Crystal packing of the c₆-type cytochrome OmcF from Geobacter sulfurreducens is mediated by an N-terminal Strep-tag II

The putative outer membrane c-type cytochrome OmcF from Geobacter sulfurreducens contains a single haem group and shows homology to soluble cytochromes c₆, a class of electron-transfer proteins that are typically found in cyanobacterial photosynthetic electron-transfer chains. OmcF was overexpressed heterologously in Escherichia coli as an N-­terminal Strep-tag II fusion protein and isolated using streptactin-affinity chromatography followed by size-exclusion chromatography. The structure was solved by Fe SAD using data collected to a resolution of 1.86 Å on a rotating copper-anode X-ray generator. In the crystal, packing interactions in one dimension were exclusively mediated through the Strep-tag II sequence. The tag and linker regions were in contact with three further monomers of OmcF, leading to a well defined electron-density map for this engineered and secondary-structure-free region of the molecule.