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

Cystatins are small protein inhibitors of cysteine proteinases. The relative importance of the N-terminal region of cystatins, and of a conserved glycine within this region, remains unclear despite several studies. It was found that deletion of the N-terminal 21 amino acids abolishes the inhibitory capacity of oryzacystatin-I. The importance of a conserved glycine residue (Gly10) was also examined by replacing it with 11 other amino acids. Three further glycine residues (Gly5, -6 and -11) in this N-terminal region of oryzacystatin-I were similarly mutated. Only those variants in which Gly10 was substituted show any significant change in inhibitory capacity compared with wild-type oryzacystatin-I. The inhibitory characteristics of hybrid cystatin molecules comprising regions of chicken egg white cystatin and oryzacystatin were also examined. It is suggested that in common with animal cystatins, the N-terminal region of the plant cystatin, oryzacystatin-I, and in particular the highly conserved Gly10 residue are important for effective inhibition of papain.