Molecular characterization of a heat-modifiable protein from the outer membrane of Escherichia coli

Establishment of a Protein Concentration Gradient in the Outer Membrane Requires Two Diffusion-Limiting Mechanisms

OmpA-like proteins are involved in the stabilization of the outer membrane, resistance to osmotic stress, and pathogenesis. In Caulobacter crescentus, OmpA2 forms a physiologically relevant concentration gradient that forms by an uncharacterized mechanism, in which the gradient orientation depends on the position of the gene locus. This suggests that OmpA2 is synthesized and translocated to the periplasm close to the position of the gene and that the gradient forms by diffusion of the protein from this point. To further understand how the OmpA2 gradient is established, we determined the localization and mobility of the full protein and of its two structural domains. We show that OmpA2 does not diffuse and that both domains are required for gradient formation. The C-terminal domain binds tightly to the cell wall and the immobility of the full protein depends on the binding of this domain to the peptidoglycan; in contrast, the N-terminal membrane β-barrel diffuses slowly. Our results support a model in which once OmpA2 is translocated to the periplasm, the N-terminal membrane β-barrel is required for an initial fast restriction of diffusion until the position of the protein is stabilized by the binding of the C-terminal domain to the cell wall. The implications of these results on outer membrane protein diffusion and organization are discussed.IMPORTANCE Protein concentration gradients play a relevant role in the organization of the bacterial cell. The Caulobacter crescentus protein OmpA2 forms an outer membrane polar concentration gradient. To understand the molecular mechanism that determines the formation of this gradient, we characterized the mobility and localization of the full protein and of its two structural domains an integral outer membrane β-barrel and a periplasmic peptidoglycan binding domain. Each domain has a different role in the formation of the OmpA2 gradient, which occurs in two steps. We also show that the OmpA2 outer membrane β-barrel can diffuse, which is in contrast to what has been reported previously for several integral outer membrane proteins in Escherichia coli, suggesting a different organization of the outer membrane proteins.

Novel Kinetic Intermediates Populated along the Folding Pathway of the Transmembrane β-Barrel OmpA

We examined the folding of the β-barrel membrane protein OmpA from Escherichia coli. Although previous studies identified several intermediate states followed by a concerted translocation mechanism across the bilayer, some aspects of the pathway were still unclear, including the extent of secondary structure formation in the intermediate states and how the mechanism gave rise to multiple exponential phases in the folding kinetics. We addressed these questions by investigating the folding kinetics of the OmpA transmembrane β-barrel domain over a range of bilayer thicknesses, allowing us to observe different regions of the folding pathway. The fastest folding into the thinnest bilayers provided information about the later stages of the process, and the slowest folding into thicker bilayers revealed early kinetic steps. Folding was monitored using sodium dodecyl sulfate-polyacrylamide gel electrophoresis and circular dichroism spectroscopy, which provide complementary information about tertiary and secondary structure formation. We globally fit the folding data to kinetic schemes and found that the same core pathway was followed under all lipid conditions. We propose a multistep folding mechanism for OmpA that includes unstructured surface-adsorbed states converting through a partially inserted state with substantial β-sheet structure to the final natively inserted barrel. Kinetic models show that all steps of the main folding pathway are accelerated by membrane defects that occur as a result of thinning the bilayer or incubation of lipids at the phase transition temperature. In addition to suppressing off-pathway states, β-barrel assembly machinery-catalyzed folding in vivo could accelerate any or all of these main folding steps to ensure efficient outer membrane protein biogenesis in vivo.

One-step on-column affinity refolding purification and functional analysis of recombinant human VDAC1

The outer mitochondrial membrane porin, voltage-dependent anion-selective channel (VDAC), is believed to play an important role in mediating mitochondria-dependent apoptosis. However, detailed structure-function studies of VDAC have been hindered by the difficulties to obtain a soluble, correctly folded, and fully active form of the recombinant VDAC and its mutant variants due to its transmembrane nature. Here we report a high-throughput one-step chromatographic procedure in purification of recombinant human VDAC1 (rhVDAC1) protein overexpressed in bacteria. The improved methodology could generate a large quantity of rhVDAC1 with correct folding in terms of the secondary structure, with full biological activities in mediating cytochrome c release and in interaction with Bcl-X(L). The method will significantly benefit genetic, biochemical, and structural studies of this critical channel protein.

Characterization of a heat modifiable protein, Escherichia coli outer membrane protein OmpA in binary surfactant system of sodium dodecyl sulfate and octylglucoside

A membrane protein, OmpA of Escherichia coli, in the process of refolding from its heat-modified form in the presence of sodium dodecyl sulfate (SDS) to its non-heated one by the addition of systematic amounts of octylglucoside (OG) was characterized by means of dynamic light scattering and the size exclusion chromatography combined with low angle laser light scattering photometry. Upon heating in the presence of SDS only, the amount of SDS bound to OmpA was increased from 1.8 to 2.3 g/g of protein and its hydrodynamic radius increased from 3.7 to 4.7 nm. On the addition of OG, the once denatured OmpA regained its original size above the weight fraction of OG in the total amount of surfactants, 0.8. During the process, the hydrodynamic radius was observed to decrease cooperatively at the weight fraction of 0.6, while no change took place in the molar mass of the protein. The refractive index increment of OmpA reflecting the amount of surfactant binding also regained the value before the heating in parallel with the change of size. Examination of the amount of surfactants bound to the membrane protein according to known properties of the binary surfactant micellar system of the surfactants showed that SDS was principally responsible for the denaturing phenomena of OmpA.

Surface topology of the Escherichia coli K-12 ferric enterobactin receptor

Monoclonal antibodies (MAb) were raised to the Escherichia coli K-12 ferric enterobactin receptor, FepA, and used to identify regions of the polypeptide that are involved in interaction with its ligands ferric enterobactin and colicins B and D. A total of 11 distinct FepA epitopes were identified. The locations of these epitopes within the primary sequence of FepA were mapped by screening MAb against a library of FepA::PhoA fusion proteins, a FepA deletion mutant, and proteolytically modified FepA. These experiments localized the 11 epitopes to seven different regions within the FepA polypeptide, including residues 2 to 24, 27 to 37, 100 to 178, 204 to 227, 258 to 290, 290 to 339, and 382 to 400 of the mature protein. Cell surface-exposed epitopes of FepA were identified and discriminated by cytofluorimetry and by the ability of MAb that recognize them to block the interaction of FepA with its ligands. Seven surface epitopes were defined, including one each in regions 27 to 37, 204 to 227, and 258 to 290 and two each in regions 290 to 339 and 382 to 400. One of these, within region 290 to 339, was recognized by MAb in bacteria containing intact (rfa+) lipopolysaccharide (LPS); all other surface epitopes were susceptible to MAb binding only in a strain containing a truncated (rfaD) LPS core, suggesting that they are physically shielded by E. coli K-12 LPS core sugars. Antibody binding to FepA surface epitopes within region 290 to 339 or 382 to 400 inhibited killing by colicin B or D and the uptake of ferric enterobactin. In addition to the FepA-specific MAb, antibodies that recognized other outer membrane components, including Cir, OmpA, TonA, and LPS, were identified. Immunochemical and biochemical characterization of the surface structures of FepA and analysis of its hydrophobicity and amphilicity were used to generate a model of the ferric enterobactin receptor's transmembrane strands, surface peptides, and ligand-binding domains.

Insertion of peptides into cell-surface-exposed areas of the Escherichia coli OmpA protein does not interfere with export and membrane assembly

Peptides, 21 amino acids (aa) in length, were inserted into cell-surface-exposed areas of the Escherichia coli outer membrane protein, OmpA, corresponding to aa positions 70 or 154 or at both sites simultaneously. The corresponding hybrid proteins were exported and normally assembled in the outer membrane. The results of protease-accessibility experiments are compatible with the presence of the peptides at the cell surface.

Role of SecA and SecY in protein export as revealed by studies of TonA assembly into the outer membrane of Escherichia coli

The growth of secAts or secYts mutants at the restrictive temperature has been shown to inhibit the export of many outer membrane proteins. We report here that in two secAts strains the rate of incorporation of newly synthesized protein into both inner and outer membrane fractions decreased by about 70% at the restrictive temperature. The export of the outer membrane protein TonA was used as a model system in which to study the effects of SecA or SecY inactivation. pre-TonA that accumulated at the restrictive temperature was found to co-sediment with the outer membrane fraction. However, the precursor was sensitive to protease and did not float up a sucrose gradient with the membrane fractions. It was therefore concluded that pre-TonA was not integrated into the outer membrane fraction but probably accumulated in the cytoplasm. Studies on the rate of processing of pre-TonA, pulse-labelled at the restrictive temperature then chased at the permissive temperature, revealed differences between secA and secY mutants. In the secAts mutant the great majority of cytoplasmic pre-TonA was not apparently processed to the mature form, whereas in the secYts mutant significant amounts of precursors were rapidly chased into mature TonA, which appeared in the outer membrane. These results suggest that SecA and SecY may act sequentially in the export of proteins to the outer membrane. In particular these data indicate that SecA is required to maintain pre-TonA in a translocationally competent form prior to interaction with the SecY export site.

Bdellovibrio bacteriovorus synthesizes an OmpF-like outer membrane protein during both axenic and intraperiplasmic growth

Outer membrane preparations of Bdellovibrio bacteriovorus grown intraperiplasmically on Escherichia coli containing OmpF were prepared by the Triton X-100 procedure of Schnaitman (J. Bacteriol. 108:545-552, 1971). They contained a protein that migrated to almost the same position as E. coli OmpF in sodium dodecyl sulfate-acrylamide gradient gel electrophoresis and to the same position as E. coli OmpF when urea was incorporated into the gel. The mobility of this protein increased relative to that of OmpC in urea-containing gels as does E. coli OmpF. However, the same protein was also produced during axenic growth and during intraperiplasmic growth on prey lacking OmpF. The peptide profile generated by partial proteolysis of this protein showed no homology to that produced from E. coli OmpF. We conclude that B. bacteriovorus synthesizes an OmpF-like protein. Previous claims that the bdellovibrio incorporates an intact E. coli OmpF are not consistent with these observations.

Influence of heat and sodium dodecyl sulfate on the endopeptidase I from Bacillus sphaericus 9602

Endopeptidase I from Bacillus sphaericus is a stable enzyme which retains its activity at 37 degrees C in the presence of sodium dodecyl sulfate. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate revealed two forms of the enzyme: an active, fast-running form, for the enzyme preheated at 37 degrees C and a denatured, slow-running form, for the enzyme preheated at 100 degrees C. Such behavior is similar to that of the "heat-modifiable" outer membrane proteins from gram-negative bacteria. In the absence of sodium dodecyl sulfate, endopeptidase I aggregated in an enzymatically active dimer, with an apparent molecular weight of 90,000 daltons, which could be the native form of the enzyme.

Estimation of molecular weights of membrane proteins in the presence of SDS by low-angle laser light scattering combined with high-performance porous silica gel chromatography. Confirmation of the trimer structure of porin of the E. coli outer membrane

An assessment study was carried out to evaluate the performance of the low-angle laser light-scattering technique combined with high-performance porous silica gel chromatography in the presence of sodium dodecyl sulfate and precision differential refractometry. It was found that the combined technique is highly promising as a reliable method for determining the molecular weight of a membrane protein solubilized by the surfactant. As a test, molecular weights of porin forming the permeability channel of the outer membrane of E. coli B in an oligomeric form were measured before and after heat treatment, which is known to cause dissociation. The results obtained indicate that the porin oligomer is a trimer with stoichiometric composition.

Interactions of cations with membrane fractions of smooth and rough strains of Salmonella typhimurium and other Gram-negative bacteria

Addition of cations (20 to 50 mM for Mg(2+) or Ca(2+) or 100 to 500 mM for Na(+)) to N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid buffer during preparation of membranes from smooth and rough strains of Salmonella typhimurium LT2, Salmonella minnesota, and Escherichia coli O8 had two effects on the composition of the membranes isolated. First, in rough strains of chemotypes Ra to Re the "total membranes" (pellets from high-speed centrifugation) were deficient in the proteins of the outer membrane. The missing proteins were found to have been sedimented in a prior low-speed centrifugation in a fraction we call "cation-aggregated membranes." Since these membranes were enriched for lipopolysaccharide and for outer membrane proteins, deficient in succinic dehydrogenase, and contained primarily the dense peak after sucrose gradient centrifugation, it appears to be relatively pure outer membrane. About 10% of the membrane protein of smooth strains and up to 50% that of rough strains were cation-aggregated membranes, appearing to contain most of the outer membrane of rough strains. Thus, cation aggregation may be a useful means of preparation of outer membrane samples. The second effect was that with cation addition, several high-molecular-weight proteins not seen when membranes were prepared without cation addition were found in the total membranes of both smooth and rough strains after high-speed centrifugation. These proteins were bound by cations to the inner membranes, since they were soluble in Triton X-100 and separated into the less dense peak upon sucrose gradient centrifugation. They originated from the cytoplasm or the periplasm, since they corresponded to soluble proteins found in the supernatant after high-speed centrifugation and were depleted from this supernatant when preparation was done in the presence of cations.

Composition and immunochemical properties of outer membrane proteins of Vibrio cholerae

The protein compositions of the outer membranes of various Vibrio cholerae strains, belonging to both biotypes (El Tor and classical) and both serotypes (Ogawa and Inaba), were analyzed by electrophoresis on polyacrylamide slab gels in the presence of sodium dodecyl sulfate. All these strains contained a major protein band of molecular weight 48,000. But they differed in the composition and proportions of minor proteins. The outer membrane protein profile was influenced by the growth medium. A protein band of molecular weight 15,000 was observed in the outer membrane when V. cholerae Ogawa 395 (classical) was grown in peptone-water, whereas a protein of molecular weight 68,000 appeared when it was grown in the synthetic medium. Under anaerobic growth conditions the proportion of the 13,000-molecular-weight protein was greatly enhanced. The outer membrane contained heat-modifiable proteins, as the protein bands with molecular weights 41,000 and 37,000 disappeared when membrane proteins were disaggregated in sodium dodecyl sulfate at or above 60 degrees C. The antisera to the outer membrane proteins of V. cholerae Ogawa 395 (classical) produced immunoprecipitation to the outer membrane proteins of both biotypes and both serotypes. Also, the antisera agglutinated bacteria of both biotypes and both serotypes, suggesting the presence of a common protein antigen in the outer membrane of V. cholerae.

Outer membrane protein composition of Yersinia pestis at different growth stages and incubation temperatures

The protein composition of the outer membrane of Yersinia pestis grown at 26 and at 37 degrees C was examined. The outer membrane was isolated by isopycnic sucrose density centrifugation, and its degree of purity was determined with known inner and outer membrane components. Using two-dimensional gel electrophoresis, we identified a large number of heat-modifiable proteins in the outer membrane of cells grown at either incubation temperature. One-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis of heated preparations indicated five proteins in the outer membrane of 37 degrees C-grown cells not evident in 26 degrees C-grown cells. Differences in the protein composition of the outer membrane due to the stage of growth were evident at both 26 degrees C and 37 degrees C, although different changes were found at each temperature. When cell envelopes were examined for the presence of peptidoglycan-associated proteins, no differences were seen as a result of stage of growth. Envelopes from 26 degrees C-grown cells yielded two peptidoglycan-associated proteins, E and J. Cells grown at 37 degrees C, however, also contained an additional protein (F) which was not found in either the bound or free form 26 degrees C. The changes in outer membrane protein composition in response to incubation temperature may relate to known nutritional and antigenic changes which occur under the same conditions.

Characterization of the cell wall and cell wall proteins of Chromatium vinosum

Highly purified cell walls of Chromatium vinosum were isolated by differential centrifugation, with or without Triton X-100 extraction. The isolated material had a protein composition similar to that of cell walls obtained by sucrose density gradient centrifugation. Twenty-two proteins were reproducibly detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. A 42-kilodalton protein was shown to account for 65% of the total cell wall protein. The majority of cell wall proteins were solubilized in sodium dodecyl sulfate at room temperature; however, they existed as high-molecular-weight complexes unless heated to 45 degrees C or above. The cell wall contained one heat-modifiable protein which migrated with an apparent molecular weight of 37,400 when solubilized at 70 degrees C or below, but which migrated with an apparent molecular weight of 52,500 if solubilized at 100 degrees C. The electrophoretic mobility of three proteins was modified by 2-mercaptoethanol. The majority of C. vinosum cell wall proteins had isoelectric points between pH 4.5 and 5.5, and the 42-kilodalton protein focused at pH 4.9. No proteins were detected which were analogous to the lipoprotein or peptidoglycan-associated proteins of the Enterobacteriaceae. Nearest-neighbor analysis with a reducible, cross-linking reagent indicated that three proteins, including the 42-kilodalton protein, associated with themselves. Most of the cell wall proteins were partially accessible to proteases in both intact cells and isolated cell walls. Protease treatment of the whole cell or isolated cell wall digested approximately an 11,000-molecular-weight portion of the 42-kilodalton protein.

Factors affecting the electrophoretic mobility of the major outer membrane proteins of Escherichia coli in polyacrylamide gels

The outer membrane proteins of Escherichia coli can be resolved by polyacrylamide gel electrophoresis in the presence of anionic detergents. Factors such as the choice of detergent and buffer system and the presence of urea in the separation gel are all shown to affect the charge and/or the configuration of the detergent-protein complexes and will affect the relative migration of these complexes to different extents. The procedures described in this paper may be of use in the determination of the relatedness of the proteins from the same or different strains. In addition, detailed examinations of the effects of these different parameters and the effect of changes in acrylamide concentrations may be useful in the detection of unusual characteristics which may indicate the presence of posttranslational modification.

Modifiable chromatophore proteins in photosynthetic bacteria

The chromatophores of Chromatium vinosum, as well as six other photosynthetic bacteria, contained two or more proteins which were insoluble when heated in the presence of sodium dodecyl sulfate (SDS) and 2-mercaptoethanol (beta-ME). When the chromatophores were dissolved at room temperature in SDS-beta-ME, these proteins were present in the SDS-polyacrylamide gel electrophoresis profiles, but when the samples were dissolved at 100 degrees C, they were absent or considerably diminished. When one-dimensional gels of chromatophores solubilized at room temperature were soaked in the SDS-beta-ME solution and heated to 100 degrees C and the gels were run in a second dimension, the proteins became immobilized in the original first-dimension gel, where they could be detected by staining. The two major proteins so affected in C. vinosum had apparent molecular weights of 28,000 and 21,000. The chromatophores of several other photosynthetic bacteria also contained predominant proteins between 30,000 and 19,000 molecular weight, which became insoluble when heated in the presence of SDS and beta-ME. In at least two of the species examined, these appeared to be reaction center proteins. The conditions causing the proteins to become insoluble were complex and involved temperature, SDS concentration, and the presence of sulfhydryl reagents. The chromatophores of four of the Chromatiaceae species and two strains of one of the Rhodospirillaceae species examined had a protein-pigment complex that was visible in SDS-polyacrylamide gel profiles of samples dissolved at room temperature but was absent in samples dissolved at 100 degrees C.

Heat-modifiable outer membrane proteins of Neisseria meningitidis and their organization within the membrane

Neisseria meningitidis group B serotype 2 strain M986 contains two predominant outer membrane proteins, with apparent molecular weights of 41,000 (protein b) and 28,000 (protein e). Heating of outer membrane vesicles at 56 degrees C for 20 min caused much of b** to disaggregate and denature into b (41,000 daltons). In contrast, protein e could be rapidly solubilized by SDS at room temperature into its monomeric state (e*), but it was not converted to its final higher apparent molecular weight of 28,000 (e) unless heated at 100 degrees C for 2 min. We propose that protein b exists in the membrane as trimers or tetramers in a transmembrane configuration and that protein e exists as subunits on the exterior surface of the outer membrane and has a highly ordered tertiary structure.

Effect of heat and 2-mercaptoethanol on intracytoplasmic membrane polypeptides of Rhodopseudomonas sphaeroides

Solubilization at 75 degrees C of Rhodopseudomonas sphaeroides chromatophores in the presence of sodium dodecyl sulfate (SDS) and 2-mercaptoethanol (beta-ME) resulted in the selective absence of reaction center B and C polypeptides from SDS-polyacrylamide gel electrophoresis profiles. A newly identified, chromatophore-specific polypeptide, with a mass of 35.2 kdaltons, was also missing under these conditions of chromatophore solubilization. Solubilization at 27 degrees C in the presence of SDS and beta-ME also resulted in the disappearance of these three polypeptides, but at much slower rates. Disappearance of either endogenous or exogenously supplied reaction center polypeptides B and C during SDS solubilization of whole chromatophores at either 27 or 75 degrees C was shown to be entirely dependent upon the presence of beta-ME. After chromatophore solubilization in the presence of beta-ME and subsequent SDS-polyacrylamide gel electrophoresis, exogenously added reaction centers B and C could be localized in a complex of no less than 100 to 200 kdaltons. However, the precise size of the complex was influenced by the stoichiometry of the reacting components. The disappearance of the 35.2-kdalton polypeptide was neither dependent upon the presence of beta-ME nor dependent upon the presence of any additional chromatophore polypeptides. The 35.2-kdalton polypeptide underwent a heat-induced oligomerization to yield several high-molecular-weight species.

Apparent molecular weights of a heat-modifiable protein from the outer membrane of Escherichia coli in gels with different acrylamide concentrations

The apparent molecular weights of the two forms of a heat-modifiable protein from the outer membrane of Escherichia coli K-12, estimated in gels with different concentrations of acrylamide, indicate that the protein binds excess amounts of sodium dodecyl sulfate, possibly due to large beta structures before boiling.

Metal ion dependence of a heat-modifiable protein from the outer membrane of Escherichia coli upon sodium dodecyl sulfate-gel electrophoresis

One heat-modifiable protein of Escherichia coli outer membrane does not completely change to the high-temperature form in the presence of magnesium ion in sodium dodecyl sulfate solution. When the metal ion complexing reagents ethylenediaminetetraacetic acid, phosphate ion, hydroxyl ion, or the competitive cations Zn2+ or Ca2+ are added to the sodium dodecyl sulfate-solubilized sample of outer membrane, and then the sample is heated to 100 degrees C and recooled to room temperature, the protein is almost completely converted to the high-temperature form. In control samples, or if sodium chloride, magnesium chloride, or manganous chloride are added to these samples and treated the same way, a large amount of the low-temperature form of the protein is preserved. beta-Mercaptoethanol additions gave the same results as the metal ion complexing reagents and may owe its activity in these solutions to metal-binding activity and not to its role as a reducing reagent. We concluded that magnesium ion may be involved with stabilization of the low-temperature form of the protein either by directly binding the magnesium or by mediating interaction with other components of the membrane.