Inhibitor of the PP1 regulatory subunit PPP1CA. Detected in heart, muscle, spinal cord, hippocampus, hypothalamus, thalamus, midbrain, brain stem, cerebellum, brain cortex and olfactory bulb. Up-regulated by morphine. The main inhibitory site appears to be Thr-72 (By similarity). Has over 600-fold higher inhibitory activity when phosphorylated, creating a molecular switch for regulating the phosphorylation status of PPP1CA substrates and smooth muscle contraction. Belongs to the PP1 inhibitor family.