Duke: Bacterial cells have an
added layer of protection, called the cell wall, that animal cells
don’t. Assembling this tough armor entails multiple steps, some of which
are targeted by antibiotics like penicillin and vancomycin. Yet one step in the process has remained a mystery because the molecular structures of the proteins involved were not known. Duke University researchers have now provided
the first close-up glimpse of a protein, called MurJ, which is crucial
for building the bacterial cell wall and protecting it from outside
attack. They published MurJ’s molecular structure on Dec. 26 in Nature
Structural and Molecular Biology.
Antibiotic researchers feel an urgent need to
gain a deeper understanding of cell wall construction to develop new
antibiotics in the face of mounting antibacterial resistance. In the
U.S. alone, an antibiotic-resistant infection called MRSA causes nearly
12,000 deaths per year.
“Until now, MurJ’s mechanisms have been
somewhat of a ‘black box’ in the bacterial cell wall synthesis because
of technical difficulties studying the protein,” said senior author
Seok-Yong Lee, Ph.D., associate professor of biochemistry at Duke
University School of Medicine. “Our study could provide insight into the
development of broad spectrum antibiotics, because nearly every type of
bacteria needs this protein’s action.”
A bacterium’s cell wall is composed of a
rigid mesh-like material called peptidoglycan. Molecules to make
peptidoglycan are manufactured inside the cell and then need to be
transported across the cell membrane to build the outer wall.
In 2014, another group of scientists had
discovered that MurJ is the transporter protein located in the cell
membrane that is responsible for flipping these wall building blocks
across the membrane. Without MurJ, peptidoglycan precursors build up
inside the cell and the bacterium falls apart.
Many groups have attempted to solve MurJ’s
structure without success, partly because membrane proteins are
notoriously difficult to work with.
In the new study, Lee’s team was able to
crystallize MurJ and determine its molecular structure to 2-angstrom
resolution by an established method called X-ray crystallography --which
is difficult to achieve in a membrane protein.
The structure, combined with follow-up
experiments in which the scientists mutated specific residues of MurJ,
allowed them to propose a model for how it flips peptidoglycan
precursors across the membrane.
After determining the first structure of
MurJ, Lee’s team is now working to capture MurJ in action, possibly by
crystallizing the protein while it is bound to a peptidoglycan
precursor.
“Getting the structure of MurJ linked to its
substrate will be key. It will really help us understand how this
transporter works and how to develop an inhibitor targeting this
transporter,” Lee said.
Lee’s group is continuing structure and
function studies of other key players in bacterial cell wall
biosynthesis as well. Last year, they published the structure of another
important enzyme, MraY, bound to the antibacterial muraymycin.
The research was supported by Duke University startup funds.
CITATION: " Crystal structure of the MOP
flippase MurJ in an inward-facing conformation," Alvin C. Y. Kuk, Ellene
H. Mashalidis, Seok-Yong Lee. Nature Structural & Molecular
Biology, December 26, 2016. DOI: 10.1038/nsmb.3346