The Wzx/Wzy pathway

The Wzx/Wzy pathway, also known as the ABC transporter-dependent pathway or the ATP-binding cassette (ABC) transporter pathway, is a mechanism involved in the biosynthesis and export of certain types of surface polysaccharides in bacteria. It is primarily responsible for the assembly of O-antigen polysaccharides, which are important components of the lipopolysaccharides (LPS) found in the outer membrane of Gram-negative bacteria.

The Wzx/Wzy pathway involves several key components and steps:

1. Biosynthesis of Polysaccharide Repeat Units: The first step in the Wzx/Wzy pathway is the synthesis of the repeating units of the polysaccharide. Enzymes encoded by specific genes catalyze the assembly of these repeat units by adding sugar residues sequentially, typically using sugar nucleotide precursors.
2. Flipping of Repeat Units: Once the repeat units are synthesized, they need to be translocated from the cytoplasmic side to the periplasmic side of the inner membrane. This is facilitated by a flippase protein, known as Wzx or O-antigen flippase. The Wzx protein recognizes the repeat units and flips them across the inner membrane to the periplasmic space.
3. Polymerization of Repeat Units: In the periplasm, the flipped repeat units are polymerized to form the O-antigen polysaccharide chain. This polymerization step is catalyzed by a glycosyltransferase protein, known as Wzy or O-antigen polymerase. Wzy transfers the repeat units from undecaprenyl phosphate carriers to the growing polysaccharide chain.
4. Export of O-Antigen Polysaccharide: The completed O-antigen polysaccharide chain needs to be transported across the outer membrane and exposed on the bacterial cell surface. This process is facilitated by the action of the Wzy-dependent ABC transporter system.
   • Periplasmic Binding Protein (PBP): A periplasmic binding protein, known as Wzm or O-antigen ligase, binds to the O-antigen polysaccharide chain and shuttles it to the ABC transporter complex.
   • ABC Transporter Complex: The ABC transporter complex consists of an ATP-binding protein (Wzt or O-antigen translocase) and one or more membrane-spanning proteins (WzmB or O-antigen translocation protein). The ATP-binding protein hydrolyzes ATP to provide energy for the transport process.
   • Export of Polysaccharide: The ABC transporter complex mediates the translocation of the O-antigen polysaccharide chain across the outer membrane using the energy derived from ATP hydrolysis. The polysaccharide is then exposed on the cell surface, contributing to the structure and properties of the bacterial outer membrane.

The Wzx/Wzy pathway ensures the efficient biosynthesis and export of O-antigen polysaccharides in a highly regulated manner. The genes involved in this pathway are typically organized in clusters known as O-antigen gene clusters or O-antigen biosynthesis gene clusters. These clusters contain the genes required for the biosynthesis, flipping, polymerization, and export of the O-antigen polysaccharide.

The Wzx/Wzy pathway is essential for the proper assembly and display of O-antigens in Gram-negative bacteria. O-antigens are important for various biological processes, including protection against immune responses, resistance to environmental stresses, and bacterial pathogenicity. The specific structures and compositions of O-antigens contribute to the diversity and antigenic properties of bacterial strains, allowing for identification and classification of different bacterial serotypes.