The cysteine proteinase inhibitor chicken cystatin is a phosphoprotein

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Abstract
Peptide maps obtained by reversed‐phase HPLC of tryptic digests of isoelectric form 1 (pI=6.5) and 2 (pI=5.6) of chicken egg white cystatin revealed that the difference was located only in a single peptide (residues Ser‐74—Lys‐91). Ser‐80 of cystatin 2 was subsequently identified as being modified by phosphorylation. Moreover, alkaline phosphatase treatment of a mixture of native cystatin forms 1 and 2 was shown by ion‐exchange chromatography to cause the disappearance of isoelectric form 2 with a concomitant increase in form 1. Thus, the existence of two isoelectric forms of chicken cystatin is due to the phosphorylated form 2 and non‐phosphorylated form 1.