Crystal structure of cyanovirin-N, a potent HIV-inactivating protein, shows unexpected domain swapping

The crystal structure of cyanovirin-N (CV-N), a protein with potent antivir al activity, was solved at 1.5 Angstrom resolution by molecular replacement using as the search model the solution structure previously determined by NMR. The crystals belong to the space group P3(2)21 with one monomer of CV- N in each asymmetric unit. The primary structure of CV-N contains 101 resid ues organized in two domains, A (residues 1 to 50) and B (residues 51 to 10 1), with a high degree of internal sequence and structural similarity. We f ound that under the conditions of the crystallographic experiments (low pH and 26 % isopropanol), two symmetrically related monomers form a dimer by d omain swapping, such that domain A of one monomer interacts with domain B' of its crystallographic symmetry mate and vice versa. Because the two swapp ed domains are distant from each other, domain swapping does not result in additional intramolecular interactions. Even though one of the protein samp le solutions that was used for crystallization clearly contained 100% monom eric CV-N molecules, as judged by various methods, we were only able to obt ain crystals containing domain-swapped dimers. With the exception of the un expected phenomenon of domain swapping, the crystal structure of CV-N is ve ry similar to the NMR structure, with a root-mean-square deviation of 0.55 Angstrom for the main-chain atoms, the best agreement reported to date for structures solved using both techniques.