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Vol. 10, Issue 11, 3729-3743, November 1999
Department of Molecular Pharmacology and Program in Cancer Biology,
Stanford University School of Medicine, Stanford, California 94305-5332
Xenopus oocyte maturation requires the
phosphorylation and activation of p42 mitogen-activated protein kinase
(MAPK). Likewise, the dephosphorylation and inactivation of p42 MAPK
are critical for the progression of fertilized eggs out of meiosis and
through the first mitotic cell cycle. Whereas the kinase
responsible for p42 MAPK activation is well characterized,
little is known concerning the phosphatases that inactivate p42 MAPK.
We designed a microinjection-based assay to examine the mechanism of
p42 MAPK dephosphorylation in intact oocytes. We found that p42 MAPK
inactivation is mediated by at least two distinct phosphatases, an
unidentified tyrosine phosphatase and a protein phosphatase 2A-like
threonine phosphatase. The rates of tyrosine and threonine
dephosphorylation were high and remained constant throughout meiosis,
indicating that the dramatic changes in p42 MAPK activity seen during
meiosis are primarily attributable to changes in MAPK kinase activity.
The overall control of p42 MAPK dephosphorylation was shared among four
partially rate-determining dephosphorylation reactions, with the
initial tyrosine dephosphorylation of p42 MAPK being the most critical
of the four. Our findings provide biochemical and kinetic insight into
the physiological mechanism of p42 MAPK inactivation.
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