Complete Primary Structure of Protein Phosphatase Inhibitor‐1 from Rabbit Skeletal Muscle

The complete primary structure of protein phosphatase inhibitor‐1 has been determined. The protein consists of a single polypeptide chain of 165 residues, molecular weight 18640. The threonine residue that must be phosphorylated for activation is at position 35 and the active cyanogen bromide peptide, CB‐1, comprises residues 2–66. The N‐terminal methionine is acetylated and 40% of the inhibitor‐1 molecules lack the C‐terminal dipeptide Ala‐Val. Serine‐67 is substantially phosphorylated in vivo, but this phosphoserine residue does not appear to influence the activity of inhibitor‐1.