Biochemistry and Regulation of a Novel Escherichia coli K-12 Porin Protein, OmpG, Which Produces Unusually Large Channels

Abstract

A novel porin, OmpG, is produced in response to a chromosomal mutation termed cog-192. Molecular characterization ofcog-192 revealed that it is a large chromosomal deletion extending from the 3′ end of pspA through to the 5′ end of an open reading frame located immediately upstream of ompG. As a result of this 13.1-kb deletion, the expression ofompG was placed under the control of the pspApromoter. Characterization of OmpG revealed that it is quite different from other porins. Proteoliposome swelling assays showed that OmpG channels were much larger than those of the OmpF and OmpC porins, with an estimated limited diameter of about 2 nm. The channel lacked any obvious solute specificity. The folding model of OmpG suggests that it is the first 16-stranded β-barrel porin that lacks the large external loop, L3, which constricts the channels of other nonspecific and specific porins. Consistent with the folding model, circular dichroism showed that OmpG contains largely a β-sheet structure. In contrast to other Escherichia coli porins, there is no evidence that OmpG exists as stable oligomers. Although ompG DNA was present in all E. coli strains examined so far, its expression under laboratory conditions was seen only due to rare chromosomal mutations. Curiously, OmpG was constitutively expressed, albeit at low levels, in Salmonella, Shigella, and Pseudomonas species.

The growth rate of Mycobacterium smegmatis depends on sufficient porin‐mediated influx of nutrients

“…Hence, the mechanism of activation of mspB and mspD transcription remains obscure. These results show that the mspB and mspD genes function as backup porins in M. smegmatis similar to several silent porin genes of E. coli (Highton et al, 1985;Blasband et al, 1986;Fajardo et al, 1998;Prilipov et al, 1998). These results indicate further that: (i) deletion of mspA may be detrimental for M. smegmatis unless expression of other porin genes compensates for the loss of MspA and (ii) the permeability defects of the mspA mutants are alleviated by additional expression of other porin genes and underestimate the real physiological role of MspA in wild-type M. smegmatis.…”

Structure of the monomeric outer-membrane porin OmpG in the open and closed conformation

“…Projection maps at 6 Å resolution obtained by electron cryo-microscopy of two different two-dimensional (2D) crystal forms revealed a ring-shaped density indicative of a monomeric b-barrel, with no evidence of oligomer formation. Initial secondary structure predictions of OmpG suggested 16 b-strands (Fajardo et al, 1998), but more recent estimates (Conlan et al, 2000;Behlau et al, 2001) put this number at 14, which agrees better with the diameter of the b-barrel in the projection maps.…”

“…Chemical crosslinking and two-dimensional electrophoresis suggested that OmpG is a monomer (Fajardo et al, 1998), unlike the classical outer-membrane porins which are trimers (Nikaido, 2003). Projection maps at 6 Å resolution obtained by electron cryo-microscopy of two different two-dimensional (2D) crystal forms revealed a ring-shaped density indicative of a monomeric b-barrel, with no evidence of oligomer formation.…”

“…In mutants where LamB is either nonfunctional or deleted, the uptake of sugars is facilitated by OmpG (Misra and Benson, 1989). The gene for OmpG encodes a polypeptide with 301 amino acids, and the mature OmpG protein shows all features of an outer-membrane porin: a signal sequence of 21 amino acids at its N-terminus, which is cleaved during export, absence of long hydrophobic stretches, lack of cystein residues (Fajardo et al, 1998) and a C-terminal phenylalanine, which is important for membrane insertion (Struyve et al, 1991). Circular dichroism spectroscopy of detergent-solubilized OmpG indicated that, like other outer-membrane porins, it consists largely of b-sheet with little, if any, a-helix content.…”

“…Circular dichroism spectroscopy of detergent-solubilized OmpG indicated that, like other outer-membrane porins, it consists largely of b-sheet with little, if any, a-helix content. Proteoliposome swelling assays have shown that OmpG is a nonselective channel for mono-, di-and trisaccharides, with an unusually large limiting pore diameter of 2 nm (Fajardo et al, 1998).…”