Crystal Structure of Flavocetin-A, a Platelet Glycoprotein Ib-Binding Protein, Reveals a Novel Cyclic Tetramer of C-Type Lectin-like Heterodimers^†,‡

Snake venom contains a number of the hemostatically active C-type lectin-like proteins, which affect the interaction between von Willebrand factor (vWF) and the platelet glycoprotein (GP) Ib or platelet receptor to inhibit/induce platelet activation. Flavocetin-A (FL-A) is a high-molecular mass C-type lectin-like protein (149 kDa) isolated from the habu snake venom. FL-A binds with high affinity to the platelet GP Ibα-subunit and functions as a strong inhibitor of vWF-dependent platelet aggregation. We have determined the X-ray crystal structure of FL-A and refined to 2.5 Å resolution. This is a first elucidation of a three-dimensional structure of the platelet GP Ib-binding protein. The overall structure reveals that the molecule is a novel cyclic tetramer (αβ)₄ made up of four αβ-heterodimers related by a crystallographic 4-fold symmetry. The tetramerization is mediated by an interchain disulfide bridge between cysteine residues at the C-terminus of the α-subunit and at the N-terminus of the β-subunit in the neighboring αβ-heterodimer. The high affinity of FL-A for the platelet GP Ib α-subunit could be explained by a cooperative-binding action through the multiple binding sites of the tetramer.