Factor X-activating glycoprotein of Russell's viper venom. Polypeptide composition and characterization of the carbohydrate moieties.

There is contradictory information regarding the molecular weight and polypeptide chain composition of RVV-X, a glycoprotein in Russell's viper venom that is capable of activating factor X to Xa. We show that RVV-X is a 92,880-Da glycoprotein. It consists of three disulfide-linked polypeptide chains, one heavy chain (alpha-chain, M(r) 57,600) and two light chains (beta- and gamma-chains, M(r) 19,400 and 16,400, respectively). The two light chains seen on SDS-polyacrylamide gel electrophoresis under reducing conditions are two distinct components of the molecule, rather than a heterogeneous mixture of a single chain as previously reported (Takeya, H., Nishida, S., Miyata, T., Kawada, S., Saisaka, Y., Morita, T., and Iwanaga, S. (1992) J. Biol. Chem. 267, 14109-14117). The following evidence supports this conclusion. (i) The two light chains of RVV-X are present in equal proportion. (ii) The estimated molecular weight of an alpha 1 beta 1 gamma 1-structure closely matches the molecular weight determined by matrix-assisted laser desorption mass spectrometry. (iii) The amino acid compositions and NH2-terminal sequences of the beta- and gamma-chains are different. (iv) Although both the beta- and gamma-chains contain one N-linked oligosaccharide chain each, they are glycosylated differentially. RVV-X contains six N-linked oligosaccharides, four in the alpha-chain and one in each of the beta- and gamma-chains. The carbohydrate structures are different from those known for other snake venom glycoproteins, and they resemble closely those in various mammalian glycoproteins. The majority of the oligosaccharides are complex bi-, tri-, and tetraantennary structures, with a small proportion of multiantennary and high mannose-type structures. Two notable structural features of RVV-X oligosaccharides are as follows. (i) Sialic acid residues are linked to beta-galactosyl residues solely by alpha 2,3-linkages, and (ii) bisecting N-acetylglucosamine residues are present in the majority of the oligosaccharides.