Pores in the outer membrane of Escherichia coli K12. Involvement of proteins b and c in the permeation of cephaloridine and ampicillin

Recent results have shown that several outer membrane proteins of Gram-negative bacteria play a role in the permeation of hydrophilic molecules through this membrane. In Escherichia coli uptake of complexed iron ions is mediated by various outer membrane proteins [1-3]. The products of the genes bfe, lamB and tsx are involved, respectively, in the uptake of vitamin B12 [4,5], of maltose and maltodextrins [6,7] and of various nucleosides [8]. Wild type E. coli K12 contains two peptidoglycan-associated major outer membrane proteins [9-11 ], which we have designated b and c [12]. (A comparison of the nomenclature used by various authors is given in the latter reference.) Recently a new peptidoglycan-associated protein, protein Ic, was found in mutants ofE. coli K12, by Henning et al. [13], which is probably identical to protein e reported by us [14]. It has recently been shown, both in vitro [15] and in vivo [14, 16-18], that the peptidoglycan-associated proteins also play a role in the permeation of hydrophilic molecules through the outer membrane. All three proteins (b, c and e) facilitate the permeation of several amino acids, sugars and ions through the outer membrane [14, 16]. However, a certain specificity exists with respect to the uptake of nucleotides, as this is only mediated by proteins b and e, and not by protein c [14]. It has recently been reported thatthe rate of diffusion of the antibiotic cephaloridine is decreased in mutants of Salmonella typhimurium, which lack both the 34K and 36K peptidoglycan-associated outer merebrane proteins [ 17]. In order to obtain greater insight into the specificity of the pore properties of the three proteins (b, c and e) ofE. coli K12, we determined which protein(s) facilitate(s) the permeation of the antibiotics cephaloridine and ampicillin through the outer membrane of this organism.