Evidence for a functional link between Rab3 and the SNARE complex.
Rab3 is a monomeric GTP-binding protein associated with secretory vesicles which has been implicated in the control of regulated exocytosis. We have exploited Rab3 mutant proteins to investigate the function of Rab3 in the process of neurotransmitter release from Aplysia neurons. A GTPase-deficient Rab3 mutant protein was found to inhibit acetylcholine release suggesting that GTP hydrolysis by Rab3 is rate-limiting in the exocytosis process. This effect was abolished by a mutation in the effector domain, and required the association of Rab3 with membranes. In order to determine the step at which Rab3 interferes with the secretory process, tetanus and botulinum type A neurotoxins were applied to Aplysia neurons pre-injected with the GTPase-deficient Rab3 mutant protein. These neurotoxins are Zn(2+)-dependent proteases that cleave VAMP/synaptobrevin and SNAP-25, two proteins which can form a ternary complex (termed the SNARE complex) with syntaxin and have been implicated in the docking of synaptic vesicles at the plasma membrane. The onset of toxin-induced inhibition of neurotransmitter release was strongly delayed in these cells, indicating that the mutant Rab3 protein led to the accumulation of a toxin-insensitive component of release. Since tetanus and botulinum type A neurotoxins cannot attack their targets, VAMP/synaptobrevin and SNAP-25, when the latter are engaged in the SNARE complex, we propose that Rab3 modulates the activity of the fusion machinery by controlling the formation or the stability of the SNARE complex.
Johannes
L
L
Service de Neurobiologie Physico-Chimique, Centre National de la Recherche Scientifique, UPR 9071, Institut de Biologie Physico-Chimique, Paris, France.
Doussau
F
F
Clabecq
A
A
Henry
J P
JP
Darchen
F
F
Poulain
B
B
D000109:Acetylcholine / Q000378:metabolism; D000818:Animals; D001048:Aplysia; D003013:Clostridium; D020558:GTP Phosphohydrolases / Q000378:metabolism; D019204:GTP-Binding Proteins / Q000737:chemistry / Q000378:metabolism*; D006160:Guanosine Triphosphate / Q000378:metabolism; D006801:Humans; D008565:Membrane Proteins / Q000737:chemistry / Q000378:metabolism*; D008969:Molecular Sequence Data; D016296:Mutagenesis; D009419:Nerve Tissue Proteins / Q000378:metabolism*; D009498:Neurotoxins / Q000494:pharmacology; D017368:Protein Prenylation; D011994:Recombinant Proteins / Q000378:metabolism; D050600:SNARE Proteins; D016415:Sequence Alignment; D033921:Vesicular Transport Proteins; D020692:rab3 GTP-Binding Proteins
D008565:Membrane Proteins; D009419:Nerve Tissue Proteins; D009498:Neurotoxins; D011994:Recombinant Proteins; D050600:SNARE Proteins; D033921:Vesicular Transport Proteins; D006160:Guanosine Triphosphate; D020558:GTP Phosphohydrolases; D019204:GTP-Binding Proteins; D020692:rab3 GTP-Binding Proteins; D000109:Acetylcholine