Research unveils structure of crucial bacterial cell wall protein

Duke University researchers have 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. The research is published in Nature Structural and Molecular Biology.

Researchers at Duke University solved the structure of an enzyme that is crucial for helping bacteria build their cell walls. The molecule, called MurJ (shown in green), must flip cell wall precursors (purple) across the bacteria’s cell membrane before these molecules can be linked together to form the cell wall. This new structure could be important to help develop new broad-spectrum antibiotics. Credit: Alvin Kuk, Duke University

“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 this 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.

Citation: Kuk, Alvin CY, Ellene H. Mashalidis, and Seok-Yong Lee. “Crystal structure of the MOP flippase MurJ in an inward-facing conformation.” Nature Structural & Molecular Biology (2016).
DOI: 10.1038/nsmb.3346
Research funding: Duke University
Adapted from press release by the Duke University.