Bothrojaracin is a potent and selective thrombin inhibitor that has been isolated from the venom of Botlzrops jaruruca. It does not interact with the catalytic site of the enzyme but binds to both anionbinding exosites 1 and 2 resulting in a potent inhibition of thrombin activity towards fibrinogen and platelets [Zingali, R. B., Jandrot-Perrus, M., Guillin, M. C. & Bon, C. (1993) Biochemistry 32, 10794-I0 8021. Bothrojaracin is a 27-kDa protein composed of two disulfide-linked polypeptide chains, A and B, of 15 kDa and 13 kDa, respectively. The sequences of A and B chains determined by molecular cloning exhibit a high degree of identity with other snake venom lectin-like proteins. In contrast to other ligands that interact with thrombin exosite 1, the amino acid sequence of bothrojaracin does not contain an acidic sequence similar to the C-terminal tail of hirudin. Expression of functional bothrojaracin was achieved in COS cells upon transfection with two pcDNA3 vectors containing the complete cDNAs. Recombinant bothrojaracin, which was secreted into the medium, was able to bind to and inhibit thrombin. When expressed alone, the B chain formed inactive dimers that were secreted into the culture medium. In contrast, no bothrojaracin-related protein was detected in conditioned media from cells transfected with the A chain.Keywords: thrombin inhibitor; snake-venom protein; C-type lectin; molecular cloning and sequencing; expression of recombinant heterodimeric protein.Snake venoms are a rich source of enzymes and inhibitors involved in blood coagulation or platelet activation [I]. Several of these venom proteins, with molecular masses of approximately 30 kDa, consist of two disulfide-linked similar subunits containing a C-type lectin motif 12, 31 and exhibit a high degree of similarity : factor IX/factor X-binding protein (IX/X-bp) interacts with coagulation factors TX and X in a calcium-dependent manner [4-61; botrocetin binds von Willebrand factor, inducing its binding to glycoprotein Ib [7, 81; and alboaggregin-B [9, 101, echicetin [11, 121, agkicetin 1131 andjararaca glycoprotein-Ib-binding protein [ 141 bind platelet glycoprotein Ib. However, most of these proteins do not bind to carbohydrates, and thus cannot be considered as true C-type lectins. We have identified and purified previously bothrojaracin, a protein from the venom of Bothvops ,juraruca, which belongs to the same protein family, as shown by the N-terminal sequences of its two chains 1251. Bothrojaracin is a 27-kDa protein composed of two disulfide-linked polypeptide chains of 13 kDa and 15 kDa and presents several isoforms. Bothrojaracin binds both anion-binding exosites 1 and 2 of thrombin, resulting in potent inhibition of thrombin activity on fibrinogen and platelets, but it does not mask the thrombin catalytic site [lS, 161, in In the present report, we describe the molecular cloning and the expression of bothrojaracin in COS cells. The entire protein sequence of the two chains of bothrojaracin, deduced from the cDNA cloner, confirmed their...