Cathepsin O
Cathepsin O (CTSO) is a papain-family lysosomal cysteine protease that remains among the least characterized members of the cathepsin group. The gene encodes a typical preproenzyme with a signal peptide, propeptide, and a catalytic cysteine-histidine-asparagine triad, consistent with other papain-like proteases in this family. Transcript analyses have detected CTSO expression in several normal tissues and in some ovarian and breast carcinoma specimens, and altered levels have been noted in tumor expression panels. Functional characterization is still limited, but the predicted acidic activity optimum and lysosomal targeting indicate that the enzyme participates in intracellular protein degradation within endolysosomal compartments, without an established unique physiological substrate to date.
