Isolation and characterization of two novel serpins from the tick Rhipicephalus haemaphysaloides

Two novel serpins with anti-chymotrypsin activity, RHS-1 and RHS-2, were identified in the tick Rhipicephalus haemaphysaloides. The complementary cDNA sequence of RHS-1 was 1286 base pairs (bp) and encoded a deduced 403-amino acid protein with a signal peptide, whereas that of RHS-2 was 1682bp and encoded a deduced 380-amino acid protein with no signal peptide. Although both RHS-1 and RHS-2 exhibited high sequence similarities to known serpins from other ticks, the level of similarity at the amino acid level between the 2 serpins characterized here was only 32.5%. Salivary gland-specific expression of RHS-1 and midgut-specific expression of RHS-2 were found by Western blot using the relevant antiserum. We tested the ability of purified recombinant rRHS-1 and rRHS-2 to inhibit various serine proteases and found that both significantly inhibited chymotrypsin (95.6% and 94.2%, respectively). We further demonstrated that RHS-1, but not RHS-2 exhibited anticoagulation activity, based on activated partial thromboplastin time (APTT). Disruption of the genes encoding the 2 serpins with RNA interference (RNAi) led to a significant decrease in tick attachment and engorgement rates. These results indicate that RHS-1 and RHS-2 are 2 novel serpins with anti-chymotrypsin activity that are involved in blood feeding of R. haemaphysaloides.