A synaptic vesicle specific GTP-binding protein from ray electric organ

A cDNA encoding a synaptic vesicle associated GTP-binding protein was identified by screening a λgt11 expression library derived from the electric lobe of Discopyge ommata with polyclonal antibodies recognizing vesicle-specific proteins of M_r 25,000. Nucleotide sequence analysis defines an open reading frame of 218 amino acids. The protein belongs to the ras superfamily and shares about 75% amino acid identity with smg-25A, B and C identified in bovine brain and rab3A characterized in rat brain. Northern blot analysis revealed a 4.5 kb transcript present only in neural tissues, the highest level of expression being observed in electric lobe. Western blot analysis of total tissue homogenates derived from D. ommata detected the protein in electric organ, forebrain and to a lesser extent in electric lobe and spinal cord. No immunoreactivity was detected in non-neuronal tissues. Blotting of subcellular fractions derived from electric ray electric organ revealed that the GTP-binding protein co-purifies with synaptic vesicles. The neural specific expression and the localization to synaptic vesicles suggest a role of this protein in synaptic vesicle trafficking and targeting.