Members of the cystatin family are important inhibitors of cathepsin-type cysteine proteases and are involved in a number of pathologies. Parasite cystatins are attractive target molecules for parasite control, but our knowledge about them is still limited. This work is focused on cystatins of two blood-feeding parasites: the common tick (Ixodes ricinus) as the main vector of Lyme disease and tick-borne encephalitis, and the liver fluke (Fasciola hepatica), the causative agent of fasciolosis. Four novel cystatins were functionally and structurally characterized to determine the structural determinants of their inhibitory specificity and describe them in the context of evolution and physiological role of cystatins.
The cystatin FhCyLS-2 from F. hepatica has broad inhibitory specificity and is suggested to play a dual role in the regulation of proteolytic systems in host tissue and the parasite gut.
FhCyLS-2 combines the characteristics of two cystatin subfamilies in a unique way and is a model representative of a novel evolutionary group of cystatins identified in several orders of parasitic flukes.
Ricistatin and iristatin are salivary cystatins of I. ricinus with immunomodulatory effects on the host caused by an exceptionally narrow inhibitory specificity. It was explained by structural modifications of the binding segments of the inhibitors. The modifications have a different pattern for ricistatin and iristatin, as was demonstrated by the crystal structure of the cystatin-protease complex and are the principal feature distinguishing two phylogenetic groups of tick salivary cystatins. Mialostatin from the tick gut contains unmodified binding segments, providing a broad inhibitory specificity that enables effective regulation of the multienzyme proteolytic digestive system of the ticks.
The thesis provides new information on structure-function relationships in parasite cystatins and their molecular evolution towards various physiological roles. The studied cystatins represent promising antigens for the development of antiparasitic vaccines and have pharmacologically interesting properties for potential biomedical applications.
