A novel cathepsin B active site motif is shared by helminth bloodfeeders

This study compared specific protein sequence motifs present within cathepsin B-like cysteine proteases from a number of helminth parasites. We have focused our efforts on cathepsin B-like proteases of Haemonchus contortus, Caenorhabditis elegans, Schistosoma mansoni, Schistosoma japonicum, Ostertagia ostertagi, and Ancylostoma caninum. The goal of this work is to correlate specific features, or proposed roles, of the cathepsin B-like proteases with primary sequence motifs discovered within the proteins. We report here a general motif for the identification of cathepsin B enzymes, and more significantly, a motif within this pattern that is found, with one exception, only in cathepsin B-like proteases of helminth bloodfeeders. We suggest that the "hemoglobinase" motif arose evolutionarily in a minimum of three independent events as a specialized response to increase the efficiency of hemoglobin degradation by these cathepsin B-like enzymes. This motif should be useful in identifying additional helminth hemoglobinases and may provide a specific target for drug design efforts.