Phosphorylation and functions of inhibitor-2 family of proteins.

Protein phosphatase-1 (PP1) is an essential protein Ser/Thr phosphatase that is extraordinarily conserved from yeast to human, and Inhibitor-2 (I-2) is the most ancient of the heat-stable proteins specific for PP1. We identified novel I-2 homologues in Caenorhabditis elegans (Ce) and Xenopus laevis (Xe) and compared them to the I-2 proteins from Homo sapiens (Hs), Saccharomyces cerevisiae (GLC8), and Drosophila melanogaster (Dm). The Ce I-2 and Dm I-2 showed the highest potency inhibition of rabbit PP1 with IC50 near 5 nM compared to Hs I-2 and Xe I-2 with IC50 between 10 and 50 nM and GLC8 with >100-fold lower activity. Inhibition of PP1 bound to Nek2 kinase activated the kinase to phosphorylate a C-Nap1 domain substrate. All the species of I-2 except GLC8 activated the Nek2::PP1 to the same extent as microcystin-LR. Only Hs I-2 and Xe I-2, not the I-2 proteins more divergent in sequence, directly activated human Aurora-A kinase. Various species of I-2 have a common PxTP phosphorylation site that showed a wide range of reactivity with GSK3, ERK, or CDC2/cyclinB1 kinases. The Suc1 subunit of CDC2/cyclinB1 enhanced reactivity with I-2, consistent with this being a site of mitotic phosphorylation. The results show species specificity among the I-2 family within the context of conserved PP1 inhibitory activity and variable phosphorylation by Pro-directed kinases.