Structural Insights into Modification and Export of the Pseudomonas aeruginosa Alginate Exopolysaccharide

Pseudomonas aeruginosa is an opportunistic human pathogen that is notorious for causing chronic lung infections in individuals with cystic fibrosis and tolerating antimicrobial treatments by forming a biofilm. Biofilms contain a variety of different components, including exopolysaccharides. During chronic lung infections, P. aeruginosa converts to a mucoid phenotype by constitutive expression of the AlgU/T regulon. As a result, P. aeruginosa overproduces the alginate exopolysaccharide, leading to worse patient outcomes.In the cytoplasm, the process of alginate biosynthesis begins with nucleotide-sugar precursor formation by AlgA, AlgC, and AlgD. These precursors are polymerized by Alg8 in the inner membrane upon cyclic diguanylate binding to Alg44. Following polymerization and translocation into the periplasm, the polymer can be modified by epimerization by AlgG, or acetylation by the concerted action of AlgI, AlgJ, AlgF, and AlgX. We proposed that AlgF is involved in protein-protein interactions mediating formation of an AlgJFX periplasmic acetylation complex. We demonstrated that AlgF can bind to AlgJ and AlgX in vitro and provide evidence for formation of an AlgIJFX complex. Once the polymer is modified, it gets exported via the outer membrane porin AlgE and the tetratricopeptide repeat-containing protein AlgK. We elucidated the crystal structure of the AlgKX complex and showed that AlgK, AlgX, and the AlgKX complex are capable of binding alginate polymer. In P. aeruginosa, disruption of the AlgKX complex resulted in abrogated alginate secretion and biofilm attachment, demonstrating a role for AlgX in alginate export. Predictive modelling of the AlgEKX outer membrane modification and secretion complex provided insight into how the process of alginate acetylation and export are coordinated in P. aeruginosa. If polymer is accumulated within the periplasm, AlgL functions as a periplasmic homeostasis enzyme to degrade aberrantly exported alginate. We identified key catalytic site residues in AlgL that result in loss of enzymatic activity, reduced cell viability, and abnormal cellular morphology when mutated. We showed that an algL deletion mutant results in accumulation of alginate within the periplasm. This was mitigated by constitutive expression of the AlgU/T regulon, suggesting that there are proteins within the regulon that can compensate for loss of AlgL.

Department dc:contributor.department

Biochemistry

Year dc:date.issued

2023

Author dc:creator

• Gheorghita, Andreea Alexandra

Advisor dc:contributor.advisor

• Howell, Patricia L

Handle dc:identifier.uri

http://hdl.handle.net/1807/129796

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/129796