Synthase-dependent exopolysaccharide secretion in Gram-negative bacteria

Abstract

The biosynthesis and export of bacterial cell-surface polysaccharides is known to occur through several distinct mechanisms. Recent advances in the biochemistry and structural biology of several proteins in synthase-dependent polysaccharide secretion systems have identified key conserved components of this pathway in Gram-negative bacteria. These components include an innermembrane-embedded polysaccharide synthase, a periplasmic tetratricopeptide repeat (TPR)-containing scaffold protein, and an outer-membrane β-barrel porin. There is also increasing evidence that many synthase-dependent systems are post-translationally regulated by the bacterial second messenger bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP). Here, we compare these core proteins in the context of the alginate, cellulose, and poly-β-D-Nacetylglucosamine (PNAG) secretion systems.

Characterization of the Pseudomonas aeruginosa Glycoside Hydrolase PslG Reveals That Its Levels Are Critical for Psl Polysaccharide Biosynthesis and Biofilm Formation

“…AlgL is proposed to be bifunctional, acting both as a lyase and an integral component of the transenvelope alginate secretion complex (12). Deletion of algL presumably prevents complex assembly, resulting in polymer accumulation in the periplasm and cell rupture.…”

“…Despite the chemical diversity of bacterial exopolysaccharides, the means by which they are synthesized and exported from the bacterium have been categorized into three distinct mechanisms (12). In each mechanism, the proteins required to polymerize, modify, and export these exopolysaccharides are typically encoded on a single operon (13)(14)(15).…”

Periplasmic de-acylase helps bacteria don their biofilm coat

“…In E. coli dPNAG is produced by a synthase-dependent pathway, one of three main mechanisms by which bacteria produce and secrete polysaccharides (6) [the other two are the Wzx/Wzy-dependent and the ATPbinding cassette (ABC) transporter-dependent pathways (6,7)]. Common components of synthase-dependent pathways are an inner membrane-spanning synthase protein, an inner membrane bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) receptor protein, an outer membrane-associated tetratricopeptide repeat (TPR)-containing protein, and an outer membrane-spanning β-barrel porin, with polysaccharide-tailoring enzymes also being present in many of the pathways (6).…”

“…Common components of synthase-dependent pathways are an inner membrane-spanning synthase protein, an inner membrane bis-(3′-5′)-cyclic dimeric guanosine monophosphate (c-di-GMP) receptor protein, an outer membrane-associated tetratricopeptide repeat (TPR)-containing protein, and an outer membrane-spanning β-barrel porin, with polysaccharide-tailoring enzymes also being present in many of the pathways (6). In the dPNAG pathway, these functions are apparently encoded on the pgaABCD genes.…”

“…Located within the outer membrane is the porin PgaA, which has a predicted C-terminal β-barrel domain and likely facilitates dPNAG export across the outer membrane. This protein has an N-terminal TPR-containing domain that is thought to protect the polysaccharide in the periplasm and may also be involved in PgaA-PgaB or PgaA-dPNAG interactions (4,6,9). Sitting in the periplasm between the two is PgaB itself, a metal-dependent, low-efficiency de-N-acetylase enzyme that modifies the PNAG precursor, partially (∼3-5%) de-N-acetylating it to form dPNAG before its export by PgaA.…”