Crystal structures of two protease inhibitors from tick saliva

P. Řezáčová1,2, J. Brynda1,2, Zuzana Kovářová1, Jiří Salát3, Jindřich Chmelař3, Michal Mareš1


1Institute of Organic Chemistry and Biochemistry, Academy of Sciences of the Czech Republic v.v.i., Flemingovo nam. 2, 166 37 Prague 6, Czech Republic

2 Institute of Molecular Genetics Academy of Sciences of the Czech Republic, v.v.i., Vídeňská 1083, 142 20 Prague 4, Czech Republic

3Institute of Parasitology, Biology Centre of the Academy of Sciences of the Czech Republic, Branišovská 31, 37005 České Budějovice, Czech Republic


The saliva of ticks is a rich source of various protease inhibitors which help the blood-feeding parasite to overcome the defense of host immune system. Two proteins (named IRS-2 and OmC2) belonging to distinct families of protease inhibitors  were identified in the saliva of two tick species and their roles in parasite-host interaction were investigated as well as their crystal structures.

Protein IRS-2 belongs to the serpin family of protease inhibitors. It is produced in the salivary glands of the hard tick Ixodes ricinus and exhibits an activity in suppressing the adaptive host immune response [1]. Recombinant protein was prepared by heterologous expression in bacterial system and the crystal structure was determined at resolution of 1.8 Å [2]. IRS-2 adopt a typical serpin fold composed of 3 large b-sheets and 9 a-helixes (Figure 1A). Interestingly, the structure represents a relaxed state of serpin, in which the loop responsible for interaction with target protease (so called reactive-centre loop, RCL) is cleaved and inserted into the central b-sheet.

Protein OmC-2 belonging to cystatin family of protease inhibitors was identified in the salive of the soft tick Ornithodorus moubata by proteomic approach. The inhibitory activity of OmC2 toward broad-range of human peptidases was confirmed as well as its immunomodulatory activity in vivo. OmC2 represents a promising target for the development of anti-tick vaccine. Crystal structure of recombinant OmC2 determined at resolution of 2.45 Å was used to explain the structure-inhibitory activity relationship [3]. OmC2 adopts a typical cystatin fold characterized by five-stranded twisted antiparallel b-sheet wrapping around a central helix (Figure 1B).



Figure 1. A. Crystal structure of serpin IRS-2 from hard tick Ixodes ricinus (PDB code 3NDA). B. Crystal structure of cystatin OmC2 from soft tick Ornithodorus moubata (PDB code 3L0R).



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