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 mod ified post-translationally by two geranylgeranyl groups and specifical ly targeted to synaptic vesicles. We have now cloned and characterized the murine gene coding for rab3A. With a size of less than 8 kb inclu ding the promoter, the rab3A gene is relatively small. It contains fiv e exons, the first of which is non-coding. The organization of the rab 3A coding sequence into exons in the gene is different from that of ra s proteins, the only other low-molecular-mass GTP-binding proteins wit h currently characterized gene structures. Nevertheless, the intron pl acement in the primary structure of rab3A may be indicative of a domai n division of the protein, since each coding exon contains one of the four major conserved rab protein sequence motifs. The epitopes of mono clonal and polyclonal antibodies to rab3A were mapped with the hypothe sis that antibody epitopes might represent distinct exposed protein do mains and correlate with exon structures. Two monoclonal antibodies, n amed 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. Comparis on of the sequence of rab3A with that of ras, whose crystal structure has been determined, revealed that the epitopes for the monoclonal ant ibodies correspond to regions in ras that are highly exposed. Taken to gether, these results suggest that exons 4 and 5 at least represent di stinct exposed protein domains that also form major natural epitopes i n rab3A.