Evolutionary conserved cathepsin E substrate specificity as defined by N-terminal and C-terminal sequencing of peptide pools

The substrate specificity of the non-lysosomal aspartic protease cathepsin E from three different species has been studied using the method of automated N-terminal sequencing and a newly developed method for C-terminal sequencing of peptides and peptide pools, The combination of N-terminal and C-terminal sequencing of peptide pools is a fast and easy method to identify and compare the substrate specificity of endopeptidases. Our analysis shows a conserved hydrolytic specificity between human, mouse and bovine cathepsin E, with only small differences in fine specificity, Furthermore, our results confirm and extend the rules governing the interactions of the substrate with the amino acid (aa) side chains of the various pockets within the enzyme's active cleft, We found that the positions flanking the scissile peptide bond P1-P1' are occupied exclusively by hydrophobic aa with both aliphatic or aromatic side chains; Val and lie, however, are not allowed in the S1 binding site, The S2 acid S2' subsites accept hydrophilic aa. Additional requirements concerning the S3' to S5' subsites were also revealed. Finally, the sequences of single peptides generated by cathepsin E from the three different species can be easily aligned to the determined cleavage motif, showing the reliability of our pool sequencing methods.