Visualizing conformation transitions of the Lipid II flippase MurJ

Area of Science:

• Structural Biology
• Microbiology
• Biochemistry

Background:

• Polysaccharide biosynthesis, including bacterial peptidoglycan and eukaryotic N-linked glycans, relies on specialized flippases to transport lipid-linked oligosaccharide (LLO) precursors across membranes.
• MurJ is the essential flippase for Lipid II, the lipid-linked peptidoglycan precursor, making it crucial for bacterial cell wall synthesis and a target for novel antibacterials.
• The precise mechanism by which MurJ and other LLO flippases transport their substrates remains poorly understood due to limited structural data.

Purpose of the Study:

• To elucidate the molecular mechanism of Lipid II flipping by the bacterial flippase MurJ.
• To provide high-resolution structural insights into MurJ's conformational changes during substrate transport.
• To identify key factors, such as essential ions, that facilitate MurJ's function.

Main Methods:

• X-ray crystallography was employed to determine the structures of MurJ in multiple distinct conformations: inward-closed, inward-open, inward-occluded, and outward-facing.
• Site-directed mutagenesis studies were conducted to investigate the functional importance of specific amino acid residues and identified ions.

Main Results:

• Crystal structures revealed MurJ undergoes significant conformational transitions, including inward-closed, inward-open, inward-occluded, and outward-facing states, essential for lipid flipping.
• Mutagenesis studies identified a critical ion required for MurJ's flippase activity.
• The elucidated conformational cycle provides a mechanistic framework for how MurJ transports Lipid II across the membrane.

Conclusions:

• The study reveals the dynamic conformational changes of MurJ that underlie its function as a lipid flippase.
• Identification of a key ion essential for MurJ activity offers new insights into the transport mechanism.
• These findings provide a structural and mechanistic basis for designing novel inhibitors targeting MurJ and bacterial cell wall synthesis.