Characterization of a ribosomal inhibitory polypeptide of protein phosphatase-1 from rat liver.
About 4% of the spontaneous phosphorylase phosphatase activity in a rat liver extract was associated with the ribosomal fraction and stemmed from both protein phosphatase-1 (PP-1) and protein phosphatase-2A (PP-2A). However, after repeated washing, only PP-1 remained bound to the ribosomes. The activity of ribosome-associated PP-1 (PP-1R) was partially latent and could be increased 2-3-fold by incubation with trypsin and an additional 50% by incubation with low concentrations of exogenous type-1 catalytic subunit. In contrast, incubation of the ribosomal fraction with MgATP resulted in a 50% drop in the activity of PP-1R. We have purified from a ribosomal extract a basic polypeptide (pI > or = 10.5) of 23 kDa that potently inhibited PP-1. This ribosomal inhibitor of PP-1, termed RIPP-1, was at least 30-times less efficient in inhibiting other major Ser/Thr protein phosphatases (PP-2A, PP-2B and PP-2C). RIPP-1 was identified as a non-competitive inhibitor of PP-1 with a substrate-dependent potency. The lowest Ki (approximately 20 nM) was obtained with phosphorylase and myelin basic protein as substrates. Besides instantaneously inhibiting the type-1 catalytic subunit, RIPP-1 also converted the catalytic subunit in a time-dependent manner (t 1/2 = 45 min at 25 degrees C) into a less active conformation. Unlike the inhibition, this slow inactivation was not reversed by the removal of RIPP-1. We propose that RIPP-1 accounts, at least in part, for the latency of PP-1R.
Beullens
M
M
Afdeling Biochemie, Faculteit Geneeskunde, Katholieke Universiteit Leuven, Belgium.
Stalmans
W
W
Bollen
M
M
D000595:Amino Acid Sequence; D000818:Animals; D002852:Chromatography, Ion Exchange; D004591:Electrophoresis, Polyacrylamide Gel; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008969:Molecular Sequence Data; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors* / Q000737:chemistry / Q000302:isolation & purification / Q000378:metabolism*; D010766:Phosphorylation; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2; D051381:Rats; D013379:Substrate Specificity; D014357:Trypsin / Q000378:metabolism
D010749:Phosphoprotein Phosphatases; D054645:Protein Phosphatase 1; D054648:Protein Phosphatase 2; D014357:Trypsin