1.B.21.1.1
Non-specific, 14 β-stranded monomeric OmpG porin (Conlan et al. 2000). pH-induced conformational changes of OmpG have been studied after reconstitution in native E. coli lipids (Mari et al., 2010).  Encoded by a gene in a gene cluster also encoding an ABC sugar uptake system (TC# 3.A.1.1.46), a glucosyl hydrolase and two oxidoreductases.  Therefore it's phsiological function may be glucoside uptake. At neutral/high pH, the channel is open and permeable to substrates of size up to 900Da. At acidic pH, loop L6 folds across the channel and blocks the pore. The channel blockage at acidic pH appears to be triggered by the protonation of a histidine pair on neighboring β-strands, which repel one another, resulting in the rearrangement of loop L6 and channel closure (Köster et al. 2015).  Crystallization and analysis by electron microscopy and X-ray crystallography revealed the fundamental mechanisms essential for channel activity.  A 28 aa extension has been added to the 14 β-TMS barrel to make a 16 β-TMS barrel with normal activity and stability but differing pH sensitivity (Korkmaz et al. 2015). A minimized OmpG porin of only 220 aas still exhibits gating, but it was 5-fold less frequent than in native OmpG. The residual gating of the minimal pore is independent of L6 rearrangements and involves narrowing of the ion conductance pathway, most probably driven by global stretching-flexing deformations of the membrane-embedded β-barrel (Grosse et al. 2014).  pH-dependent gating is controlled by an electrostatic interaction network formed between the gating loop and charged residues in the lumen (Perez-Rathke et al. 2018). 3-d structures of the protein in lipid bilayers have been solved (Retel et al. 2017). OmpG may provide a route for D-lactate/D-3-hydroxybutyrate oligo-ester secretion as well as sugar uptake (Utsunomia et al. 2017). OmpG lacks a central constriction  and has an exceptionally wide pore diameter of about 13 Å. The equatorial plane of OmpG harbors an annulus of four alternating basic and acidic patches, and manipulation of charge distribution in the arginine and glutamate clusters alters sugar specificity and ion selectivity (Schmitt et al. 2019).