THE MOUSE B-RAF GENE ENCODES MULTIPLE PROTEIN ISOFORMS WITH TISSUE-SPECIFIC EXPRESSION

The c-Rmil/B raf proto-oncogene is a member of the mil/raf family enco ding serine/threonine protein kinases shown to be involved in signal t ransduction from the membrane to the nucleus. We isolated from a mouse brain library B-raf cDNAs containing a previously unidentified 36-bas e pair alternatively spliced exon located between exons 8 and 9 and, t herefore, designated exon 8b. Human and mouse B raf mRNAs also contain the 120-base pair alternatively spliced exon 10 previously described in the avian c-Rmil gene. Independent splicing of these two exons, loc ated between the conserved region 2 (CR2) and the catalytic domain (CR 3) gives rise to mRNAs potentially encoding four distinct proteins. By using specific sera generated against different portions of B-Raf, we identified at least 10 protein isoforms in adult mouse tissues. Some isoforms, in the range of 69-72 kDa, are not recognized by antisera di rected against peptides encoded by exons 1 and 2, indicating the exist ence of B-Raf proteins with two different NH2 extremities. The other i soforms, in the range of 79-99 kDa, contain the amino acids encoded by exons 1 and 2, by either or both of the alternatively spliced exons, and, possibly, by another unidentified exon. Analysis of B-raf mRNA ex pression by reverse transcriptase polymerase chain reaction and immuno characterization of B-Raf proteins in different tissues of the adult m ouse showed a tissue-specific pattern of B-Raf isoforms expression. In terestingly, isoforms containing amino acids encoded by exon 10 are sp ecifically expressed in neural tissues. Taken together, these results suggest that distinct B-Raf proteins could be involved, in a tissue-sp ecific manner, in signal transduction pathways.