Cyanovirin-N (CV-N) is a potent 11 kDa HIV-inactivating protein that binds with high affinity to the HIV surface envelope protein gp120. A double mutant P51S/S52P of CV-N was engineered by swapping two critical hinge-region residues Pro51 and Ser52. This mutant has biochemical and biophysical characteristics equivalent to the wild-type CV-N and its structure resembles that of wild-type CV-N. However, the mutant shows a different orientation in the hinge region that connects two domains of the protein. The observation that this double mutant crystallizes under a wide variety of conditions challenges some of the current hypotheses on domain swapping and on the role of hinge-region proline residues in domain orientation. The current structure contributes to the understanding of domain swapping in cyanovirins, permitting rational design of domain-swapped CV-N mutants. (C) 2002 Published by Elsevier Science (USA).