The CopC Family: Structural and Bioinformatic Insights into a Diverse Group of Periplasmic Copper Binding Proteins

The
CopC proteins are periplasmic copper binding proteins believed
to play a role in bacterial copper homeostasis. Previous studies have
focused on CopCs that are part of seven-protein Cop or Pco systems
involved in copper resistance. These canonical CopCs contain distinct
Cu­(I) and Cu­(II) binding sites. Mounting evidence suggests that CopCs
are more widely distributed, often present only with the CopD inner
membrane protein, frequently as a fusion protein, and that the CopC
and CopD proteins together function in the uptake of copper to the
cytoplasm. In the methanotroph Methylosinus trichosporium OB3b, genes encoding a CopCD pair are located adjacent to the particulate
methane monooxygenase (pMMO) operon. The CopC from this organism (Mst-CopC) was expressed, purified, and structurally characterized.
The 1.46 Å resolution crystal structure of Mst-CopC reveals a single Cu­(II) binding site with coordination somewhat
different from that in canonical CopCs, and the absence of a Cu­(I)
binding site. Extensive bioinformatic analyses indicate that the majority
of CopCs in fact contain only a Cu­(II) site, with just 10% of sequences
corresponding to the canonical two-site CopC. Accordingly, a new classification
scheme for CopCs was developed, and detailed analyses of the sequences
and their genomic neighborhoods reveal new proteins potentially involved
in copper homeostasis, providing a framework for expanded models of
CopCD function.