Processing of pro-thrombostasin by a recombinant subtilisin-like proprotein convertase derived from the salivary glands of horn flies ( Haematobia irritans)

Thrombostasin (TS) is a thrombin inhibitor found in the salivary glands of horn flies ( Haematobia irritans). It is produced as an inactive form with a 76-amino acid propeptide in the N-terminus preceding the mature TS. A minimal recognition sequence by subtilisin-like proprotein convertases, Arg-Xaa-Xaa-Arg, is localized C-terminal to the propeptide. This study demonstrated that a gene cloned from the salivary glands of the horn fly encodes a new convertase, subsequently named horn fly proprotein convertase (HFPC), and that the recombinant HFPC expressed in insect HighFive cell culture specifically cleaves recombinant pro-thrombostasin, produced in E. coli, at the expected site. The relative cleavage efficiency of rHFPC was compared with that of recombinant human furin, a commercially available proprotein convertase. The result indicated that this newly identified proprotein convertase is of importance for the proteolytic maturation of thrombostasin, a protein secreted in horn fly saliva and used by the insect to counteract its host's haemostatic response.