Molecular cloning of a novel multidomain Kunitz-type proteinase inhibitor from the hookworm Ancylostoma caninum

Degenerate oligonucleotide primers derived from conserved serine protease inhibitors were used to amplify a 90-base pair (bp) amplicon from an Ancylostoma caninum adult-stage complementary deoxyribonucleic acid (cDNA) library by polymerase chain reaction (PCR). The amplicon was labeled and used as a probe to screen the library, and a 2,300-bp cDNA clone was identified. The 5' end of the molecule was obtained from adult cDNA by 5'-RACE. The complete sequence named A. caninum Kunitz-type protease inhibitor (Ac-kpi-1) was 2,371 bp and encoded a 759-amino acid open reading frame. The deduced amino acid sequence had a calculated molecular weight of 84,886 Da and contained an amino terminal signal peptide, suggesting that the protein is secreted. Analysis of the predicted protein sequence indicates 12 highly conserved Kunitz-type serine protease inhibitor domains connected by short, conserved spacers. On the basis of sequence analysis, the first 11 domains are predicted to be active serine protease inhibitors based on the P1 amino acid. Domains 5-8 have identical amino acid sequences, and the remaining domains are 38-88% identical. Domain 12 lacks several of the conserved cysteine residues and has an atypical amino acid in the P1 position, suggesting that it is nonfunctional. Reverse transcriptase-PCR indicated that the Ac-kpi-1 messenger ribonucleic acid is present in egg, L1, L3, and adult stages but is most abundant in the adult stage. Ac-KPI-1 is most similar in domain architecture to several extracellular matrix proteins involved in cellular remodeling during insect development. In addition, there are 44 nematode proteins containing one or more Kunitz domains in GenBank, including several with multiple domains.