C. Papin et al., MODULATION OF KINASE-ACTIVITY AND ONCOGENIC PROPERTIES BY ALTERNATIVESPLICING REVEALS A NOVEL REGULATORY MECHANISM FOR B-RAF, The Journal of biological chemistry, 273(38), 1998, pp. 24939-24947
Members of the raf oncogene family encode serine/ threonine protein ki
nases, 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 activatio
n, regulation, and kinase activity. In particular, B-Raf was shown to
display higher MEK kinase activity than Raf-1. By using both two-hybri
d analysis and coimmunoprecipitation experiments, we demonstrate here
that B-Raf also markedly differs from Raf-1 by a higher affinity for M
EK. We previously reported that the B-raf gene encodes multiple protei
n isoforms resulting from complex alternative splicing of two exons (e
xons 8b and 10) located upstream of E-Raf kinase domain. In the presen
t study, we show that these naturally occurring modifications within t
he protein sequence markedly modulate both the biochemical and oncogen
ic 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 presenc
e of exon 8b sequences seems to have opposite effects. Therefore, alte
rnative splicing represents a novel regulatory mechanism for a protein
of the Raf family.