Modulation of kinase activity and oncogenic properties by alternative splicing reveals a novel regulatory mechanism for B-Raf

Members of the raf oncogene family encode serine/threonine protein kinases, which activate the mitogen-activated protein kinase kinase MEKs (MAPK or ERK kinases) through direct interaction and phosphorylation. Several recent studies have revealed interesting differences between two members of this family, Raf-1 and B-Raf, regarding their activation, regulation, and kinase activity. In particular, B-Raf was shown to display higher MEK kinase activity than Raf-1. By using both two-hybrid analysis and coimmunoprecipitation experiments, we demonstrate here that B-Raf also markedly differs from Raf-1 by a higher affinity for MEK. We previously reported that the B-raf gene encodes multiple protein isoforms resulting from complex alternative splicing of two exons (exons 8b and 10) located upstream of B-Raf kinase domain. In the present study, we show that these naturally occurring modifications within the protein sequence markedly modulate both the biochemical and oncogenic properties of B-Raf. The presence of exon 10 sequences enhances the affinity for MEK, the basal kinase activity, as well as the mitogenic and transforming properties of full-length B-Raf, whereas the presence of exon 8b sequences seems to have opposite effects. Therefore, alternative splicing represents a novel regulatory mechanism for a protein of the Raf family.