Evidence for a Widespread Third System for Bacterial Polysaccharide Export across the Outer Membrane Comprising a Composite OPX/β-Barrel Translocon

Abstract

Bacteria secrete a wide variety of polysaccharides that have critical functions in, e.g., fitness, surface colonization, and biofilm formation and in beneficial and pathogenic human-, animal-, and plant-microbe interactions. In Gram-negative bacteria, export of these chemically diverse polysaccharides across the outer membrane depends on two known translocons, i.e., an outer membrane OPX protein in Wzx/Wzy- and ABC transporter-dependent pathways and an outer membrane 16- to 18-stranded β-barrel protein in synthase-dependent pathways. Here, using a combination of experiments in Myxococcus xanthus , phylogenomics, and computational structural biology, we provide evidence supporting that a third type of translocon can export polysaccharides across the outer membrane.

Molecular Dynamics of Outer Membrane-Embedded Polysaccharide Secretion Porins Reveals Closed Resting-State Surface Gates Targetable by Virtual Fragment Screening for Drug Hotspot Identification

“…These findings support the hypothesis that an internal triggering mechanism is needed for the opening of these β-barrels; this would be consistent with nascent PNAG/EPS/MASC/BPS polymers (arriving from the periplasm) being translocated through the interior of OM-spanning PgaA/WzpX/WzpS/WzpB, with this translocation step contributing to the opening of the extracellular loops of the respective porins. For WzpX/WzpS/WzpB, this opening may happen in conjunction with the proposed interactions , between the β-barrels and the periplasmic Class-3 OPX proteins WzaX/WzaS/WzaB for the respective EPS/MASC/BPS pathways.…”

“…Importantly, our investigation has demonstrated the utility of using membrane-embedded integrated MD simulations to evaluate the quality of neural network-based membrane protein model structures, which are becoming widely reported in the scientific literature. In addition, our protocol will allow for the rigorous testing of more complex questions such as the proposed interaction dynamics between the EPS/MASC/BPS secretion porins (WzpX/S/B) and other proteins such as modeled octamers of the WzaX/S/B Class-3 OPX proteins required for the secretion of each polymer. , …”

“…The WzpX/WzpS/WzpB OM porins , were respectively found to be part of the EPS/MASC/BPS Wzx/Wzy-dependent pathways ,, in which undecaprenyl pyrophosphate-linked sugar repeats are flipped across the inner membrane to its periplasmic leaflet (by Wzx), where they are then polymerized (by Wzy) and secreted across the cell envelope to the cell exterior. However, WzpX/WzpS/WzpB were identified as expanded structural homologues of the porin PgaA required for the secretion of poly- N -acetyl- d -glucosamine (PNAG) in Escherichia coli.…”

Microbial Niche Differentiation during Nitrite-Dependent Anaerobic Methane Oxidation

“…sinica” at high nitrite input, providing materials for polymerization to EPS . EPS is then secreted by the mechanism associated with increased expression of wza and kpsM providing a barrier from nitrite toxicity . Similar genes are expressed at a low level (about 100–1000-fold less) in “ Ca.…”

“…Genes associated with nucleoside diphosphate saccharide production from the serine pathway were highly expressed in "M. sinica" at high nitrite input, providing materials for polymerization to EPS. 52 EPS is then secreted by the mechanism associated with increased expression of wza 53 and kpsM 54 providing a barrier from nitrite toxicity. 55 Similar genes are expressed at a low level (about 100−1000-fold less) in "Ca.…”