J. Biol. Chem., Vol. 264, Issue 1, 151-156, 01, 1989
Purification and characterization of a novel protein phosphatase highly specific for ribosomal protein S6
JL Andres and JL Maller
Department of Pharmacology, University of Colorado School of Medicine, Denver 80262.
Ribosomal protein S6 is the principal phosphoprotein of the eucaryotic
ribosome that becomes multiply phosphorylated on serine residues in
response to a wide variety of mitogenic stimuli. In this paper the
principal protein phosphatases able to dephosphorylate S6 were
characterized in Xenopus laevis ovary and eggs. Two enzymes termed peak I
and peak II were found to account for most S6 phosphatase activity in both
oocytes and eggs. The peak I enzyme had an apparent Mr of 200,000 on gel
filtration, dephosphorylated the beta subunit of phosphorylase kinase and
phosphorylase a, and was inhibited by inhibitor 1 and inhibitor 2,
suggesting it was similar to protein phosphatase 1. The peak II enzyme was
purified over 12,000-fold and had an apparent Mr = 55,000 on glycerol
gradient centrifugation. This phosphatase could dephosphorylate all sites
in S6 but was unable to dephosphorylate phosphorylase a or phosphorylase
kinase. However, it was inhibited by nanomolar concentrations of inhibitor
1 and inhibitor 2. These results indicate the peak II enzyme represents a
new class of highly specific protein phosphatase and suggest that
inhibition of dephosphorylation in cellular extracts by inhibitor 1 and
inhibitor 2 is not a sufficient criterion for implicating protein
phosphatase 1 in a cellular process.
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