Domain swapping between protein monomers in crystals. (a and b) Original demonstration of domain swapping of diphtheria toxin in a protein crystal (26). The receptor-binding domain of a monomer (shown in yellow in a) can become rotated and translated to form contacts with the other two domains of the toxin, as in b, where the blue receptor-binding domain of the subunit at the left has become associated with the other two domains of the green subunit at the right, and vice versa. This demonstrates the substitution of interchain contacts for intrachain ones as proposed by Goldberg (see Figure 2). R, receptor binding domain. Reproduced with permission from Proceedings of the National Academy of Sciences of the United States of America (26). (c) Models for mechanics of higher-order domain swapping. Reproduced with permission from Protein Science (27).