Molecular insights into the capsular polysaccharide transporter Wza-Wzc complex

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Capsular polysaccharides (CPS) are crucial virulence factors in bacterial pathogens, forming a protective capsule.
• Understanding the secretion machinery of CPS is essential for deciphering bacterial pathogenesis and developing novel antimicrobial strategies.

Purpose of the Study:

• To elucidate the complete cryo-electron microscopy (cryo-EM) structure of the Gram-negative bacterial CPS secretion machinery, specifically the E. coli K12 Wza-Wzc complex.
• To investigate the mechanistic details of CPS transport and complex formation within the bacterial envelope.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine the high-resolution structure of the Wza-Wzc complex.
• Biochemical analysis of isolated Wza translocon and Wzc co-polymerase.
• Structural analysis of intermediate conformations captured during complex assembly.

Main Results:

• The complete structure reveals an elongated, continuous channel spanning the bacterial envelope, facilitating CPS secretion.
• Multiple structural snapshots captured dynamic intermediate conformations of the double membrane assembly.
• The Wzc jellyroll domain was identified as a CPS-binding module, guiding repeat units into a polymerization platform.

Conclusions:

• This study provides unprecedented structural and functional insights into the bacterial CPS synthesis and transport machinery.
• The findings advance our understanding of bacterial capsule formation, a key virulence mechanism.
• The detailed mechanism of CPS secretion offers potential targets for future therapeutic interventions.