Cysteine proteases are essential to the biological function of parasitic organisms and are gaining interest as the target of chemotherapeutics aimed at their control. We report a cysteine protease gene from the myxozoan Myxobolus cerebralis, the cause of whirling disease in salmonid fish. The mature gene (MyxCP-1) encodes a 248-amino acid polypeptide that includes catalytic residues similar to the papain family of cysteine proteases. MyxCP-1 features a propeptide region and sequence insertions that are characteristics of cathepsin Z proteases. Phylogenetic comparisons of M. cerebralis to other eukaryotes based on full-length cathepsin-like genes show that MyxCP-1 is the earliest lineage in the cathepsin Z group and separated from cathepsin L, B, and C-like proteases. Using TaqMan PCR differential levels of transcription of the cathepsin Z-like protease were found in earlier and later developmental stages of the parasite in experimentally infected rainbow trout (Oncorhynchus mykiss).
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