Targeting of porin to the outer membrane of Escherichia coli. Rate of trimer assembly and identification of a dimer intermediate

Porin, a transmembrane protein in the outer membrane of Escherichia coli, exists in a trimeric structure which is not dissociated during sodium dodecyl sulfate-polyacrylamide gel electrophoresis at 25 degrees C. This unusual stability was utilized in the study of the conformational changes which accompany the targeting of porin to the outer membrane. A delay of 16-44 s between completion of synthesis of a monomer and its assembly into a trimer was found from the ratio of monomers to trimers found in exponentially growing cells. Pulse-chase experiments showed that rapid processing of precursor OmpF molecules was followed by assembly into sodium dodecyl sulfate-resistant oligomers with a half-time of 20 s at 30 degrees C. An intermediate in assembly was isolated by immunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis below 10 degrees C and was identified as a metastable dimer.     

Maltose transport and starch binding in phage-resistant point mutants of maltoporin. Functional and topological implications

The relationships between the bacteriophage lambda binding site, the starch binding site and the pore formed by maltoporin (LamB protein, lambda receptor protein) were investigated. Bacteria with single amino acid substitutions in the maltoporin sequence, which were previously shown to be strongly reduced in phage lambda sensitivity, were assayed for maltose- (and maltodextrin) selective pore functions. Maltose transport assays was performed at low substrate concentrations, under conditions where LamB is limiting for transport. It revealed three classes of mutants. Class A is composed of mutants with no effect on transport (substitutions at amino acid residues 154, 155, 259, 382 and 401); class B corresponds to mutants with a significant but variable reduction in transport (sites 148, 151, 152, 163, 164, 245, 247 and 250); class C is represented by a single mutant for which transport is almost completely abolished (site 18). Starch binding was assayed by two different methods that gave compatible results. In class A mutants, binding was normal, while no binding was observed in the class C mutant. Binding was impaired to various extents in category B mutants. There was a correlation between the level of impairment of starch binding and impairment of maltose transport, consistent with the notion that the residues influencing starch binding are inside, or in close proximity to, the pore. These results, together with previous data on starch-binding mutants that were not affected in phage binding (substitutions at residues 8, 74, 82, 118 and 121), suggest that the binding sites for starch and phage lambda overlap but are distinct. Mutations affecting transport and starch binding are located in the first third of the protein and in the region of residues 245 to 250. Mutations affecting phage adsorption are located mainly in the last two-thirds of the protein. The topological constraints suggested by the results with the available mutants altered in the lamB gene were used to propose a revised model of maltoporin folding across the outer membrane as well as to define the outlines of footprints of macromolecular binding sites (phage, starch and monoclonal antibodies) on the surface of the protein.     

Molecular cloning and chromosome mapping of a mouse DNA sequence homologous to homeotic genes of Drosophila

Some of the homeotic genes of Drosophila, involved in the control of segmental development, form a diverged multigene family. A conserved DNA sequence common to these genes has been used to isolate a clone (Mo-10) from the mouse genome which contains a sequence coding for a protein domain that is homologous to the domain conserved in the Drosophila homeotic genes. By structural analogy, this sequence may be involved in the control of metameric pattern formation in the mouse. Mo-10 has been mapped to the proximal portion of mouse chromosome 6, and its position in relationship to genes known to influence mouse morphogenesis is discussed.     

Activation and partial proteolysis of variant glucocorticoid receptors, studied by two-phase partitioning

In cultured lines of mouse lymphoma cells, resistant to glucocorticoids is frequently associated with the occurrence of glucocorticoid receptors with an abnormally low affinity (nt-) or an abnormally high affinity (nti) for nuclei and DNA. We have investigated whether the abnormal affinities for DNA are correlated with alterations in charge and surface properties of the receptors, that would be revealed through the partition coefficient in aqueous dextran/poly(ethylene glycol) two-phase systems. We have found that none of the receptor variants is defective in the activation step per se, and that only the nti receptors are abnormal in partition properties. Partial proteolysis of wild-type and nt- receptors with alpha-chymotrypsin produces forms which are indistinguishable from the nti receptors with respect to partition coefficients. Upon alpha-chymotrypsin treatment the wild-type receptors attain DNA-binding properties identical to those of the nti receptors, while the nt- receptors, in spite of some increase in DNA affinity, still bind less firmly to DNA than the alpha-chymotrypsin-treated wild-type receptors. alpha-Chymotrypsin treatment of the various receptor types also produces an increase in the binding to dextran sulphate, but the dextran sulphate affinity is higher and varies less between different receptor types than the DNA affinity. Trypsin-treated receptors were found to be devoid of affinity for DNA and dextran sulphate.     

Mass spectrometry of acetylated,permethylated and reduced oligopeptides

The amino acid sequence of oligopeptides can be determined from the mass spectra of their volatile derivatives. New peptide derivatives are described which are prepared from permethylated peptides by reduction with borane-tetrahydrofuran. These derivatives have been found to be more volatile than the corresponding permethylated ones. The procedure is illustrated by application of this technique to representative oligopeptides. As little as 10–100 nmol of these peptides have been sequenced using this technique.     

Comparison of enzyme-linked immunomagnetic chemiluminescence with U.S. Food and Drug Administration's Bacteriological Analytical Manual method for the detection of Escherichia coli O157:H7

Escherichia coli O157:H7, a major foodborne pathogen, has been associated with numerous cases of foodborne illnesses. Rapid methods have been developed for the screening of this pathogen in foods in order to circumvent timely plate culture techniques. Unfortunately, many rapid methods are presumptive and do not claim to confirm the presence of E. coli O157:H7. The previously developed method, enzyme-linked immunomagnetic chemiluminescence (ELIMCL), has been improved upon to allow for fewer incidences of false positives when used to detect E. coli O157:H7 in the presence of mixed cultures. The key feature of this assay is that it combines the highly selective synergism of both anti-O157 and anti-H7 antibodies in the sandwich immunoassay format. This work presents application of a newly semi-automated version of ELIMCL to the detection of E. coli O157:H7 in pristine buffered saline yielding detection limits of approximately 1 x 10(5) to 1 x 10(6) of live cells/mL. ELIMCL was further demonstrated to detect E. coli O157:H7 inoculated into artificially contaminated ground beef at ca. 400 CFU/g after a 5 h enrichment and about 1.5 h assay time for a total detection time of about 6.5 h. Finally, ELIMCL was compared with USFDA's Bacteriological Analytical Manual method for E. coli O157:H7 in a double-blind study. Using McNemar's treatment, the two methods were determined to be statistically similar for the detection of E. coli O157:H7 in ground beef inoculated with mixed cultures of select bacteria.     

β-Amyloid carrying the Dutch mutation has diverse effects on calpain-mediated toxicity in hippocampal neurons

Hereditary cerebral hemorrhage with amyloidosis-Dutch type is a disorder associated with a missense mutation (E693Q) in the β-amyloid (Aβ)-coding region of the amyloid precursor protein (APP). This familial disease is characterized by cognitive deficits secondary to intracerebral hemorrhage and, in some cases, progressive Alzheimer's disease (AD)-like dementia. Although this mutation was the first ever reported in the human APP gene, little is known about the molecular mechanisms underlying the direct toxic effects of this mutated Aβ on central neurons. In the present study, we assessed the role of calpain-mediated toxicity in such effects using an AD primary culture model system. Our results showed that Dutch mutant Aβ (E22Q) induced calpain-mediated cleavage of dynamin 1 and a significant decrease in synaptic contacts in mature hippocampal cultures. These synaptic deficits were similar to those induced by wild-type (WT) Aβ. In contrast, calpain-mediated tau cleavage leading to the generation of a 17-kDa neurotoxic fragment, as well as neuronal death, were significantly reduced in E22Q Aβ-treated neurons when compared with WT Aβ-treated ones. This complex regulation of the calpain-mediated toxicity pathway by E22Q Aβ could have some bearing in the pathobiology of this familial AD form.