Alginate polymerization and export

Function

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Alg8 is assigned the catalytic glycosyltransferase role from P. aeruginosa envelope-fraction GDP-mannuronate incorporation, native loss/rescue and active-site mutagenesis. The in vitro assay does not isolate purified Alg8, and some substitutions also affect protein stability. KT2440 chemistry is inferred from the characterized ortholog; Azotobacter native deletion establishes requirement without a purified target assay.

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c-di-GMP-binding regulatory copolymerase that contributes to the Alg8 synthase complex; it is not independently assigned GDP-mannuronate polymerase catalysis. P. aeruginosa ligand-binding stoichiometry is directly measured; transfer to KT2440 and Azotobacter remains explicit homology inference. Upstream Azotobacter regulation is not replaced with the Pseudomonas regulatory circuit.

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A separable protein-dependent role protects nascent polymer from periplasmic degradation in the characterized P. aeruginosa context. In FRD1, deleting algG releases AlgL digestion products, whereas an epimerase-deficient AlgG allele permits polymer secretion. Required structural status is not transferred to Azotobacter: its low AlgG epimerization contribution and extracellular-epimerase-export mutants do not test an algG-null background.

Function

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Optional chemical conversion of polymer mannuronate residues to guluronate. Optionality concerns epimerase catalysis, not automatic dispensability of the whole protein. Azotobacter AlgG is active despite the larger native contribution of extracellular AlgE1–7, which lie outside this envelope module. Physiological polymer block patterns are not an absolute limit on purified AlgG catalytic capacity.

Processes

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TPR-rich export-associated scaffold. KT2440 AlgK has a direct AlgX complex structure and intrinsic polymer binding; Pa localization and AlgE-mislocalization experiments support envelope organization. The 2010 AlgK structure is from P. fluorescens WCS374r, not the Pa protein used in localization studies. Native Azotobacter genetics supports involvement without a purified scaffold assay. The shared scaffold annoton remains without a universal molecular-function term: GO:0030247 is reviewed for KT2440 AlgK, whereas the Pa and Azotobacter member reviews do not assert that activity.

Processes

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AlgX supports polymer binding and passage through the AlgK-associated export machinery. The KT2440 AlgKX structure and binding measurements are direct; Pa native interface mutants distinguish protein organization from acetyltransferase chemistry. Azotobacter handling is inferred from homolog architecture and locus evidence. Its O-acetyltransferase reaction is represented in the adjacent acetylation module. GO:0030247 is reviewed for the two Pseudomonas AlgX proteins, but the Azotobacter review does not yet assert a separate binding MF. The shared handling role is retained without extending that term to every representative.

Function

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Clears misplaced or accumulated periplasmic polymer in the characterized P. aeruginosa system; analogous clearance in KT2440 and Azotobacter is inferred. Native Azotobacter studies establish lyase chemistry and molecular-mass control without a periplasmic accumulation assay. This is genuine catabolism even when it supports biosynthetic homeostasis rather than nutrient acquisition. Pa viability depends on production and regulatory background; a native nonpolar Azotobacter mutant remains viable and produces higher-molecular-mass alginate. AlgL is not asserted to be an obligatory polymerase component.

Function

Processes

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Outer-membrane channel for alginate export. Pa structure, anion-flux assays and native loop mutants jointly support the mechanism; detailed gating derives partly from modeling. KT2440 and Azotobacter transport chemistry is inferred from specific homolog identity and envelope/locus evidence rather than a direct native conductance assay.