Sirtilins – the new old members of the vitamin K‐dependent coagulation factor family

ESSENTIALS: Blood coagulation is driven by vitamin K (VK)‐dependent proteases. We have identified and characterized ‘sirtilin’ as an additional VK‐dependent protease. Sirtilins emerged early in the evolution of the coagulation system of vertebrates. Ubiquitous occurrence might indicate an important functional role of sirtilins. SUMMARY: BACKGROUND: Vitamin K (VK)‐dependent proteases are major players in blood coagulation, including both the initiation and the regulation of the cascade. Five different members of this protease family have been described, comprising the following coagulation factors: factor VII, FIX, FX, protein C (PC), and prothrombin (FII). FVII, FIX, FX and PC share a typical domain architecture, with an N‐terminal γ‐carboxyglutamate (Gla) domain, two epidermal growth factor‐like (EGF) domains, and a C‐terminal trypsin‐like serine protease (SP) domain. OBJECTIVES: We have identified uncharacterized proteins in snake genomes showing the typical Gla–EGF1–EGF2–SP domain architecture but relatively low sequence conservation compared to known VK‐dependent proteases. On the basis of sequence analysis, we hypothesized that these proteins are functional members of the VK‐dependent protease family. METHODS/RESULTS: Using phylogenetic analyses, we confirmed the so‐called ‘sirtilins’ as an additional VK‐dependent protease class. These proteases were found in several vertebrates, including jawless fish, cartilaginous fish, bony fish, reptiles, birds, and marsupials, but not in other mammals. The recombinant zymogen form of Thamnophis sirtalis sirtilin was produced by in vitro renaturation, and was activated with human activated FXI. The activated form of sirtilin proteolytically cleaved peptide and protein substrates, including prothrombin. Mass spectrometry‐based substrate profiling of sirtilin revealed a narrower sequence specificity than those of FIX and FX. CONCLUSIONS: The ubiquitous occurrence of sirtilins in many vertebrate classes might indicate an important functional role. Understanding the detailed functions of sirtilins might contribute to a deeper understanding of the evolution and function of the vertebrate coagulation system.