Edited by Ruma Banerjee Copper is critically important for methanotrophic bacteria because their primary metabolic enzyme, particulate methane monooxygenase (pMMO), is copper-dependent. In addition to pMMO, many other copper proteins are encoded in the genomes of methanotrophs, including proteins that contain periplasmic copper-A chaperone (PCu A C) domains. Using bioinformatics analyses, we identified three distinct classes of PCu A C domain-containing proteins in methanotrophs, termed PmoF1, PmoF2, and PmoF3. PCu A C domains from other types of bacteria bind a single Cu(I) ion via an HX n MX 21/22 HXM motif, which is also present in PmoF3, but PmoF1 and PmoF2 lack this motif entirely. Instead, the PCu A C domains of PmoF1 and PmoF2 bind only Cu(II), and PmoF1 binds additional Cu(II) ions in a His-rich extension to its PCu A C domain. Crystal structures of the PmoF1 and PmoF2 PCu A C domains reveal that Cu(II) is coordinated by an N-terminal histidine brace HX 10 H motif. This binding site is distinct from those of previously characterized PCu A C domains but resembles copper centers in CopC proteins and lytic polysaccharide monooxygenase (LPMO) enzymes. Bioinformatics analysis of the entire PCu A C family reveals previously unappreciated diversity, including sequences that contain both the HX n MX 21/22 HXM and HX 10 H motifs, and sequences that lack either set of copper-binding ligands. These findings provide the first characterization of an additional class of copper proteins from methanotrophs, further expand the PCu A C family, and afford new insight into the biological significance of histidine bracemediated copper coordination. Copper enzymes, including cytochrome c oxidase, nitrous oxide reductase, nitrite reductase, superoxide dismutase, and particulate methane monooxygenase (pMMO), 4 play important roles in bacterial metabolism (1). In Gram-negative bacteria, these enzymes are located in the inner membrane and/or periplasm, where copper loading likely takes place via periplasmic chaperone proteins or directly from extracytoplasmic pools (2). One family of periplasmic copper-binding proteins implicated in assembly of the cytochrome c oxidase copper centers is the periplasmic copper-A chaperone (PCu A C) proteins. These proteins have been proposed to play a role in loading the cytochrome c oxidase Cu A and/or Cu B sites (3-8). Genes encoding PCu A Cs are often found neighboring genes encoding Sco1 proteins (3, 9), and transfer of Cu(I) from PCu A C to Sco1 has been demonstrated for PCu A Cs from Streptomyces lividans (7) and Rhodobacter capsulatus (10). However, it remains unclear whether PCu A C is absolutely required for metalation of cytochrome c oxidase because multiple gene disruption studies indicate that it is not critical for cytochrome c oxidase activity (5, 7, 11). In addition, there is evidence that some PCu A C homologs have other functions. For example, the PCu A C homolog AccA is essential for copper nitrite reductase activity in pathogenic Neisseria strains (12). Consistent with a broader ...