A novel cysteine protease inhibitor of the egg of chum salmon, containing a cysteine-rich thyroglobulin-like motif.

ECI, one of three isoforms of cysteine protease inhibitor found in chum salmon eggs was purified to homogeneity, and its complete amino acid sequence was determined. The primary structure of ECI did not resemble those of other cysteine protease inhibitors of the cystatin superfamily but did resemble that of a cysteine-rich motif found as a repetitive structural element in thyroglobulin and several other proteins. The function of the cysteine-rich motif is not yet hypothesis. Two cysteine motif proteins, thyroglobulin and entactin, were tested for papain inhibitory activity and found to have none.