Transmembrane permeability channels in vesicles reconstituted from single species of porins from Salmonella typhimurium.

Aggregates of the "major" outer membrane proteins, "porins," of Salmonella typhimurium form diffusion channels in reconstituted vesicle membranes. The aggregate consists of three species of porins with apparent molecular weights of 34,000, 35,000, and 36,000 when active aggregates are subjected to sodium dodecyl sulfate-acrylamide gel electrophoresis after heating in the presence of sodium dodecyl sulfate (Nakae, J. Biol. Chem. 251:2176-2178, 1976). Single species of porins were isolated by solubilization of membranes and subsequent gel filtration in the presence of sodium dodecyl sulfate from the mutant strains of Salmonella typhimurium that produced only single species of porin. The single species of porins of either 34,000, 35,000, or 36,000 daltons formed diffusion channels when assayed for sucrose permeability in the vesicle membranes reconstituted from porins, phospholipids, and lipopolysaccharides. The exclusion limits of the pores made of single species of porins were not distinguishable from each other and from the exclusion limits of the pores made of the porin aggregates from the wild-type strain, when the permeability of vesicle membranes to radioactive di-, tri-, and tetrasaccharides and to various sizes of radioactive polyethylene glycol was determined. Porin-deficient mutants produced residual amounts of porin amounting to 1 to 5% that produced by the parent strain. This residual porin made diffusion channels when the isolated porins were incorporated into the vesicle membrane and assayed for permeability of saccharides.     

Porin from Rhodopseudomonas sphaeroides

A protein homooligomer was purified from both the cell envelope fractions and the saline extracts of Rhodopseudomonas sphaeroides cells. This oligomer exhibited strong porin activity when reconstituted into proteoliposomes with egg phosphatidylcholine. In the saline extracts of both chemotrophically and phototrophically grown cells, the porin oligomer was the most predominant polypeptide, which produced pores whose behavior toward various sugars could be approximated by hollow cylinders of 0.62 nm in radius. The oligomer was dissociated, in the presence of EDTA, into monomers that migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis as though their molecular weight was about 47,000. The monomer was active in the reconstitution assay and produced pores with sizes comparable to those produced by the oligomer. Circular dichroism spectra indicated the predominance of beta-sheet structure in both the oligomeric and EDTA-dissociated monomeric forms. Drastic conditions, for example, precipitation with 10% trichloroacetic acid or heating for a few hours at 100 degrees C in sodium dodecyl sulfate, were necessary to denature the protein into a form with a reduced content of beta-sheet structure.     

Pore-forming activity of OmpA protein of Escherichia coli.

Escherichia coli outer membrane protein OmpA was purified to homogeneity, as a monomer, from a K12 derivative deficient in both OmpF and OmpC porins. When proteoliposomes reconstituted from the purified OmpA, phospholipids, and lithium dodecyl sulfate were tested for permeability to small molecules by osmotic swelling, it was found that OmpA produced apparently nonspecific diffusion channels that allowed the penetration of various solutes. The pore-forming activity was destroyed by the heat denaturation of the OmpA protein, and the use of an OmpA-deficient mutant showed that the activity was not caused by copurifying contaminants. The size of the OmpA channel, estimated by comparison of diffusion rates of solutes of different sizes, was rather similar to that of E. coli OmpF and OmpC porins, i.e. about 1 nm in diameter. The rate of penetration of L-arabinose caused by a given amount of OmpA protein, however, was about a hundredfold lower than the rate produced by the same amount of E. coli OmpF porin. The addition of large amounts of lithium dodecyl sulfate to the reconstitution mixture increased the permeability through the OmpA channel, apparently by facilitating the correct insertion of OmpA into the bilayer.     

Determinants of OmpF porin antigenicity and structure.

Sixty-six murine hybridomas raised to Escherichia coli B/r porin were used to identify and differentiate the epitopes of this outer membrane protein. Anti-porin monoclonal antibodies (mAb) were raised against outer membrane fragments, purified native trimeric porin (trimer), and purified sodium dodecyl sulfate-denatured monomeric porin (monomer). Immunochemical and flow cytometric methods identified five distinct cell surface-exposed determinants on OmpF. The peptide composition of porin epitopes was determined by analysis of mAb reactivity with cyanogen bromide-generated peptide fragments. Four of 43 anti-monomer mAb reacted with surface exposed sites on OmpF, defining epitopes that consist of residues within CNBr peptides d2, d3, and B. The anti-porin mAb panel was also used to evaluate changes in porin antigenic structure in strains with short ompF deletions. Flow cytometric experiments indicated that despite changes in porin permeability, little if any alteration of surface epitopes occurred in these strains. Western immunoblot analysis of the mutant porins showed loss of reactivity with numerous mAb, which was caused by changes in three spatially distinct epitopes at residues 108-111, 118-123, and 124-129. Our findings indicate that in these ompF mutants the residues responsible for altering porin permeability are not exposed on the cell surface, but are buried within the tertiary structure of the protein. One of these regions, which is apparently involved in the determination of channel permeability characteristics, is conserved among 15 of 16 different porin molecules which were screened with the anti-OmpF mAb panel.     

Bordetella pertussis major outer membrane porin protein forms small, anion-selective channels in lipid bilayer membranes.

The major outer membrane protein of molecular weight 40,000 (the 40K protein) of a virulent isolate of Bordetella pertussis was purified to apparent homogeneity. The purified protein formed an oligomer band (of apparent molecular weight 90,000) on sodium dodecyl sulfate-polyacrylamide gels after solubilization at low temperatures. The porin function of this protein was characterized by the black lipid bilayer method. The 40K protein formed channels smaller than all other constitutive major outer membrane porins studied to date. The average single-channel conductance in 1 M KCl was 0.56 nS. This was less than a third of the conductance previously observed for Escherichia coli porins. Zero-current potential measurements made of the porin to determine its ion selectivity revealed the porin to be more than 100-fold selective for anions over cations. The single-channel conductance was measured as a function of salt concentration. The data could be fitted to a Lineweaver-Burk plot suggesting an anion binding site with a Kd of 1.17 M Cl- and a maximum possible conductance through the channel of 1.28 nS.     

Characterization of porins from the outer membrane of Salmonella typhimurium. 2. Physical properties of the functional oligomeric aggregates.

We have purified to homogeneity, from mutant strains of Salmonella typhimurium, the small oligomers of porin that confer permeability channels to artificial vesicle membranes reconstituted from phospholipids and lipopolysaccharide. The molecular weights of the porin oligomers from the strains SH5551 and SH6017 appeared to be 130000 and 125000, respectively, and those of the monomers were 41000 and 37500, respectively, when determined by sedimentation equilibrium in the presence of dodecylsulfate. It was thus concluded that the functional porin oligomers consisted of three identical subunits. The Stokes' radius of the trimer . dodecylsulfate complex was around 5 nm. The trimer bound less dodecylsulfate than the monomer. The trimer . dodecylsulfate complex retained at room temperature the native conformation of porin, which is rich in beta-structure. When the trimers were dissociated further by various treatments, only the porin monomers were recovered in significant amounts, and the permeability-conferring activity was lost simultaneously. We propose, therefore, that the trimer is the minimal functional unit of porin that is capable of forming permeability channels in the outer membrane of Salmonella typhimurium.     

Outer membrane porin proteins F, P, and D1 of Pseudomonas aeruginosa and PhoE of Escherichia coli: chemical cross-linking to reveal native oligomers.

Native oligomers of three Pseudomonas aeruginosa outer membrane porin proteins and one Escherichia coli porin were demonstrated by using a chemical cross-linking technique. P. aeruginosa protein F, the major constitutive outer membrane porin, was cross-linked to dimers in outer membrane and whole-cell cross-linking experiments. Purified preparations of P. aeruginosa proteins F, D1 (glucose induced), and P (phosphate starvation induced) and E. coli protein PhoE (Ic) were also cross-linked to reveal dimers and trimers upon two-dimensional sodium dodecyl sulfate-polyacrylamide electrophoretic analysis. Cross-linking of protein F was abolished by pretreatment of the protein with sodium dodecyl sulfate, indicating that the cross-linked products were due to native associations in the outer membrane.     

The major outer membrane protein of Acidovorax delafieldii is an anion-selective porin.

The major outer membrane protein (Omp34) of Acidovorax delafieldii (formerly Pseudomonas delafieldii) was purified to homogeneity and was characterized biochemically and functionally. The polypeptide has an apparent molecular weight (Mr) of 34,000, and it forms stable oligomers at pH 9.0 in the presence of 10% octylpolyoxyethylene or 2% lithium dodecyl sulfate below 70 degrees C. The intact protein has a characteristic secondary structure composition, as revealed by Fourier transforming infrared spectroscopy (about 60% beta sheet). These features and the amino acid composition are typical for porins. The purified Omp34 is associated with 1 to 2 mol of lipopolysaccharide per mol of the monomer. Pore-forming activity was demonstrated with lipid bilayer experiments. Single-channel and selectivity measurements showed that the protein forms highly anion-selective channels. The unusual dependence of the single-channel conductance on salt concentration suggests that the porin complexes bear positive surface charges, accumulating negatively charged counterions at the pore mouth.     

Outer membrane of Pseudomonas aeruginosa: heat- 2-mercaptoethanol-modifiable proteins.

A number of polyacrylamide gel systems and solubilization procedures were studied to define the number and nature of "major" polypeptide bands in the outer membrane of Pseudomonas aeruginosa. It was shown that five of the eight major outer membrane proteins were "heat modifiable" in that their mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis was determined by the solubilization temperature. Four of these heat-modifiable proteins had characteristics similar to protein II of the Escherichia coli outer membrane. Addition of lipopolysaccharide subsequent to solubilization caused reversal of the heat modification. The other heat-modifiable protein, the porin protein F, was unusually stable to sodium dodecyl sulfate. Long periods of boiling in sodium dodecyl sulfate were required to cause conversion to the heat-modified form. This was demonstrated both with outer membrane-associated and purified lipopolysaccharide-depleted protein F. Furthermore, lipopolysaccharide treatment had no effect on the mobility of heat-modified protein F. Thus it is concluded that protein F represents a new class of heat-modifiable protein. It was further demonstrated that the electrophoretic mobility of protein F was modified by 2-mercaptoethanol and that the 2-mercaptoethanol and heat modification of mobility were independent of one another. The optimal conditions for the examination of the outer membrane proteins of P. aeruginosa by one-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis are discussed.     

Pore forming activity of the major outer membrane protein of Rhodobacter capsulatus in lipid bilayer membranes

Porin of the outer membrane of Rhodobacter capsulatus St. Louis (ATCC 23782) was isolated and reconstituted into lipid bilayer membranes. The porin was obtained either by the sodium dodecyl sulfate treatment of cell envelopes (SDS-porin) or by saline extraction of whole cells (NaCl-porin). Nanomolar concentrations of both porin preparations resulted in a strong conductance increase of the lipid bilayer membranes by many orders of magnitude. At small protein concentrations the conductance increased in a stepwise fashion, the average single channel conductance being about 0.35 nS in 0.1 M KCl for SDS-porin and NaCl-porin as well. The single channel conductance was a linear function of the specific conductance of the aqueous phase. The results were consistent with the assumption that the porin formed large water-filled transmembrane channels in the membrane. From the average value of the single channel conductance in 0.1 M KCl an effective channel diameter of about 1.5 nm was estimated for both types of porins.Abbreviations EDTA ethylenediamine tetraacetic acid - SDS sodium dodecyl sulfate