Mutation of the critical pH-gating residues histidine 231 to glutamate increase open probability of outer membrane protein G in planar lipid bilayer

Dear Editor,Outer-membrane protein G(OmpG)is a nonspecificβ-barrel porin in the outer membrane of Escherichia coli(E.coli),allowing the passage of ions and molecules up to 900 Da(Fajardo et al.,1998).It comprises of 280 amino acids that form 14-strandedβ-sheets with seven long loops(L1-L7)on the extra-cellular side and six short turns on the periplasmic side(Subbarao and van den Berg,2006;Yildiz et al.,2006;Liang and Tamm,2007).Despite that the OmpG gene exists in the genome of several E.coli strains(Nikaido,1999),expres-sion of OmpG was only observed in E.coli mutants lacking OmpF and LamB(Fajardo et al.,1998)to enable the diffu-sion of maltodextrins across the bacte-rial outer membrane.Very interestingly,unlike usual trimeric channel-forming porins,OmpG exhibits fascinating characteristics of a functional monomer in physiological and structural studies(Conlan and Bayley,2003;Mari et al.,2010).