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. Biochemical and bilayer recording suggest that OmpG forms a monomeric rather than the usual trimeric porin (Conlan et al., 2000). 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, has been studied by Anbazhagan et al. (2008).
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). Based on sequence comparisons, the OmpG family may be distantly related to the cyclodextrin porin (CDP) family (1.B.26).
The OmpG family is distantly related to the KdgM family (1.B.35) (condemine et al., 2005).
