Primary Structure and Functional Characterization of Bilitoxin-1, a Novel Dimeric P-II Snake Venom Metalloproteinase from Agkistrodon bilineatus Venom

Abstract

The amino acid sequence of the hemorrhagic toxin, bilitoxin-1, isolated from the venom of Agkistrodon bilineatus was determined by the Edman sequencing procedure of peptides derived from digests utilizing cyanogen bromide, clostripain, lysyl endopeptidase, and Staphylococcus aureus V8 protease. A molecular mass of 80,000 Da was observed in the nonreduced state and 48,000 Da was observed in the reduced state, as demonstrated by sodium dodecyl sulfate–polyacrylamide gel electrophoresis. Each subunit consists of 291 amino acid residues and has a calculated molecular mass of 32,276 Da. The toxin contains fucose, galactosamine, glucosamine, galactose, mannose, and N-acetylneuraminic acid and three N-linked glycosylation consensus sites. Hydrazinolysis and ESI mass spectrometry revealed that asparagine was the carboxyl-terminal amino acid. The disintegrin-like domain of bilitoxin-1 lacks the RGD cell-binding sequence, which is substituted by the MGD sequence. Under certain conditions, the disintegrin domain is autoproteolytically processed from the native protein. Studies with the bilitoxin disintegrin demonstrated that it lacks platelet aggregation inhibitory activity, probably reflecting the substitution of RGD by MGD. The hemorrhagic activity of the asialobilitoxin-1 was only 25% of bilitoxin-1, while proteolytic activity was unaffected. The three-dimensional structure of this toxin was modeled and was shown to likely possess a structure similar to that of adamalysin II (Gomis-Rüth et al., EMBO J. 12, 151–157 (1993)) and the disintegrin kistrin (Adler et al., Biochemistry 32, 282–289 (1993)). In summary, here we report the first primary structure of a dimeric, P-II snake venom metalloproteinase and the biological role of bilitoxin-1 glycosylation and the disintegrin domain.