MOLECULAR-CLONING OF NIPP-1, A NUCLEAR INHIBITOR OF PROTEIN PHOSPHATASE-1, REVEALS HOMOLOGY WITH POLYPEPTIDES INVOLVED IN RNA PROCESSING

NIPP-1 was originally isolated as a potent and specific nuclear inhibi tory polypeptide (16-18 kDa) of protein phosphatase-1. We report here the cDNA cloning of NIPP-1 from bovine thymus and show that the native polypeptide consists of 351 residues and has a calculated mass of 38. 5 kDa. The bacterially expressed central third of NIPP-1 completely in hibited the type-1 catalytic subunit, but displayed a reduced inhibito ry potency after phosphorylation by protein kinase A and casein kinase 2, Translation of NIPP-1 mRNA in reticulocyte lysates resulted in the accumulation of both intact NIPP-1 and a smaller polypeptide generate d by alternative initiation at the codon corresponding to Met(143), A data base search showed that the COOH terminus of NIPP-1 is nearly ide ntical to the human ard-1 protein (13 kDa), which has been implicated in RNA processing (Wang, M., and Cohen, S. N. (1994) Proc. Natl. Acad. Sci, U. S. A. 91, 10591-10595), Comparison of the cDNAs encoding ard- 1 and NIPP-1 suggests that their mRNAs are generated by alternative sp licing of the same pre-mRNA. Western blotting with antibodies against the COOH terminus of NIPP-1, however, showed a single polypeptide of 4 7 kDa, which was enriched in the nucleus. Northern analysis revealed a single transcript of 2.2 kilobases in bovine thymus and of 2.4 kiloba ses in various human tissues.