Wild-type and mutant B-RAF activate C-RAF through distinct mechanisms involving heterodimerization.
The protein kinase B-RAF is mutated in approximately 7% of human cancers. Most mutations are activating, but, surprisingly, a small number have reduced kinase activity. However, the latter can still stimulate cellular signaling through the MEK-ERK pathway because they activate the related family member C-RAF. We examine the mechanism underlying C-RAF activation by B-RAF. We show that C-RAF is activated in the cytosol in a RAS-independent manner that requires activation segment phosphorylation and binding of 14-3-3 to C-RAF. We show that wild-type B-RAF forms a complex with C-RAF in a RAS-dependent manner, whereas the mutants bind independently of RAS. Importantly, we show that wild-type B-RAF can also activate C-RAF. Our data suggest that B-RAF activates C-RAF through a mechanism involving 14-3-3 mediated heterooligomerization and C-RAF transphosphorylation. Thus, we have identified a B-RAF-C-RAF-MEK-ERK cascade that signals not only in cancer but also in normal cells.
Garnett
Mathew J
MJ
Signal Transduction Team, The Institute of Cancer Research, Cancer Research UK Centre of Cell and Molecular Biology, London.
Rana
Sareena
S
Paterson
Hugh
H
Barford
David
D
Marais
Richard
R
D048948:14-3-3 Proteins / Q000378:metabolism; D019898:Autocrine Communication; D003593:Cytoplasm / Q000201:enzymology; D019281:Dimerization; D004789:Enzyme Activation; D006133:Growth Substances / Q000378:metabolism; D006801:Humans; D007527:Isoenzymes / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D020935:MAP Kinase Signaling System / Q000502:physiology*; D008958:Models, Molecular; D046912:Multiprotein Complexes; D009154:Mutation; D009369:Neoplasms / Q000201:enzymology; D011487:Protein Conformation; D048493:Proto-Oncogene Proteins B-raf / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018631:ras Proteins / Q000235:genetics / Q000378:metabolism
D048948:14-3-3 Proteins; D006133:Growth Substances; D007527:Isoenzymes; D046912:Multiprotein Complexes; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf; D018631:ras Proteins