AGiardia duodenalisgene encoding a protein with multiple repeats of a toxin homologue

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Abstract
SUMMARY: A gene,CRP136,from the intestinal protozoan parasiteGiardia duodenalis,expressed at a high level in a metronidazole-resistant line, encodes a 136 kDa protein with 23 copies of a 40 amino acid repeat. The protein is cysteine-rich and has the typical membrane-spanning region and CXXC amino acid motifs of a family ofGiardiacysteine-rich surface proteins (CRSPs). The repeat unit inCRP136, shares 57% homology with the gene encoding the precursor of the sarafotoxins, a group of snake toxins from the burrowing adder known to cause symptoms similar to those of humans acutely infected withGiardia.The sarafotoxins are low molecular weight sulphydryl cross-linked peptides which are proteolytically cleaved from a precursor polyprotein.CRP136has homology over the entire length of the sarafotoxin precursor, and the repeats are of the same length. ThusCRP136represents the first evidence for a potentialGiardiatoxin. The genomic copy number ofCRP136appears to be the same in both the parent and drug-resistant lines and expression of this gene, and at least one other, is associated with a conserved partial duplication, but not amplification, of one chromosome.