Abstract
C-type cytochromes are important electron transporters in all domains of life. They possess a common fold hallmarked by three α-helices that surround a covalently attached heme. An intriguing feature of many c-type cytochromes is their capacity to form oligomers by exchanging at least one of their α-helices, which is often referred to as 3D domain swapping. Here, we have determined the crystal structure of NirC, a c-type cytochrome co-encoded with other proteins involved in nitrite reduction by the opportunistic pathogen Pseudomonas aeruginosa. Crystals diffracted anisotropically to a maximum resolution of 2.12 Å (spherical resolution 2.83 Å) and initial phases were obtained by Fe-SAD phasing, revealing the presence of eleven NirC chains in the asymmetric unit. Surprisingly, these protomers arrange into one monomer and two different types of 3D domain-swapped dimers, one showing pronounced asymmetry. While the simultaneous observation of monomers and dimers probably reflects the interplay between high protein concentration required for crystallization and the structural plasticity of c-type cytochromes, the identification of conserved structural motifs in the monomer together with a comparison to similar proteins may offer new leads to unravel the unknown function of NirC.
Synopsis The crystal structure of the c-type cytochrome NirC from Pseudomonas aeruginosa has been determined and reveals the simultaneous presence of monomers and 3D domain-swapped dimers in the same asymmetric unit.