Regulator that plays a central role in regulation of apoptosis via its interaction with p53/TP53 (By similarity). Regulates TP53 by enhancing the DNA binding and transactivation function of TP53 on the promoters of proapoptotic genes in vivo (By similarity). Interacts with P53/TP53; the interaction promotes pro-apoptotic activity. Predominantly cytoplasmic (By similarity). Some fraction is nuclear. The ankyrin repeats and the SH3 domain are required for a specific interactions with TP53. In contrast to its official gene name, it is not a regulatory subunit of protein phosphatase 1. This name was given due to its similarity with a protein that binds to protein phosphatase 1. Belongs to the ASPP family.