Regulates myosin phosphatase activity. Augments Ca(2+) sensitivity of the contractile apparatus. 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. Along actomyosin filaments. Detected in skeletal muscle, fetal and adult heart, brain, placenta, kidney, spleen, thymus, pancreas and lung. Isoform 3 and isoform 4 are heart specific. Produced by alternative promoter usage. Produced by alternative splicing of isoform 1. Produced by alternative splicing of isoform 4. Produced by alternative promoter usage.