Structural basis for alginate secretion across the bacterial outer membrane

Area of Science:

• Microbiology
• Structural Biology
• Biochemistry

Background:

• Pseudomonas aeruginosa is a major cause of chronic lung infections in cystic fibrosis patients.
• Alginate overproduction by P. aeruginosa leads to a mucoid phenotype, contributing to lung colonization.
• The outer membrane protein AlgE is hypothesized to mediate alginate secretion across the bacterial outer membrane.

Purpose of the Study:

• To determine the crystal structure of the outer membrane protein AlgE.
• To elucidate the mechanism of alginate export through AlgE.
• To investigate the role of specific AlgE loops in transport regulation.

Main Methods:

• X-ray crystallography to obtain the 2.3 Å structure of AlgE.
• Site-directed mutagenesis to create loop deletion mutants (ΔT8-AlgE, ΔL2-AlgE).
• Anion flux assays and in vivo complementation experiments.

Main Results:

• The crystal structure revealed AlgE as a monomeric 18-stranded β-barrel with an electropositive pore constriction.
• An arginine-rich conduit in the pore likely acts as a selectivity filter for alginate.
• Deletion of periplasmic loop T8 significantly increased anion flux, suggesting its regulatory role in transport.
• Complementation studies with ΔT8-AlgE impaired alginate production in vivo.

Conclusions:

• AlgE forms a regulated transport pathway for alginate export across the outer membrane.
• Periplasmic loop T8 plays a critical role in regulating AlgE-mediated transport.
• This export mechanism differs from canonical capsular polysaccharide export systems like Wza.