Structure of the murine rab3A gene: correlation of genomic organization with antibody epitopes

Rab3A is a neuronal low-molecular-mass GTP-binding protein that is modified post-translationally by two geranylgeranyl groups and specifically targeted to synaptic vesicles. We have now cloned and characterized the murine gene coding for rab3A. With a size of less than 8 kb including the promoter, the rab3A gene is relatively small. It contains five exons, the first of which is non-coding. The organization of the rab3A coding sequence into exons in the gene is different from that of ras proteins, the only other low-molecular-mass GTP-binding proteins with currently characterized gene structures. Nevertheless, the intron placement in the primary structure of rab3A may be indicative of a domain division of the protein, since each coding exon contains one of the four major conserved rab protein sequence motifs. The epitopes of monoclonal and polyclonal antibodies to rab3A were mapped with the hypothesis that antibody epitopes might represent distinct exposed protein domains and correlate with exon structures. Two monoclonal antibodies, named 42.1 and 42.2, were found to recognize epitopes with a different degree of conservation between different rab3 isoforms. These epitopes were mapped to relatively short amino acid sequences corresponding to exons 4 and 5 respectively, whereas a polyclonal antibody recognized a complex epitope that required the presence of intact rab3A. Comparison of the sequence of rab3A with that of ras, whose crystal structure has been determined, revealed that the epitopes for the monoclonal antibodies correspond to regions in ras that are highly exposed. Taken together, these results suggest that exons 4 and 5 at least represent distinct exposed protein domains that also form major natural epitopes in rab3A.