Organelle Markers
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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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