The trimeric periplasmic chaperone Skp of Escherichia coli forms 1:1 complexes with outer membrane proteins via hydrophobic and electrostatic interactions.

The interactions of outer membrane proteins (OMPs) with the periplasmic chaperone Skp from Escherichia coli are not well understood. We have examined the binding of Skp to various OMPs of different origin, size, and function. These were OmpA, OmpG, and YaeT (Omp85) from Escherichia coli, the translocator domain of the autotransporter NalP from Neisseria meningitides, FomA from Fusobacterium nucleatum, and the voltage-dependent anion-selective channel, human isoform 1 (hVDAC1) from mitochondria. Binding of Skp was observed for bacterial OMPs, but neither for hVDAC1 nor for soluble bovine serum albumin. The Skp trimer formed 1:1 complexes, OMP.Skp(3), with bacterial OMPs, independent of their size or origin. The dissociation constants of these OMP.Skp(3) complexes were all in the nanomolar range, indicating that they are stable. Complexes of Skp(3) with YaeT displayed the smallest dissociation constants, complexes with NalP the largest. OMP binding to Skp(3) was pH-dependent and not observed when either Skp or OMPs were neutralized at very basic or very acidic pH. When the ionic strength was increased, the free energies of binding of Skp to OmpA or OmpG were reduced. Electrostatic interactions were therefore necessary for formation and stability of OMP.Skp(3) complexes. Light-scattering and circular dichroism experiments demonstrated that Skp(3) remained a stable trimer from pH 3 to pH 11. In the OmpA.Skp(3) complex, Skp efficiently shielded tryptophan residues of the transmembrane strands of OmpA against fluorescence quenching by aqueous acrylamide. Lipopolysaccharide (LPS), a major component of the outer membrane of Gram-negative bacteria, bound to OmpA.Skp(3) complexes at low stoichiometries. Acrylamide quenching of fluorescence indicated that in this ternary complex, the tryptophan residues of the transmembrane domain of OmpA were located closer to the surface than in binary OmpA.Skp(3) complexes. This may explain previous observations that folding of Skp-bound OmpA into lipid bilayers is facilitated in presence of LPS.

A ToxR-based two-hybrid system for the detection of periplasmic and cytoplasmic protein-protein interactions in Escherichia coli: minimal requirements for specific DNA binding and transcriptional activation.

The Vibrio cholerae transcriptional regulator ToxR is anchored in the cytoplasmic membrane by a single transmembrane segment, its C-terminal domain facing the periplasm. Most of its N-terminal cytoplasmic domain shares sequence similarity with the winged helix-turn-helix (wHTH) motif of OmpR-like transcriptional regulators. In the heterologous host Escherichia coli ToxR activates transcription at the V.cholerae ctx promoter in a dimerization-dependent manner, which has led to its employment as a genetic indicator for protein-protein interactions. However, although offering a broader potential application range than other prokaryotic two-hybrid systems described to date, ToxR has so far only been used to study interactions between heterologous transmembrane segments or to monitor homodimerization of C-terminal fusion partners in the periplasm and the cytoplasm of E.coli. Here we show that the ToxR-system also allows the detection of heterodimerization in both cellular compartments of E.coli. In addition, to better understand ToxR's mode of action at ctx in E.coli, we have investigated the minimal requirements for its function as a transcriptional activator. We show that the wHTH motif of ToxR's N-terminal domain constitutes the minimal structural element required to activate transcription at ctx in E.coli when fused to a dimerizing protein module.

The periplasmic chaperone SurA exploits two features characteristic of integral outer membrane proteins for selective substrate recognition.

The Escherichia coli periplasmic chaperone and peptidyl-prolyl isomerase (PPIase) SurA facilitates the maturation of outer membrane porins. Although the PPIase activity exhibited by one of its two parvulin-like domains is dispensable for this function, the chaperone activity residing in the non-PPIase regions of SurA, a sizable N-terminal domain and a short C-terminal tail, is essential. Unlike most cytoplasmic chaperones SurA is selective for particular substrates and recognizes outer membrane porins synthesized in vitro much more efficiently than other proteins. Thus, SurA may be specialized for the maturation of outer membrane proteins. We have characterized the substrate specificity of SurA based on its natural, biologically relevant substrates by screening cellulose-bound peptide libraries representing outer membrane proteins. We show that two features are critical for peptide binding by SurA: specific patterns of aromatic residues and the orientation of their side chains, which are found more frequently in integral outer membrane proteins than in other proteins. For the first time this sufficiently explains the capability of SurA to discriminate between outer membrane protein and non-outer membrane protein folding intermediates. Furthermore, peptide binding by SurA requires neither an active PPIase domain nor the presence of proline, indicating that the observed substrate specificity relates to the chaperone function of SurA. Finally, we show that SurA is capable of associating with the outer membrane. Together, our data support a model in which SurA is specialized to interact with non-native periplasmic outer membrane protein folding intermediates and to assist in their maturation from early to late outer membrane-associated steps.

Periplasmic chaperones--preservers of subunit folding energy for organelle assembly.

The periplasmic PapD-like chaperones have long been known to be necessary for the assembly of bacterial surface organelles. New structural work now suggests that they control assembly by arresting subunit folding. This step may be required to preserve energy for fiber formation.

Folding and insertion of the outer membrane protein OmpA is assisted by the chaperone Skp and by lipopolysaccharide.

We have studied the folding pathway of a beta-barrel membrane protein using outer membrane protein A (OmpA) of Escherichia coli as an example. The deletion of the gene of periplasmic Skp impairs the assembly of outer membrane proteins of bacteria. We investigated how Skp facilitates the insertion and folding of completely unfolded OmpA into phospholipid membranes and which are the biochemical and biophysical requirements of a possible Skp-assisted folding pathway. In refolding experiments, Skp alone was not sufficient to facilitate membrane insertion and folding of OmpA. In addition, lipopolysaccharide (LPS) was required. OmpA remained unfolded when bound to Skp and LPS in solution. From this complex, OmpA folded spontaneously into lipid bilayers as determined by electrophoretic mobility measurements, fluorescence spectroscopy, and circular dichroism spectroscopy. The folding of OmpA into lipid bilayers was inhibited when one of the periplasmic components, either Skp or LPS, was absent. Membrane insertion and folding of OmpA was most efficient at specific molar ratios of OmpA, Skp, and LPS. Unfolded OmpA in complex with Skp and LPS folded faster into phospholipid bilayers than urea-unfolded OmpA. Together, these results describe a first assisted folding pathway of an integral membrane protein on the example of OmpA.

Periplasmic chaperones--new structural and functional insights.

Although chaperones exist in the periplasmic compartment of Gram-negative bacterial cells, how they function is not well understood. New intriguing functional insights are provided by the solved crystal structure of the periplasmic chaperone SurA.

[Development of an in vitro assay for the detection of Botulinum Neurotoxins]

Botulism is a severe, often lethal intoxication, which affects man and animal. It is caused by the neurotoxins (BoNT) of Clostridium botulinum, an anaerobic spore-former. At present the toxin types A-G have been identified. The extreme toxicity of these substances still make the animal testing inevitable. No alternative method could reach the sensitivity of the mouse bioassay. Based on the investigations presented, a serological test shall be developed, which concentrates the sample and detects the toxin in one step. Preliminary results of an in vitro test detecting BoNT/D are discussed.