CDNA CLONING AND DEDUCED AMINO-ACID-SEQUENCE OF PROTHROMBIN ACTIVATOR(ECARIN) FROM KENYAN ECHIS-CARINATUS VENOM

The complete amino acid sequence of ecarin is deduced from the nucleot ide sequence of a cDNA clone isolated by screening a venomous gland cD NA library of Kenyan Echis carinatus. The cDNA sequence with 2379 base pairs encodes an open reading frame of 616 amino acids with a remarka ble sequence homology to the putative precursor protein of trigramin f rom Trimeresurus gramineus venom (61% identity) and a large hemorrhagi n, jararhagin, from the pit viper Bothrops jararaca venom (62% identit y). Thus, ecarin, as well as jararhagin and trigramin, is translated a s a precursor protein, which may be processed posttranslationally. The ecarin proprotein has a ''cysteine switch'' motif (-Pro-Lys-Met-Cys-G ly-Val-) similar to that involved in the activation of matrix metallop roteinase zymogens. The processed mature protein consists of 426 amino acid residues (residues 191-616), showing the strongest sequence simi larity with that of Russell's viper venom factor X activator (RVV-X) h eavy chain (64% identity). Like RVV-X heavy chain, ecarin contains met alloproteinase, disintegrin, and cysteine-rich domains. The metallopro teinase domain has a typical zinc-chelating sequence (-His-Glu-Xaa-Xaa -His-Xaa-Xaa-Gly-Xaa-Xaa-His-), as found in crayfish astacin. In the d isintegrin domain of ecarin, the Arg-Gly-Asp sequence is replaced by A rg-Asp-Asp, as found in the disintegrin domains of RVV-X heavy chain ( Arg-Asp-Glu) and a guinea pig sperm fusion protein, PH-30 beta (Thr-As p-Glu). These findings show that while there are structural and evolut ionary relationships among these proteins, each has a unique functiona l activity.