The protein p130 was originally isolated from rat brain as an inositol 1,4,5-trisphosphate-binding protein with a domain organization similar to that of phospholipase C-delta1 but which lacks phospholipase C activity. Yeast two-hybrid screening of a human brain cDNA library for clones that encode proteins that interact with p130 has now led to the identification of the catalytic subunit of protein phosphatase 1 alpha (PP1c alpha) as a p130-binding protein. The association between p130 and PP1c alpha was also confirmed in vitro by an overlay assay, a "pull-down" assay, and surface plasmon resonance analysis. The interaction of p130 with PP1c alpha resulted in inhibition of the catalytic activity of the latter in a p130 concentration-dependent manner. Immunoprecipitation and immunoblot analysis of COS-1 cells that stably express p130 and of mouse brain extract with antibodies to p130 and to PP1c alpha also detected the presence of a complex of p130 and PP1c alpha, The activity of glycogen phosphorylase, which is negatively regulated by dephosphorylation by PP1c alpha, was higher in COS-1 cells that stably express p130 than in control COS-1 cells. These results suggest that, in addition to its role in inositol 1,4,5-trisphosphate and Ca2+ signaling, p130 might also contribute to regulation of protein dephosphorylation through its interaction with PP1c alpha.