BIOCHEMISTRY AND REGULATION OF A NOVEL ESCHERICHIA-COLI K-12 PORIN PROTEIN, OMPG, WHICH PRODUCES UNUSUALLY LARGE CHANNELS

A novel porin, OmpG, is produced in response to a chromosomal mutation termed cog-192, Molecular characterization of cog-192 revealed that i t is a large chromosomal deletion extending from the 3' end of pspA th rough to the 5' end of an open reading frame located immediately upstr eam of ompG. As a result of this 13.1-kb deletion, the expression of o mpG was placed under the control of the pspA promoter. Characterizatio n of OmpG revealed that it is quite different from other porins, Prote oliposome swelling assays showed that OmpG channels were much larger t han those of the OmpF and OmpC porins, with an estimated limited diame ter of about 2 nm, The channel lacked any obvious solute specificity, The folding model of OmpG suggests that it is the first 16 stranded be ta-barrel porin that lacks the large external loop, L3, which constric ts the channels of other nonspecific and specific porins. Consistent w ith the folding model, circular dichroism showed that OmpG contains la rgely a beta-sheet structure. In contrast to other Escherichia coli po rins, there is no evidence that OmpG exists as stable oligomers, Altho ugh ompG DNA was present in all E. coli strains examined so far, its e xpression under laboratory conditions was seen only due to rare chromo somal mutations. Curiously, OmpG was constitutively expressed, albeit at low levels, in Salmonella, Shigella, and Pseudomonas species.