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. Interacts with phosphorylated CDC5L, SF3B1 and MELK. Interacts with EED. Part of a complex consisting of PPP1R8, EED, HDAC2 and PP-1. Part of the spliceosome. Interacts with PPP1CA, PPP1CB and PPP1CC (By similarity). Mainly, but not exclusively, nuclear. Has a basic N- and C-terminal and an acidic central domain. The FHA domain mediates interactions with threonine-phosphorylated MELK. The N-terminus is blocked. Inactivated by phosphorylation on Ser-199 or Ser-204. Expressed in baculovirus. A synthetic peptide, NIPP-1(191-200), is able to inhibit PP-1. Alanine substitution of Val-201 and Phe-203 in NIPP-1(191-210) prevents PP-1 binding (far-western assay) but do not affect PP-1 inhibition. Phosphorylation of Ser-199 or Ser-204 prevents PP-1 binding (far-western assay) and reduces PP-1 inhibition.