High impact information on Rab3a

• COS cell cotransfection studies demonstrate that rSec8 colocalizes with the GTP-binding protein Rab3a and syntaxin 1a, two proteins involved in synaptic vesicle docking and fusion at the presynaptic terminal [1].
• Rim1, a brain-specific Rab3a-binding protein, localizes to the presynaptic cytomatrix and plays an important role in synaptic transmission and synaptic plasticity [2].
• Many different types of experiments indicate that Rab3a is a negative regulator of exocytosis and that its GTP-bound form interacts with Rabphilin3, a possible effector [3].
• Although Rab3a and Rabphilin3 are located on the same secretory granule or secretory vesicle and interact both in vitro and in situ, it is concluded that the inhibition of secretion by overexpression of Rab3a is unrelated to its ability to interact with Rabphilin3 [3].
• To eliminate the possibility that the staining was due to a Rab3a-related protein, we investigated by immunoelectron microscopy the localization of an epitope-tagged Rab3a expressed in rat PC12 cells [4].

Biological context of Rab3a

• We investigated the roles of two Rab-family proteins, Rab3a and Rab5a, in hippocampal synaptic transmission using real-time fluorescence imaging [5].
• However, overexpression of a mutant form of Rab3a that was truncated at the C-terminus to remove the sites for geranylgeranylation failed to inhibit degranulation [6].
• The effect of Rab3a is specific to secretion, and we observe no effect of Rab3a on receptor-mediated endocytosis [6].
• Using the sciatic nerve as a model, we have studied the axonal transport of Rab3a by quantitative immunocytochemistry and immunoblotting and electron microscopy [7].

Anatomical context of Rab3a

• During synaptic activity, Rab3a dissociated from synaptic vesicles and dispersed into neighbouring axonal regions [5].
• Subcellular fractionation of unstimulated high and low secretor RBL clones revealed that in both cell types Rab3a has a cytoplasmic location while Rab3d is present in a membrane/organelle fraction containing secretory vesicles [8].
• Double-labelling showed a high degree of colocalization between Rab3a and rabphilin-3A in spinal roots and sciatic nerves [9].
• The GTPase Rab3a is associated with large dense core vesicles in bovine chromaffin cells and rat PC12 cells [4].
• Consistent with such a role for Rab3a in regulated exocytosis was the previously reported specific association of Rab3a with synaptic vesicles and with secretory granules in adrenal chromaffin cells [4].

Associations of Rab3a with chemical compounds

• The association of GTP with the guanine nucleotide-free form of modified Rab3A is stimulated by the Rab3 GDP/GTP exchange protein (Rab3 GEP), and the dissociation of GTP from the GTP-bound form is markedly reduced by synaptic vesicle phospholipid [10].
• The Rab3a-induced block in degranulation can be bypassed by stimulation of streptolysin-O-permeabilized cells with guanosine 5'-[gamma-thio]triphosphate [6].

Physical interactions of Rab3a

• Furthermore, as it colocalized in most, but not all, structures with Rab3a, results support the hypothesis that rabphilin-3A is interacting with Rab3a [9].

Co-localisations of Rab3a

• Rab3a was found to colocalize partially with dopamine beta-hydroxylase, a chromaffin granule marker [4].
• Rab3a immunoreactivity (-IR) was always colocalized with synaptophysin-IR and synapsin I-IR in nerve terminals of the spinal cord and peripheral nerve endings [11].

Regulatory relationships of Rab3a

• Moreover, these results - together with the fact that Rabphilin-3A associated with synaptic vesicles inhibits the activity of Rab3 GTPase-activating protein - suggest that the GTP-bound form of modified Rab3A is associated with synaptic vesicles through both Rabphilin-3A and the vesicle phospholipid [10].

Other interactions of Rab3a

• While overexpression of Rab3a did not affect vesicle recycling, overexpression of Rab5a reduced the recycling pool size by 50% [5].
• During differentiation, expression of the vesicular SNARE VAMP-2, the small GTP-binding protein Rab3a, and the target SNARE syntaxin-4 were up-regulated [12].
• The pattern for accumulation of rabphilin-3A was similar to that of Rab3a accumulation, including the poor recycling [9].
• It binds selectively to the GTP-bound form of Rab3a as well as of Rab3c [9].
• Immunoreactivities of the synaptic vesicle-associated proteins Rab3a and SNAP25 were also observed in the cells [13].

Analytical, diagnostic and therapeutic context of Rab3a

• RT-PCR using isoform-specific primers was successfully applied to the quantitation of Rab3a mRNA [8].
• Double in situ hybridization revealed that Rab3a and Rab3b mRNAs were co-localized in most neurons, in all brain areas examined [14].
• Immunogold electron microscopy revealed that organelles labeled for Rab3a were mainly small round vesicles with an average diameter of 50 to 60 nm, indistinguishable from those labeled for the synaptic vesicle protein synaptophsin [7].
• Ligation of the sciatic nerve resulted in a progressive accumulation of Rab3a proximal of the ligation site in virtually all axons with an onset of less than 1 h [7].

References