CHARACTERIZATION OF A RIBOSOMAL INHIBITORY POLYPEPTIDE OF PROTEIN PHOSPHATASE-1 FROM RAT-LIVER

About 4% of the spontaneous phosphorylase phosphatase activity in a ra t 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 parti ally latent and could be increased 2-3-fold by incubation with trypsin and an additional 50% by incubation with low concentrations of exogen ous 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 grea ter than or equal to 10.5) of 23 kDa that potently inhibited PP-1. Thi s ribosomal inhibitor of PP-1, termed RIPP-1, was at least 30-times le ss efficient in inhibiting other major Ser/Thr protein phosphatases (P P-2A, PP-2B and PP-2C). RIPP-1 was identified as a non-competitive inh ibitor of PP-1 with a substrate-dependent potency. The lowest K-i (app roximately 20 nM) was obtained with phosphorylase and myelin basic pro tein as substrates. Besides instantaneously inhibiting the type-1 cata lytic subunit, RIPP-1 also converted the catalytic subunit in a time-d ependent manner (t(1/2) = 45 min at 25 degrees C) into a less active c onformation. Unlike the inhibition, this slow inactivation was not rev ersed by the removal of RIPP-1. We propose that RIPP-1 accounts, at le ast in part, for the latency of PP-1R.