Involvement of the NH2-terminal region of oryzacystatin-I in cysteine proteinase inhibition

Cystatins are small protein inhibitors of cysteine protein-ases.The relative importance of the N-terminal region of cystatins,and of a conserved glycine within this region, remains uncleardespite several studies. It was found that deletion of the N-terminal21 amino acids abolishes the inhibitory capacity of oryzacystatin-I.The importance of a conserved glycine residue (GlylO) was alsoexamined by replacing it with 11 other amino acids. Three furtherglycine residues (Gly5, -6 and -11) in this N-terminal regionof oryzacystatin-I were similarly mutated. Only those variantsin which GlylO was substituted show any significant change ininhibitory capacity compared with wild-type oryzacystatin-I.The inhibitory characteristics of hybrid cystatin moleculescomprising regions of chicken egg white cystatin and oryzacystatinwere also examined. It is suggested that in common with animalcystatins, the N-terminal region of the plant cystatin, oryzacystatin-I,and in particular the highly conserved GlylO residue are importantfor effective inhibition of papain