The structure and biochemical properties of protease inhibitors from the thyropin family are poorly understood in parasites and pathogens. Here, we introduce a novel family member, Ir-thyropin (IrThy), which is secreted in the saliva of Ixodes ricinus ticks, vectors of Lyme borreliosis and tick-borne encephalitis. The IrThy molecule consists of two consecutive thyroglobulin type-1 (Tg1) domains with an unusual disulfide pattern. Recombinant IrThy was found to inhibit human host-derived cathepsin proteases with a high specificity for cathepsins V, K, and L among a wide range of screened cathepsins exhibiting diverse endo- and exopeptidase activities. Both Tg1 domains displayed inhibitory activities, but with distinct specificity profiles. We determined the spatial structure of one of the Tg1 domains by solution NMR spectroscopy and described its reactive center to elucidate the unique inhibitory specificity. Furthermore, we found that the inhibitory potency of IrThy was modulated in a complex manner by various glycosaminoglycans from host tissues. IrThy was additionally regulated by pH and proteolytic degradation. This study provides a comprehensive structure-function characterization of IrThy-the first investigated thyropin of parasite origin-and suggests its potential role in host-parasite interactions at the tick bite site.

1st Faculty of Medicine Charles University Katerinska 32 12108 Praha Czech Republic

Biopticka Laborator Mikulasske Namesti 4 32600 Plzen Czech Republic

Department of Biochemistry Faculty of Science Charles University Hlavova 8 12800 Praha Czech Republic

Institute of Microbiology Czech Academy of Sciences Prumyslova 595 25250 Vestec Czech Republic

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences Flemingovo n 2 16610 Praha Czech Republic

Institute of Parasitology Biology Centre Czech Academy of Sciences Branisovska 31 37005 Ceske Budejovice Czech Republic

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