Cloning and characterization of the murine antithrombin gene

A novel serpin with antithrombin-like activity in Branchiostoma japonicum: implications for the presence of a primitive coagulation system

Serine protease inhibitors, or serpins, are a group of widely distributed proteins with similar structures that use conformational change to inhibit proteases. Antithrombin (AT) is a member of the serine protease inhibitor superfamily and a major coagulation inhibitor in all vertebrates, but its evolutionary origin remains elusive. In this study we isolated for the first time a cDNA encoding an antithrombin homolog, BjATl, from the protochordate Branchiostoma japonicum. The deduced protein BjATl consisted of 338 amino acids sharing 36.7% to 41.1% identity to known vertebrate ATs. BjATl contains a potential N-linked glycosylation site, two potential heparin binding sites and the reactive center loop with the absolutely conserved sequence Gly-Arg-Ser; all of these are features characteristic of ATs. All three phylogenetic trees constructed using Neighbor-Joining, Maximum-Likelihood and Bayesian-Inference methods also placed BjATl together with ATs. Moreover, BjATl expressed in yeast cells was able to inhibit bovine thrombin activity by forming a SDS-stable BjATl-thrombin complex. It also displays a concentration-dependent inhibition of thrombin that is accelerated by heparin. Furthermore, BjATl was predominantly expressed in the hepatic caecum and hind-gut, agreeing with the expression pattern of AT in mammalian species. All these data clearly demonstrate that BjATl is an ortholog of vertebrate ATs, suggesting that a primitive coagulation system emerged in the protochordate.

Presence and localization of antithrombin and its regulation after acute lipopolysaccharide exposure in amphioxus, with implications for the origin of vertebrate liver

Antithrombin (AT), which is mainly synthesized in the liver, is an acute-phase plasma protein in mammalian species. Here, we demonstrated that sheep anti-human AT antibody cross-reacted with the humoral fluids in amphioxus Branchiostoma belcheri tsingtauense as well as human serum. The concentration of AT in the humoral fluids in amphioxus decreased slightly at first and then increased after the acute challenge with lipopolysaccharide, while the level of total proteins remained unchanged. These suggest the presence of the same acute-phase response pattern in amphioxus, as observed in some mammalian species. Immunohistochemically, AT was localized in the hepatic diverticulum. It is clear that the hepatic diverticulum in amphioxus is homologous to the vertebrate liver with respect to AT synthesis. This lends support to the hypothesis originally suggested by Müller that the vertebrate liver evolved from the hepatic diverticulum of an amphioxus-like ancestor during early chordate evolution.