Dynamic and structural insights into tick serpin from Ixodes ricinus

Abstract

Ixodid ticks have a crucial impact on people and domestic animals worldwide. These parasites also pose a serious threat to livestock. To date, vaccination of hosts against ticks is a safer, more sustainable alternative to chemical control of ticks and the disease agents they transmit. Because of their roles in tick physiology, serpins (serine protease inhibitors) from tick saliva are among the candidates for anti-tick vaccines. Inhibitory serpins employ a suicide inhibition mechanism to inhibit proteases, where the serpin reactive centre loop (RCL) is cleaved, by the targeted protease, and then inserted into the main β-sheet of the serpin. This causes a massive conformational change called the ‘stressed to relaxed’ (S→R) transition, leading to the breakdown of serpin into two regions (core domain and cleaved polypeptide). Recently, the first tick serpin crystal structure from Ixodes ricinus in R-state was reported. We thus employed molecular dynamics simulations to better understand serpin structure and dynamics in atomic detail. Overall, R-state serpin showed high rigidity, especially the core domain. The most flexible region is the terminal of the cleaved polypeptide, due to its high-water exposure, while the rest of the cleaved polypeptide is stably trapped behind the core domain. T363, D367 and N375 are found to play a vital role in protein–protein attachment. This finding can be used to explain the high stability of the R-state serpin at the atomic level and provides insight into this tick serpin which will be useful for rational anti-tick vaccine development.

Abbreviations
MD	=	Molecular Dynamics
RCL	=	Reactive centre loop

Communicated by Ramaswamy H. Sarma

Keywords:

• Ticks
• serpins
• molecular dynamics simulations
• Ixodes ricinus

Acknowledgements

We thank for the computer facilities from the Department of Computer Engineering, Kasetsart University, Thailand.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

PP would like to thank for financial supports from Kasetsart University Research and Development Institute (KURDI). This publication was supported by Thailand Innovation 4.0 (KU-VAC as an anti-tick vaccine developed for commercial use).