Purification and characterization of a novel protein phosphatase highly specific for ribosomal protein S6.
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.
Andres
J L
JL
Department of Pharmacology, University of Colorado School of Medicine, Denver 80262.
Maller
J L
JL
D000818:Animals; D002850:Chromatography, Gel / Q000379:methods; D002852:Chromatography, Ion Exchange / Q000379:methods; D005260:Female; D007700:Kinetics; D008970:Molecular Weight; D010053:Ovary / Q000201:enzymology; D010749:Phosphoprotein Phosphatases / Q000302:isolation & purification* / Q000378:metabolism; D054645:Protein Phosphatase 1; D012270:Ribosomes / Q000201:enzymology*; D013379:Substrate Specificity; D014982:Xenopus laevis
D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1