Molecular cloning of glycoprotein Ib-binding protein, flavocetin-A, which inhibits platelet aggregation.

Flavocetin-A is a strong platelet aggregation inhibitor isolated from the venom of Trimeresurus flavoviridis. It binds specifically to platelet glycoprotein Ib with high affinity and inhibits von Willebrand factor-dependent platelet aggregation. The apparent molecular weight of flavocetin-A is 149 kDa. It consists of two subunits, alpha (17 kDa) and beta (14 kDa). The amino acid sequences of the alpha and beta subunits were determined from cloned cDNAs. Deduced amino acid sequences showed signal peptide-sequences of 23 amino acids for both alpha and beta subunits, mature peptide sequences of 135 amino acids for the alpha subunit, and 125 amino acids for the beta subunit. The amino acid sequences of alpha and beta subunits show high degrees of homology to those of C-type lectin-like venom proteins such as habu coagulation factors IX/X-binding protein (IX/X-bp), botrocetin, and alboaggregin-B. The cysteinyl residues of flavocetin-A, IX/X-bp, and botrocetin are conserved, except that flavocetin-A contains Cys 135 in the alpha subunit and Cys 3 in the beta subunit. We assumed that the arrangements of disulfide bridges in flavocetin-A are similar to those of IX/X-bp and botrocetin, and the additional Cys 135 of the alpha subunit and Cys 3 of the beta subunit are involved in novel disulfide bridges. These findings suggested that the additional disulfide bridges formed with Cys 135 of the alpha subunit and Cys 3 of the beta subunit cause polymerization of C-type lectin-like heterodimers.