ORGANELLE MARKERS - Endosome

EEA1 - Early endosomal antigen

Early endosomal antigen 1 (EEA1) is a 162 kDa membrane bound protein component specific to the early endosome and is essential for fusion between early endocytic vesicles. Early endosomes are cytoplasmic compartments which fuse with endocytic vesicles for redistribution of extracellular compounds to alternate destinations. Zinc-finger-like domains, reminiscent of those found in nucleic acid binding proteins, are located in the amino and carboxyl-terminal domains of EEA1. The carboxyl-terminal zinc-finger-like-domain is conserved in several other non-nuclear proteins, some of which are also involved in intracellular protein trafficking. In addition, this domain is an authentic zinc-binding domain and is critical to the intracellular localization of EEA1. Membrane association of EEA1 has been shown dependent on phosphatidyl 3-kinase activity, inhibitors of which cause EEA1 to dissociate from early endosomes.

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Rab4

Rab4 is a 25 kDa GTP-binding protein that regulates recycling of proteins from the early endosomes to the cell surface. Overexpression of Rab4 causes a redistribution of receptors on plasma membrane versus endocytic compartments. The presence of excessive Rab4 accumulates transferrin receptors in non-acidic postendosomal recycling vesicles, which is believed to be an intermediate compartment between endosomes and plasma membranes. Rab4 is also involved in the translocation of glucose transporter (Glu4) in adipocytes in response to insulin. Association of Rab4 with transferrin receptor-containing early endosomes is mediated through the geranylgeranyl groups at its carboxyl-terminus. Membrane association is also cell cycle dependent, as phosphorylation at its carboxyl-terminus cdc2 kinase consensus sequence in mitotic cells leads to dissociation of Rab4 into the cytosol.

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Rab5

Rab5 is a 24 kDa GTP-binding protein that regulates the fusion of plasma membrane-derived clathrin-coated vesicles with early endosomes and homotypic fusion among early endosomes. It is localized to the cytoplasmic side of the plasma membrane, clathrincoated vesicles, and early endosomes. Rab5 is believed to regulate vesicle fusion through a cycle of GDP/GTP exchange and GTP hydrolysis. The different guanine nucleotide binding states of rab5 is postulated to affect its ability to associate or dissociate with membranes during endocytotic membrane traffic. Its GTP-bound form, which represents the active form of Rab5, associates with membrane and regulates vesicle docking and fusion. Studies using Rab5 mutant that hydrolysed xanthosine 5.-triphosphate (XTP) indicated that nucleotide hydrolysis occurs even in the absence of membrane fusion. GTP hydrolysis by Rab5 is postulated to determine the frequency of membrane docking and fusion events.

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Syntaxin 13

Syntaxin 13 is an integral membrane protein that belongs to the t- SNARE family, a group of proteins involved in protein transport. Confocal immunofluoresence and electron microscopy studies have shown that syntaxin 13 is primarily localized to tubular early and recycling endosomes, where it colocalizes with transferrin receptor, and it is also localized in endosomal vacuoles. Syntaxin 13 has been found to be expressed in all tissues, with higher levels of the protein found in brain, lung, spleen, thymus and testes. Immunoprecipitation studies show that syntaxin 13 complexes with SNAP, VAMP2/3, and SNAP25. The binding of this complex to SNAP and NSF is terminated in the presence of ATP . These results suggest that syntaxin 13 is a SNARE protein which mediates the recycling protein flow through tubulo-vesicular recycling endosomes.

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Rap1/Krev1

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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