The cystatins are tightly binding, but reversible, inhibitors of cysteine proteinases, which constitute a superfamily of evolutionary related proteins. They have been subdivided into three families: the cystatin family which contain two disulfide bonds, the stefin family which lack disulfide bonds, and the kininogen family composed of large glycoproteins containing three repeats similar to those found in the cystatin family. Members of the cystatin superfamily occurring in plants are currently known as phytocystatins, defined as proteins lacking disulfide bonds but possessing a conserved N-terminal amino acid sequence (L-A-R-[FY]-A-[VI]-X(3)-N). We have used the protein sequence deduced from seven phytocystatins (from the Arabidopsis thaliana genome project) and from the SUgarCane EST project (SUCEST) database to identify 25 possible sugarcane phytocystatins. Phylogenetic analysis has allowed us to cluster these phytocystatins into four distinct groups: (i) those with a characteristic N-terminal consensus, (ii) those with the same consensus plus a long C-terminal extension; (iii) those that lack the consensus but contain the highly conserved QxVxG motif found in all members of the superfamily and (iv) those that lack both the consensus and the QxVxG motif.
