The intestinal contortin structure in Haemonchus contortus: An immobilised anticoagulant?

Contortin was the first intestinal antigen of the sheep parasite Haemonchus contortus which induced significant levels of protection when used to vaccinate lambs. This antigen is present in the intestine of L4 and adult worms as a helical polymeric structure attached to the luminal surface of the intestinal cells. However, the nature of the protein itself and its function have never been reported. In the present study, contortin was isolated and analysed by peptide mass fingerprint and LC/MS–MS. These analyses indicated that contortin comprises two major proteins, Hc-PCP1 and Hc-PCP2, with homology to prolyl-carboxypeptidases. The two proteins show 64% amino acid sequence identity to each other and both are comprised of two prolyl-carboxypeptidase S28 type domains organised in a tandem repeat. The transcripts of both genes are present from the L4 stage onwards, coinciding with the onset of blood-feeding. Addition of contortin to a fibrinogen solution significantly inhibited blood coagulation in a dose-dependent manner. Mass-spectrometry indicated that the contortin-enriched fraction degraded the C-terminal end of the fibrinogen alpha-chain, which was shown previously to be essential for clot formation. The process happens within seconds after addition and can be inhibited by the dipeptidyl-peptidase IV inhibitors Diprotin A and Bt-PEG-Glu-ProP(OPh)2. These data suggest that the prolyl-carboxypeptidases are intestinal anticoagulants used by H. contortus to interfere with blood coagulation.