Blood coagulation factors IX/X-binding protein (IX/X-bp) is a heterodimer consisting of two homologous subunits. The amino acid sequence of each subunit is homologous to the carbohydrate-recognition domain of C-type lectin (C-type CRD), although IX/X-bp has no lectin activity. Crystal structure of IX/X-bp shows that a central loop projects into the adjoining subunit and contributes to an intertwinned dimer in a manner similar to 3D domain swapping. The exchange of the loop complete the C-type CRD fold, and forces a large conformational change on the hinge region classically concerned in Ca
2+ and carbohydrate-binding. This may result in the disruption of the lectin active site. On the other hand, a concave surface created by the dimerization provides a potential binding site for factors IX and X. The domain swapping seen here provides a new function which is not expected from the original C-type CRD.
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