Inhibitor subunit of the major nuclear protein phosphatase-1 (PP-1). It has RNA-binding activity but does not cleave RNA and may target PP-1 to RNA-associated substrates. May also be involved in pre-mRNA splicing. Binds DNA and might act as a transcriptional repressor. Seems to be required for cell proliferation. Isoform Gamma is a site-specific single-strand endoribonuclease that cleaves single strand RNA 3' to purines and pyrimidines in A+U-rich regions. It generates 5'-phosphate termini at the site of cleavage. This isoform does not inhibit PP-1. May be implicated in mRNA splicing. Endoribonuclease function is magnesium-dependent. Interacts with phosphorylated CDC5L, SF3B1 and MELK. Interacts with EED, in a nucleic acid-stimulated manner. Part of a complex consisting of PPP1R8, EED, HDAC2 and PP-1. Part of the spliceosome. Interacts with PPP1CA, PPP1CB and PPP1CC. Primarily, but not exclusively, nuclear. Found mainly in the cytoplasm. Ubiquitously expressed, with highest levels in heart and skeletal muscle, followed by brain, placenta, lung, liver and pancreas. Less abundant in kidney. The concentration and ratio between isoforms is cell-type dependent. Isoform Alpha (>90%) and isoform Beta were found in brain, heart and kidney. Isoform Gamma is mainly found in B-cells and T-lymphocytes, and has been found in 293 embryonic kidney cells. Has a basic N- and C-terminal and an acidic central domain. The FHA domain mediates interactions with threonine-phosphorylated MELK. May be inactivated by phosphorylation on Ser-199 or Ser-204 (By similarity). Phosphorylated by Lyn in vitro on Tyr-264, and also on Tyr-335 in the presence of RNA. A synthetic peptide, NIPP-1(330-351), is able to inhibit PP-1. Phosphorylation of Tyr-335 reduces PP-1 inhibition, whereas phosphorylation of Thr-346 or Ser-348 has no effect.