Structure and Function of a Bacterial Microcompartment Shell Protein Engineered to Bind a [4Fe-4S] Cluster

Bacterial microcompartments (BMCs)
are self-assembling organelles
composed of a selectively permeable protein shell and encapsulated
enzymes. They are considered promising templates for the engineering
of designed bionanoreactors for biotechnology. In particular, encapsulation
of oxidoreductive reactions requiring electron transfer between the
lumen of the BMC and the cytosol relies on the ability to conduct
electrons across the shell. We determined the crystal structure of
a component protein of a synthetic BMC shell, which informed the rational
design of a [4Fe-4S] cluster-binding site in its pore. We also solved
the structure of the [4Fe-4S] cluster-bound, engineered protein to
1.8 Å resolution, providing the first structure of a BMC shell
protein containing a metal center. The [4Fe-4S] cluster was characterized
by optical and EPR spectroscopies; it has a reduction potential of
−370 mV vs the standard hydrogen electrode (SHE) and is stable
through redox cycling. This remarkable stability may be attributable
to the hydrogen-bonding network provided by the main chain of the
protein scaffold. The properties of the [4Fe-4S] cluster resemble
those in low-potential bacterial ferredoxins, while its ligation to
three cysteine residues is reminiscent of enzymes such as aconitase
and radical S-adenosymethionine (SAM) enzymes. This
engineered shell protein provides the foundation for conferring electron-transfer
functionality to BMC shells.