Repo-Man recruits PP1γ to chromatin and is essential for cell viability

Protein phosphatase 1 (PP1) is a ubiquitous serine/ threonine phosphatase regulating many cellular processes. PP1 alpha and -gamma are closely related isoforms with distinct localization patterns, shown here by time-lapse microscopy of stably expressed fluorescent protein fusions. A pool of PP1 gamma is selectively loaded onto chromatin at anaphase. Using stable isotope labeling and proteomics, we identified a novel PP1 binding protein, Repo-Man, which selectively recruits PP1 gamma onto mitotic chromatin at anaphase and into the following interphase. This approach revealed both novel and known PP1 binding proteins, quantitating their relative distribution between PP1 alpha and -gamma In vivo. When overexpressed, Repo-Man can also recruit PP1 alpha to chromatin. Mutating Repo-Man's PP1 binding domain does not disrupt chromatin binding but abolishes recruitment of PP1 onto chromatin. RNA interference-induced knockdown of Repo-Man caused large-scale cell death by apoptosis, as did overexpression of this dominant-negative mutant. The data Indicate that Repo-Man forms an essential complex with PP1 gamma and is required for the recruitment of PP1 to chromatin.