The C-terminus of NIPP1 (nuclear inhibitor of protein phosphatase-1) contains a novel binding site for protein phosphatase-1 that is controlled by tyrosine phosphorylation and RNA binding

Nuclear inhibitor of protein phosphatase-1 (NIPP1; 351 residues) is a nuclear RNA-binding protein that also contains in its central domain two contiguous sites of interaction with the catalytic subunit of protein phosphatase-1 (PP1_C). We show here that mutation of these phosphatase-interaction sites did not completely abolish the ability of NIPP1 to bind and inhibit PP1_C. This could be accounted for by an additional inhibitory phosphatase-binding site in the C-terminal region (residues 311Ő351), with an inhibitory core corresponding to residues 331Ő337. Following mutation of all three PP1_C-binding sites in the central and C-terminal domains, NIPP1 no longer interacted with PP1_C. Remarkably, while both NIPP1 domains inhibited the phosphorylase phosphatase activity of PP1_C independently, mutation of either domain completely abolished the ability of NIPP1 to inhibit the dephosphorylation of myelin basic protein. The inhibitory potency of the C-terminal site of NIPP1 was decreased by phosphorylation of Tyr-335 and by the addition of RNA. Tyr-335 could be phosphorylated by tyrosine kinase Lyn, but only in the presence of RNA. In conclusion, NIPP1 contains two phosphatase-binding domains that function co-operatively but which are controlled independently. Our data are in agreement with a shared-site model for the interaction of PP1_C with its regulatory subunits.