Maintains low levels of EIF2S1 phosphorylation in unstressed cells by promoting its dephosphorylation by PP1. Interacts with PP1. Part of a complex containing PPP1R15B, PP1 and NCK1/2. Mice are born at the expected Mendelian ratio, but exhibit severe growth retardation and impaired erythropoiesis. None survive the first day of postnatal life. PPP1R15A-PPP1R15B double-knockout embryos do not develop past the preimplantation period. Belongs to the PPP1R15 family.