The crystal structure of the Shethna Protein II (FeSII) from Azotobacter vinelandii suggests a domain swap

The Azotobacter vinelandii FeSII protein forms an oxygen resistant complex with the nitrogenase MoFe and Fe proteins. FeSII is an adrenodoxin type ferredoxin, forming a dimer in solution. Previously, the crystal structure was solved (Schlesier et al., 2016), with 5 copies in the asymmetric unit. One copy is a normal adrenodoxin domain, forming a dimer with its crystallographic symmetry mate. The other four copies are in an "open" conformation with a loop flipped out exposing the 2Fe-2S cluster. The open and closed conformations were interpreted as oxidised and reduced, and the large conformational change in the open configuration allowed binding to nitrogenase. We have independently solved the structure of FeSII in the same crystal form. Our positioning of the atoms in the unit cell is similar to the earlier report. However, our interpretation of the structure is different. We interpret the "open" conformation as the product of a crystallization-induced domain swap. The 2Fe-2S cluster is not exposed to solvent, but in the crystal its interacting helix is replaced by the same helix residues from a crystal symmetry mate. The domain swap is complicated, as it is unusual in being in the middle of the protein rather than at a terminus, and it creates arrangements of molecules that can be interpreted in multiple ways. We caution that crystal structures should be interpreted in terms of the contents of the entire crystal rather than one asymmetric unit.