The YcnI protein from Bacillus subtilis contains a copper-binding domain

Abstract

Bacteria require a precise balance of copper ions to ensure that essential cuproproteins are fully metalated while also avoiding copper-induced toxicity. The Gram positive bacterium Bacillus subtilis maintains appropriate copper homeostasis in part through its ycn operon. The ycn operon comprises genes encoding three proteins: the putative copper importer YcnJ, the copper-dependent transcriptional repressor YcnK, and the uncharacterized DUF1775 domain-containing YcnI. DUF1775 domains are found across bacterial phylogeny and bioinformatics analyses indicate that they frequently neighbor domains implicated in copper homeostasis and transport. Here, we investigated whether YcnI can interact with copper and, using electron paramagnetic resonance (EPR) and inductively-coupled plasma-mass spectrometry (ICP-MS) find that it can bind a single Cu(II) ion. We determine the structure of both the apo and copper-bound forms of the protein by X-ray crystallography, uncovering a copper binding site featuring a unique mono-histidine brace ligand set that is highly conserved among DUF1775 domains. These data suggest a possible role for YcnI as a copper chaperone and that DUF1775 domains in other bacterial species may also function in copper homeostasis.