Revisiting the CooJ family, a potential chaperone for nickel delivery to [NiFe]‑carbon monoxide dehydrogenase

Nickel insertion into nickel-dependent carbon monoxide dehydrogenase (CODH) represents a key step in the enzyme activation. This is the last step of the biosynthesis of the active site, which contains an atypical heteronuclear NiFe4S4 cluster known as the C-cluster. The enzyme maturation is performed by three accessory proteins, namely CooC, CooT and CooJ. Among them, CooJ from Rhodospirillum rubrum is a histidine-rich protein containing two distinct and spatially separated Ni(II)-binding sites: a N-terminal high affinity site (HAS) and a histidine tail at the C-terminus. In 46 CooJ homologues, the HAS motif was found to be strictly conserved with a H(W/F)XXHXXXH sequence. Here, a proteome database search identified at least 150 CooJ homologues and revealed distinct motifs for HAS, featuring 2, 3 or 4 histidines. The purification and biophysical characterization of three representative members of this protein family showed that they are all homodimers able to bind Ni(II) ions via one or two independent binding sites. Initially thought to be present only in R. rubrum, this study strongly suggests that CooJ could play a significant role in CODH maturation or in nickel homeostasis.