MARKERS - Endosome
- 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.
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.
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.
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.
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