Ras-like GTPases are ubiquitously
expressed, evolutionarily conserved molecular switches that couple extracellular
signals to various cellular responses. Rap1 is primarily found at the
cytosolic side of intracellular membranes and has two isoforms: Rap1a
and 1b. Both isoforms have a molecular mass of 21 kDa and are isoprenylated
at the carboxyl-terminal and phosphorylated by the cAMP-dependent protein
kinase A (PKA).
Rap1 cycles between a GTP-bound active form and a GDP-bound inactive
form that is mediated by GTPase activating protein (GAP) and GDP dissociation
stimulator (GDS). Activation occurs by a variety of extracellular stimuli
through several conserved guanine nucleotide exchange factors (GEFs)
and GTPase activating proteins (GAPs). Rap1 is proposed to regulate Ras-mediated
signalling and may also be involved in the regulation of integrin-mediated
cell adhesion although the mechanism of regulation is not known.
Overexpression of Rap reverses the transformed phenotype induced by ras,
possibly by competing with ras for interaction with ras-GAP. Rap has
been shown to participate in MAP kinase cascade activated by growth factor
and maintaining human T cell anergic state by blocking IL-2 expression.
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