Regulates myosin phosphatase activity. Augments Ca(2+) sensitivity of the contractile apparatus (By similarity). PP1 comprises a catalytic subunit, PPP1CA, PPP1CB or PPP1CC, and one or several targeting or regulatory subunits. PPP1R12B mediates binding to myosin. Isoform 3 and isoform 4 bind PPP1R12A, but not isoform 1 of PPP1R12B itself. Binds IL16 (By similarity). Along actomyosin filaments.