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Microbiology
Electron Swapping Keeps Proteins in Shape
Bacteria like
E. coli, with their ability to grow rapidly, seem like
an ideal vehicle for mass producing human and genetically engineered
proteins such as insulin. Slip in a foreign gene, and the new protein
is churned out. But the reality is more complicated. The protein must
not only be expressed but must be folded correctly, and in a foreign
environment, even a difference of an electron can result in a protein
that does not function.
Jonathan Beckwith (left) and Federico Katzen have discovered a mechanism that may help correct problems in using E. coli to express foreign proteins. Photo by Graham Ramsay
A research team led by Jonathan Beckwith, the
American Cancer Society professor of microbiology and molecular
genetics at HMS, has found a system for protein folding in
E. coli
that may be exploited to help correct some of these problems.
The system involves a series of proteins that help form disulfide
bonds, covalent links between pairs of cysteines, that are necessary
for keeping many extracellular and secreted proteins in their correct
conformation. These bonds are not found in proteins within the cell
cytoplasm, but are formed in the periplasmic space between the two
membranes of
E. coli bacteria. Studies have shown that
E. coli have
difficulty producing certain foreign proteins, often because
incorrect disulfide bonds are formed.
The DsbD system uses a novel mechanism to transfer electrons across the membrane. A series of disulfide exchanges—interactions between disulfide bonds (S–S) and sulfhydryl groups (–SH)—shuttles electrons from one protein to another. DsbD, shown separated into its three domains, receives electrons from the cytoplasm protein thioredoxin (TrxA). The electrons travel through DsbD across the membrane to the periplasm, where they are transferred to DsbC. Adapted from original drawing by Federico Katzen
In order to form correct disulfide bonds, the system requires a
steady supply of electrons. The team's current research, detailed in
the Nov. 22
Cell, focuses on the membrane protein DsbD, which
provides this flow of electrons by acquiring them from a protein
within the cytoplasm and transferring the electrons through the
membrane into the periplasm.
The DsbD system uses a series of disulfide exchanges to shuttle
electrons from on protein to another, a novel mechanism for electron
transfer across a membrane. With a more complete view of how this
system regulates disulfide bond formation, the team hopes to be able
to enhance the process and improve the production of foreign and
genetically engineered proteins.
