1.B.21 The OmpG Porin (OmpG) Family

The OmpG family consists of two distantly related functionally characterized E. coli proteins, OmpG and OmpL. The OmpG channel appears to be much larger than the E. coli OmpC or OmpF channels (estimated limited diameter of about 2 nm) (Fajardo et al., 1998). The channel lacks solute specificity, and a folding model suggests a 16-stranded β-barrel porin lacking the large external loop, L3, that constricts the pores in other porins. However, Liang and Tamm (2007) found a 14-stranded β-barrel based on NMR analyses. OmpG has been reconstituted in planar bilayers where it exhibits uniform sized channels. Results suggested that OmpG forms a monomeric rather than the usual trimeric porin (Conlan et al., 2000).  The pH-gating conformations of the beta-barrel have been solved. When the pH changes from neutral to acidic, the flexible extracellular loop L6 folds into and closes the OmpG pore (Damaghi et al. 2010).

Voltage-induced closure occurred in a single step, and channel block by Gd³⁺ lacked cooperativity seen with trimeric porin OmpF. Incorporation of OmpG into lipid membranes revealing protein-lipid interactions and β-barrel orientation, as studied by Anbazhagan et al. (2008). OmpG has been intensively studied using physical approaches, providing data on its possible structure and biogenesis (Damaghi et al., 2010; Korkmaz-Ozkan et al., 2010). Refolding pathways of the sequential β-hairpins and kinetics of OmpG folding have been reported (Damaghi et al., 2011). By replacing β1-β6 strands of OmpF that lack sequence motifs that predominantly interact with the cell membrane and appear in more than 75% of transmembrane strands, with β1-β6 strands of OmpG enriched with these motifs and computational verification of increased stability of its transmembrane section, Lin et al. 2017 engineered a novel βMP called OmpGF.

OmpL has been purified and reconstituted. It allowed diffusion of small solutes including sugars (Dartigalongue et al., 2000). Contrary to an earlier report, it does not influence Dsb-mediated redox potential in the periplasm (Sardesai et al., 2003). The OmpG family is related to the Cyclodextrin Porin (CDP; 1.B.26) and the Oligogalacturonate Porin (KdgM; 1.B.35) families (Condemine et al., 2005). 

β-barrel porins have potential as nanosensors for single-molecule detection. However, they have inflexible biophysical properties and are limited in their pore geometry, hindering their applications in sensing molecules of different sizes and properties. By replacing beta1-beta6 strands of the protein OmpF that lack these motifs with beta1-beta6 strands of OmpG enriched with these motifs and computational verification of increased stability of its transmembrane region, Lin et al. 2017 engineered a novel porin called OmpGF. OmpGF forms a monomer with a stable transmembrane region. It can refold in vitro with a predominant beta-sheet structure, as confirmed by circular dichroism. Evidence of OmpGF membrane insertion was provided by intrinsic tryptophan fluorescence spectroscopy, and its pore-forming property was determined by a dye-leakage assay. Single-channel conductance measurements confirmed that OmpGF function as a monomer and exhibits increased conductance relative to OmpG or OmpF (Lin et al. 2017).

OmpG of E. coli is a robust, monomeric, transmembrane β-barrel without ion selectivity. Kahlstatt et al. 2018 presented a photocaged diethylaminocoumarin (DEACM) hybrid of OmpG. Blockage of the pore by DEACM was confirmed by measuring the reduced conductivity. An optimal effect was obtained when two bulky butyl-substituted coumarin cages were attached on the inside of the pore. Irradiation at 385 nm removed the photocages, leading to a restoration of channel conductivity (Kahlstatt et al. 2018).

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