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 Ca2+ 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.
The mitochondrial ADP/ATP carrier, which mediates transport of ADP and ATP via the inner membrane, functions as a dimer, each subunit being located in the same orientation. The carrier has three large loops (M1, M2, M3) facing the matrix side. M1 loop is suggested to function as an outer gate, M2 as a substrate binding site and M3 as an auxiliary gate by interacting with other loops. The transport function of the carrier is characterized largely by the cooperative swing of these paired loops, in which location of loops changes repeatedly between outside and inside of the membrane.