A novel protein phosphatase-1 inhibitory protein potentiated by protein kinase C. Isolation from porcine aorta media and characterization.

A novel phosphorylation-dependent inhibitory protein (IP) of porcine aorta myosin light chain phosphatase (PA-MLCP) was purified to homogeneity from porcine aorta media. The molecular mass of IP was 20 kDa. IP phosphorylated by endogenous potentiating kinase (IP-K) inhibited not only PA-MLCP activity, but also that of the catalytic subunit of protein phosphatase-1. The amino acid sequence of a peptide derived from IP phosphorylated with IP-K, RHARVTVK, shared one of the consensus sequences phosphorylatable by protein kinase C (PKC), where T was phosphorylated. IP was phosphorylated by PKC and the phosphorylated product inhibited PA-MLCP as strongly as IP phosphorylated with IP-K.