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.     

Schistosoma mansoni adult microsomal antigens, a serologic reagent. I. Systematic fractionation, quantitation, and characterization of antigenic components

A systematic and quantitative search was conducted to identify and isolate a serologically pertinent antigen with high specific activity and low cross-reactivity from adult worms of Schistosoma mansoni. Adult worms of S. mansoni quickly thawed from liquid N2 temperatures were disrupted by controlled homogenization in isotonic buffered sucrose. Differential centrifugation of the homogenate yielded three particulate and one soluble fractions: the 480 x G pellet (nuclear), the 7650 x G pellet (mitochondrial), the 360,000 x G pellet (microsomal), and the 360,000 x G supernatant (cytosol). Quantitative analysis indicated a major concentration of specific antigenic activities in the microsomal fraction. Further purifications of the urea-solubilized, n-butanol-treated microsomal particles by gel filtration and ionic-exchange chromatography resulted in a microsomal antigen (MAMA) possessing high specific activity and low cross-reactivity. The final purification post-ionic exchange chromatograph showed a 30-fold increase of specific antigen activity over that of the cytosol fraction. Sodium dodecyl sulfate/polyacrylamide gel electrophoresis (SDS-PAGE) and enzyme-linked immunotransfer blot by the "Western blot" technique (EITB) indicated specific antigenic activities in association with several different m.w. bands (heterogeneous m.w. by extrapolation = 4.3 to 11.9 x 10(6), 2.0 x 10(5), and 2.0 x 10(4) daltons) of the MAMA fraction. When compared with other reported serologic antigens, MAMA showed substantially higher specific activity and lower cross-reactivity.     

Formation of isoaspartate at two distinct sites during in vitro aging of human growth hormone

In vitro aging at pH 7.4, 37 degrees C causes natural sequence recombinant human growth hormone (rhGH), methionyl rhGH, and human pituitary growth hormone to become substrates for bovine brain protein carboxyl methyltransferase, an enzyme that modifies the "side chain" alpha-carboxyl group present at atypical isoaspartyl linkages. The substrate capacity of rhGH increased at a rate of 1.8 methyl-accepting sites/day/100 molecules of hormone. Reversed-phase high performance liquid chromatography (HPLC) of trypsin digests of aged rhGH revealed two altered peptides not present in digests of control rhGH. These two fragments, which had the amino acid compositions of residues 128-134 (Leu-Glu-Asp-Gly-Ser-Pro-Arg) and 146-158 (Phe-Asp-Thr-Asn-Ser-His-Asn-Asp-Asp-Ala-Leu-Leu-Lys), contained the majority of the induced methylation sites, 22 and 58%, respectively. Isoaspartate can result from deamidation of asparagine or isomerization of aspartate. Isomerization of Asp-130, the only candidate site in 128-134, was corroborated by coelution of the altered fragment with the synthetic isoaspartyl peptide upon reversed-phase HPLC. Evidence is presented that the altered 146-158 fragment is a mixture of two peptides resulting from deamidation of Asn-149 to form 70-80% isoaspartate and 20-30% aspartate at this position. The position of isoaspartate in the altered 146-158 fragment was deduced from mass spectrometry, which indicated a single deamidated asparagine; from methylation stoichiometry, which indicated only one methylation site; and from automated Edman degradation, which showed an absence of asparagine and a low yield of aspartate at position 149. These results show that isoaspartate formation from both aspartate and asparagine is a significant, and possibly the major, source of spontaneous covalent alteration of rhGH and that enzymatic carboxyl methylation provides a powerful tool for assessing this type of modification.     

Characterization of lipid A from Pseudomonas aeruginosa O-antigenic B band lipopolysaccharide by 1D and 2D NMR and mass spectral analysis.

The Lipid A from the lipopolysaccharide of Pseudomonas aeruginosa was examined by high-field nuclear magnetic resonance spectroscopy (NMR) and mass spectrometry (MS). The backbone structure and the position of phosphate substituents were unambiguously established by one- and two-dimensional 1H, 13C, and 31P NMR techniques on a de-O-acylated Lipid A sample. The Lipid A has a beta-(1,6)-glucosamine disaccharide structure which is substituted by phosphomonoesters through glycosidic bonds at C-1 and at C-4'. The configuration of the glycosidically linked phosphate at position C-1 was identified as alpha which is the same as that of Enterobacterial Lipid A. Chemical analysis revealed that the Lipid A contained 2-hydroxydodecanoic, 3-hydroxydodecanoic, dodecanoic, 3-hydroxydecanoic, and hexadecanoic acids in the approximate molar ratios 2.2:2.0:0.2:0.8:0.4. From 1H NMR and fast atom bombardment (FAB) mass spectrometry on the de-O-acylated Lipid A, it was established that both glucosamine residues were N-acylated by 3-hydroxydodecanoic acid. The identity and positions of the ester bound fatty acids in the intact Lipid A were investigated by negative ion FAB-MS. In addition to the hexaacyl and pentaacyl Lipid A species, a tetraacyl species was identified. Heterogeneity due to hydroxylated and nonhydroxylated dodecanoic acid esters could be uniquely localized to the nonreducing beta-glucosamine residue from the fragmentation pattern observed in the negative ion FAB-MS. The complete structure of the Lipid A from P. aeruginosa will be useful in understanding the determinants responsible for the endotoxin activity of this molecule.     

Extracellular lipase from Pseudomonas aeruginosa is an amphiphilic protein.

Lipase (triacylglycerol acylhydrolase, EC 3.1.1.3) secreted by Pseudomonas aeruginosa PAC1R was purified from cell-free growth medium by preparative isoelectric focusing. After blotting the N-terminal amino acid sequence and the amino acid composition were determined and compared to P. fragi and P. cepacia lipases yielding significant homology between all three species. Additionally, a consensus sequence K-Y-P-i-v-l-V-H-G was identified residing at the N-terminus of Pseudomonas lipases and in the central part of Staphylococcus lipases. Treatment of lipase with the serine-specific inhibitor diethyl p-nitrophenyl phosphate caused a rapid and complete inhibition of enzyme activity indicating the presence of a serine at the catalytic site as expected from lipase consensus sequences. Upon charge-shift electrophoresis the electrophoretic mobility of purified lipase was shifted either anodally or cathodally in the presence of sodium deoxycholate and cetyltrimethylammoniumbromide, respectively. This result demonstrates that extracellular lipase of P. aeruginosa exhibits an amphiphilic character like intrinsic membrane proteins.     

Membrane topology of the outer membrane protein OprH from Pseudomonas aeruginosa: PCR-mediated site-directed insertion and deletion mutagenesis.

The 21-kDa outer membrane protein OprH from Pseudomonas aeruginosa is overexpressed under Mg2+ starvation conditions and when overproduced causes resistance to polymyxin B, gentamicin, and EDTA. By circular dichroism analysis, OprH revealed a calculated beta-sheet structure content of 47.3%. PCR-based site-directed deletion and epitope insertion mutagenesis was used to test a topological model of OprH as an eight-stranded beta-barrel. Three permissive and seven nonpermissive malarial epitope insertion mutants and four permissive and four nonpermissive deletion mutants confirmed the general accuracy of this model. Thus, OprH is the smallest outer membrane protein to date to be confirmed as a beta-stranded protein.     

Recombinant Treponema pallidum rare outer membrane protein 1 (Tromp1) expressed in Escherichia coli has porin activity and surface antigenic exposure.

We recently reported the cloning and sequencing of the gene encoding a 31-kDa Treponema pallidum subsp. pallidum rare outer membrane porin protein, designated Tromp1 (D. R. Blanco, C. I. Champion, M. M. Exner, H. Erdjument-Bromage, R. E. W. Hancock, P. Tempst, J. N. Miller, and M. A. Lovett, J. Bacteriol. 177:3556-3562, 1995). Here, we report the stable expression of recombinant Tromp1 (rTromp1) in Escherichia coli. rTromp1 expressed without its signal peptide and containing a 22-residue N-terminal fusion resulted in high-level accumulation of a nonexported soluble protein that was purified to homogeneity by fast protein liquid chromatography (FPLC). Specific antiserum generated to the FPLC-purified rTromp1 fusion identified on immunoblots of T. pallidum the native 31-kDa Tromp1 protein and two higher-molecular-mass oligomeric forms of Tromp1 at 55 and 80 kDa. rTromp1 was also expressed with its native signal peptide by using an inducible T7 promoter. Under these conditions, rTromp1 fractionated predominantly with the E. coli soluble and outer membrane fractions, but not with the inner membrane fraction. rTromp1 isolated from the E. coli outer membrane and reconstituted into planar lipid bilayers showed porin activity based on average single-channel conductances of 0.4 and 0.8 nS in 1 M KCl. Whole-mount immunoelectron microscopy using infection-derived immune serum against T. pallidum indicated that rTromp1 was surface exposed when expressed in E. coli. These findings demonstrate that rTromp1 can be targeted to the E. coli outer membrane, where it has both porin activity and surface antigenic exposure.