The cysteine proteinase inhibitor chicken cystatin is a phosphoprotein.

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.