title: The dual roles of AlgG in C-5-epimerization and secretion of alginate polymers in Pseudomonas aeruginosa.
abstract: Pseudomonas aeruginosa strains causing chronic pulmonary infections in cystic fibrosis patients produce high levels of alginate, an exopolysaccharide that confers a mucoid phenotype. Alginate is a linear polymer of d-mannuronate (M) and variable amounts of its C-5-epimer, l-guluronate (G). AlgG is a periplasmic C-5-epimerase that converts poly d-mannuronate to the mixed M+G sequence of alginate. To understand better the role and mechanism of AlgG activity, a mutant was constructed in the mucoid strain FRD1 with a defined non-polar deletion of algG. Instead of producing poly mannuronate, the algG deletion mutant secreted dialysable uronic acids, as does a mutant lacking the periplasmic protein AlgK. High levels of unsaturated ends and the nuclear magnetic resonance spectroscopy pattern revealed that the small, secreted uronic acids were the products of extensive polymer digestion by AlgL, a periplasmic alginate lyase co-expressed with AlgG and AlgK. Thus, AlgG is bifunctional with (i) epimerase activity and (ii) a role in protecting alginate from degradation by AlgL during transport through the periplasm. AlgK appears to share the second role. AlgG and AlgK may be part of a periplasmic protein complex, or scaffold, that guides alginate polymers to the outer membrane secretin (AlgE). To characterize the epimerase activity of AlgG further, the algG4 allele of poly mannuronate-producing FRD462 was shown to encode a protein lacking only the epimerase function. The sequence of algG4 has a Ser-272 to Asn substitution in a serine-threonine-rich and conserved region of AlgG, which revealed a critical residue for C-5-epimerase activity.
authors.affiliation: Department of Microbiology, Medical College of Virginia Campus of Virginia Commonwealth University, 1101 E. Marshall St., Rm. 5-051, Richmond, VA 23298-0678, USA.
authors.forename: Sumita
authors.initials: S
authors.lastname: Jain
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Franklin
authors.forename: Helga
authors.initials: H
authors.lastname: Ertesvåg
authors.forename: Svein
authors.initials: S
authors.lastname: Valla
authors.forename: Dennis E
authors.initials: DE
authors.lastname: Ohman
mesh_terms: D000464:Alginates / Q000737:chemistry / Q000378:metabolism*; D000595:Amino Acid Sequence; D001426:Bacterial Proteins / Q000235:genetics / Q000378:metabolism; D002238:Carbohydrate Epimerases / Q000235:genetics / Q000378:metabolism; D002240:Carbohydrate Sequence; D003550:Cystic Fibrosis / Q000150:complications; D017353:Gene Deletion; D005798:Genes, Bacterial; D005816:Genetic Complementation Test; D020723:Glucuronic Acid; D006603:Hexuronic Acids; D006801:Humans; D008969:Molecular Sequence Data; D009894:Opportunistic Infections / Q000209:etiology / Q000382:microbiology; D017875:Phosphotransferases (Phosphomutases) / Q000235:genetics / Q000378:metabolism*; D011552:Pseudomonas Infections / Q000209:etiology / Q000382:microbiology; D011550:Pseudomonas aeruginosa / Q000235:genetics / Q000378:metabolism* / Q000472:pathogenicity; D017386:Sequence Homology, Amino Acid; D013237:Stereoisomerism; D014574:Uronic Acids / Q000378:metabolism
chemical_list: C104798:AlgK protein, Pseudomonas aeruginosa; D000464:Alginates; D001426:Bacterial Proteins; D006603:Hexuronic Acids; D014574:Uronic Acids; D020723:Glucuronic Acid; D002238:Carbohydrate Epimerases; C086519:mannuronan c-5-epimerase; D017875:Phosphotransferases (Phosphomutases); C052799:phosphomannomutase