Hemalin from Haemaphysalis flava ticks: cloning, expression and antithrombogenicity.

Hemalin, initially described in Haemaphysalis longicornis, is a protein with anticoagulant activity. We retrieved a gene fragment functionally annotated as hemalin from H. flava salivary gland transcriptomic library, but its full-length complementary DNA (cDNA) and antithrombogenicity have not been investigated in the species. Here we cloned the full length of hemalin (Hf-hemalin) by 3'-end rapid-amplification of cDNA ends, and the open reading frame (ORF) of Hf-hemalin was expressed in Escherichia coli. The recombinant protein (rHf-Hemalin) was tested for antithrombogenicity. The full-length of Hf-hemalin was 607 bp with an ORF of423 bp. Protein encoded by Hf-hemalin was predicted to contain 2 Kunitz domains and a signal peptide. The expression of Hf-hemalin in salivary glands, midguts and ovaries was higher in the semi-engorged than the fully engorged. Prokaryotic expression yielded a product of 40 kDa containing a glutathione S-transferase (GST) tag. Incubation of rHf-Hemalin with rat plasma significantly extended prothrombin time and activated partial thromboplastin time compared with normal saline and GST controls. Our data demonstrated that Hemalin from H. flava shared a similar primary structure with that from H. longicornis, and was also anticoagulant. Further investigations are needed to test its feasibility to be an antigen candidate for the development of vaccines against ticks.