Mixed-lineage kinase 3 regulates B-Raf through maintenance of the B-Raf/Raf-1 complex and inhibition by the NF2 tumor suppressor protein.
The Ras --> Raf --> MEK1/2 --> extracellular signal-regulated kinase (ERK) mitogen-activated protein kinase (MAPK) pathway couples mitogenic signals to cell proliferation. B-Raf and Raf-1 function within an oligomer wherein they are regulated in part by mutual transactivation. The MAPK kinase kinase (MAP3K) mixed-lineage kinase 3 (MLK3) is required for mitogen activation of B-Raf and cell proliferation. Here we show that the kinase activity of MLK3 is not required for support of B-Raf activation. Instead, MLK3 is a component of the B-Raf/Raf-1 complex and is required for maintenance of the integrity of this complex. We show that the activation of ERK and the proliferation of human schwannoma cells bearing a loss-of-function mutation in the neurofibromatosis 2 (NF2) gene require MLK3. We find that merlin, the product of NF2, blunts the activation of both ERK and c-Jun N-terminal kinase (JNK). Finally, we demonstrate that merlin and MLK3 can interact in situ and that merlin can disrupt the interactions between B-Raf and Raf-1 or those between MLK3 and either B-Raf or Raf-1. Thus, MLK3 is part of a multiprotein complex and is required for ERK activation. The levels of this complex may be negatively regulated by merlin.
Chadee
Deborah N
DN
Molecular Cardiology Research Institute, Tufts-New England Medical Center, Tufts University School of Medicine, 750 Washington Street, Boston, MA 02111, USA.
Xu
Dazhong
D
Hung
Gene
G
Andalibi
Ali
A
Lim
David J
DJ
Luo
Zhijun
Z
Gutmann
David H
DH
Kyriakis
John M
JM
D000818:Animals; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D006801:Humans; D020930:MAP Kinase Kinase Kinases / Q000037:antagonists & inhibitors* / Q000378:metabolism*; D009097:Multienzyme Complexes / Q000378:metabolism; D009154:Mutation; D025581:Neurofibromin 2 / Q000235:genetics / Q000378:metabolism*; D048493:Proto-Oncogene Proteins B-raf / Q000378:metabolism*; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism*; D051381:Rats; D025521:Tumor Suppressor Proteins / Q000235:genetics / Q000378:metabolism*; D000097767:Mitogen-Activated Protein Kinase Kinase Kinase 11
D048049:Extracellular Signal-Regulated MAP Kinases; D020930:MAP Kinase Kinase Kinases; D009097:Multienzyme Complexes; D025581:Neurofibromin 2; D048493:Proto-Oncogene Proteins B-raf; D019908:Proto-Oncogene Proteins c-raf; D025521:Tumor Suppressor Proteins; D000097767:Mitogen-Activated Protein Kinase Kinase Kinase 11; C482119:BRAF protein, human