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Protein Stores Copper For Methane-Digesting Bacteria

Structural Biology: Tetramer of four-helix bundles holds a total of 52 copper ions

by Celia Henry Arnaud
August 26, 2015

Copper storage protein with 13 copper ions bound.
Credit: Nature
Each monomer in Csp1 binds 13 copper ions (dark gray spheres) down the center of a four-helix bundle.

Engineered methanotrophic bacteria could help us cut atmospheric levels of methane, a greenhouse gas. But scientists first need to understand how the microbes handle the copper required for their methane-oxidizing enzymes.

Scientists have known how copper gets into the bacteria, but they haven’t known how the bacteria store the copper they take up. A newly identified protein is a good candidate for that storage space.

Christopher Dennison of Newcastle University, in England, and coworkers discovered a new protein, called Csp1, that can bind up to 52 Cu(I) ions (Nature 2015, DOI: 10.1038/nature14854). Using bioinformatics, the researchers discovered two other copper storage proteins belonging to the same family in the same bacterium.

The X-ray crystal structure and in vitro studies of Csp1 show that the tetrameric protein binds 13 Cu(I) ions in each of its four four-helix bundles. The ions line up down the middle of each bundle bound by cysteine residues that point into the bundle’s core.

“This report shows that Cu(I) ‘copper sponges’ do indeed exist and provides atomic-level details on how they might function in copper homeostasis,” says Amy C. Rosenzweig, an expert on metalloproteins at Northwestern University.

And such copper sponges might not be limited to methanotrophs. Additional bioinformatics suggests that many other bacteria have similar copper storage proteins.

“There is a broadly accepted idea in the community that bacteria have very little in the way of an intracellular copper requirement,” says David P. Giedroc, an expert on bacterial transition metal homeostasis at Indiana University. “This paper challenges that view.”

The findings also could help bioengineers working with methane monooxygenases (MMOs), says Ramon Gonzalez, who studies metabolic engineering at Rice University. “It has been pretty much impossible to recombinantly express MMOs in a foreign host,” he says. Adding Csp1 to these strains could pave the way for industrial microbes that use methane as a chemical feedstock, Gonzalez says.



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