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Mol. Cell. Biol., 10 1995, 5214-5225, Vol 15, No. 10
AD Catling, HJ Schaeffer, CW Reuter, GR Reddy and MJ Weber
Mammalian MEK1 and MEK2 contain a proline-rich (PR) sequence that is absent
both from the yeast homologs Ste7 and Byr1 and from a recently cloned
activator of the JNK/stress-activated protein kinases, SEK1/MKK4. Since
this PR sequence occurs in MEKs that are regulated by Raf family enzymes
but is missing from MEKs and SEKs activated independently of Raf, we sought
to investigate the role of this sequence in MEK1 and MEK2 regulation and
function. Deletion of the PR sequence from MEK1 blocked the ability of MEK1
to associate with members of the Raf family and markedly attenuated
activation of the protein in vivo following growth factor stimulation. In
addition, this sequence was necessary for efficient activation of MEK1 in
vitro by B- Raf but dispensable for activation by a novel MEK1 activator
which we have previously detected in fractionated fibroblast extracts.
Furthermore, we found that a phosphorylation site within the PR sequence of
MEK1 was required for sustained MEK1 activity in response to serum
stimulation of quiescent fibroblasts. Consistent with this observation, we
observed that MEK2, which lacks a phosphorylation site at the corresponding
position, was activated only transiently following serum stimulation.
Finally, we found that deletion of the PR sequence from a constitutively
activated MEK1 mutant rendered the protein nontransforming in Rat1
fibroblasts. These observations indicate a critical role for the PR
sequence in directing specific protein-protein interactions important for
the activation, inactivation, and downstream functioning of the MEKs.
Copyright © 1995, American Society for Microbiology
A proline-rich sequence unique to MEK1 and MEK2 is required for raf binding and regulates MEK function
Department of Microbiology and Cancer Center, University of Virginia Health Sciences Center, Charlottesville 22908, USA.
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