An extended ToxR POSSYCCAT system for positive and negative selection of self-interacting transmembrane domains

Assay systems based on the ToxR protein are widely used to investigate interaction of transmembrane domains that come from natural proteins or are isolated from combinatorial libraries. The principle of this method is that self-interaction of any given transmembrane domain, which is expressed within a ToxR chimeric protein, drives ToxR-ToxR assembly in a bacterial inner membrane. In current versions of the system, ToxR-ToxR interaction drives transcription activation of the cholera toxin (ctx) promoter and thereby induces expression of downstream reporter genes in appropriately constructed bacterial strains. Here, we describe the application of other known ToxR-regulated promoters. We show that interacting transmembrane domains also promote ToxR-driven activation of the ompU promoter. Conversely, these interactions efficiently repress transcription from the constitutively active ompT promoter. We present novel Escherichia coli strains whose chromosomes harbor fusions of ompU or ompT promoters with different reporter genes. Depending on the used promoter, self-interaction of transmembrane domains induces or represses reporter enzyme expression in these cells. These strains extend current applications of the ToxR protein and may find use in mapping transmembrane helix-helix interfaces and selection of transmembrane domains with medium affinities.

Different subcellular locations of secretome components of Gram-positive bacteria

Gram-positive bacteria contain different types of secretion systems for the transport of proteins into or across the cytoplasmic membrane. Recent studies on subcellular localization of specific components of these secretion systems and their substrates have shown that they can be present at various locations in the cell. The translocons of the general Sec secretion system in the rod-shaped bacteriumBacillus subtilishave been shown to localize in spirals along the cytoplasmic membrane, whereas the translocons in the coccoidStreptococcus pyogenesare located in a microdomain near the septum. In both bacteria the Sec translocons appear to be located near the sites of cell wall synthesis. The Tat secretion system, which is used for the transport of folded proteins, probably localizes in the cytoplasmic membrane and at the cell poles ofB. subtilis. InLactococcus lactisthe ABC transporter dedicated to the transport of a small antimicrobial peptide is distributed throughout the membrane. Possible mechanisms for maintaining the localization of these secretion machineries involve their interaction with proteins of the cytoskeleton or components of the cell wall synthesis machinery, or the presence of lipid subdomains surrounding the transport systems.

Molecular Organization in Striated Domains Induced by Transmembrane α-Helical Peptides in Dipalmitoyl Phosphatidylcholine Bilayers

Transmembrane (TM) alpha-helical peptides with neutral flanking residues such as tryptophan form highly ordered striated domains when incorporated in gel-state 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers and inspected by atomic force microscopy (AFM) (1). In this study, we analyze the molecular organization of these striated domains using AFM, photo-cross-linking, fluorescence spectroscopy, nuclear magnetic resonance (NMR), and X-ray diffraction techniques on different functionalized TM peptides. The results demonstrate that the striated domains consist of linear arrays of single TM peptides with a dominantly antiparallel organization in which the peptides interact with each other and with lipids. The peptide arrays are regularly spaced by +/-8.5 nm and are separated by somewhat perturbed gel-state lipids with hexagonally organized acyl chains, which have lost their tilt. This system provides an example of how domains of peptides and lipids can be formed in membranes as a result of a combination of specific peptide-peptide and peptide-lipid interactions.

Photo-Crosslinking Analysis of Preferential Interactions between a Transmembrane Peptide and Matching Lipids

In this study, a novel method is presented by which the molecular environment of a transmembrane peptide can be investigated directly. This was achieved by incorporating a photoactivatable crosslinking probe in the hydrophobic segment of a model transmembrane peptide. When this peptide was incorporated into lipid bilayers and irradiated with UV light, a covalent bond was formed between the crosslinking probe and a lipid. This crosslinking reaction could be visualized by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and the resulting product could be characterized by mass spectrometry. By use of phospholipases, it was demonstrated that the peptide crosslinks to both acyl chains of the lipids. The peptide showed a clear preference to partition into fluid lipids and was excluded from lipids in the gel phase. However, when the peptide was incorporated into bilayers containing two lipid species with different acyl chain lengths, molecular sorting of the lipids around the peptide based on hydrophobic matching was not observed. It is proposed that the size of the transmembrane part plays an important role in the dynamic interactions of membrane proteins with the surrounding lipids and hence in determining whether molecular sorting can occur.

The transmembrane domain of surfactant protein C precursor determines the morphology of the induced membrane compartment in CHO cells

Surfactant protein C (SP-C) is a small lipopeptide of which the main part consists of a typical valyl-rich transmembrane domain. The protein is expressed as a propeptide (proSP-C) which is processed and sorted via the regulated secretory pathway to the lamellar body, where mature SP-C is stored before secretion into the alveolar space. In this study we investigated the identity of the compartment to which proSP-C is sorted in cells that do not have a regulated secretory pathway, such as CHO cells. By electron microscopy we determined that proSP-C was localized in an uncommon membrane compartment with very regular morphology, which was not present in control cells. This membrane compartment is not influenced by the palmitoylation of proSP-C and is probably derived from the endoplasmic reticulum. However, proSP-C chimeras with artificial transmembrane domains induced a membrane compartment with a different morphology. Therefore we propose that the typical amino acid sequence of the transmembrane domain of proSP-C plays a role in membrane formation and morphology, which may be relevant under physiological conditions.

Analysis of DNA binding and transcriptional activation by the LysR-type transcriptional regulator CbbR of Xanthobacter flavus

The LysR-type transcriptional regulator CbbR controls the expression of the cbb and gap-pgk operons in Xanthobacter flavus, which encode the majority of the enzymes of the Calvin cycle required for autotrophic CO2 fixation. The cbb operon promoter of this chemoautotrophic bacterium contains three potential CbbR binding sites, two of which partially overlap. Site-directed mutagenesis and subsequent analysis of DNA binding by CbbR and cbb promoter activity were used to show that the potential CbbR binding sequences are functional. Inverted repeat IR1 is a high-affinity CbbR binding site. The main function of this repeat is to recruit CbbR to the cbb operon promoter. In addition, it is required for negative autoregulation of cbbR expression. IR3 represents the main low-affinity binding site of CbbR. Binding to IR3 occurs in a cooperative manner, since mutations preventing the binding of CbbR to IR1 also prevent binding to the low-affinity site. Although mutations in IR3 have a negative effect on the binding of CbbR to this site, they result in an increased promoter activity. This is most likely due to steric hindrance of RNA polymerase by CbbR since IR3 partially overlaps with the -35 region of the cbb operon promoter. Mutations in IR2 do not affect the DNA binding of CbbR in vitro but have a severe negative effect on the activity of the cbb operon promoter. This IR2 binding site is therefore critical for transcriptional activation by CbbR.

Importance of hydrophobic matching for spontaneous insertion of a single-spanning membrane protein

In this study, we have investigated the effect of hydrophobic mismatch between the thickness of the membrane and a transmembrane segment of a protein that directly inserts into the membrane bilayer. For this purpose we used mutants of the single-spanning Pf3 coat protein that can spontaneously insert into Escherichia coli membrane vesicles and large unilamellar vesicles (LUVs). The thickness of the liposomal bilayer could be altered by using lipids with different acyl chain lengths or by incorporation of cholesterol. The insertion efficiency of the protein clearly depended on the bilayer thickness, with most efficient insertion under hydrophobic matching conditions. To discriminate between effects of length and hydrophobicity, mutants with different synthetic transmembrane segments were constructed. These mutants inserted into LUVs in a mismatch-dependent manner. However, in particular for longer and less hydrophobic mutants, most efficient insertion was generally observed in thinner bilayers than expected on the basis of hydrophobic matching.

Structural requirements for palmitoylation of surfactant protein C precursor

Pulmonary surfactant protein C (SP-C) propeptide (proSP-C) is a type II transmembrane protein that is palmitoylated on two cysteines adjacent to its transmembrane domain. To study the structural requirements for palmitoylation of proSP-C, His-tagged human proSP-C and mutant forms were expressed in Chinese hamster ovary cells and analysed by metabolic labelling with [3H]palmitate. Mutations were made in the amino acid sequence representing mature SP-C, as deletion of the N- and C-terminal propeptide parts showed that this sequence by itself could already be palmitoylated. Substitution of the transmembrane domain by an artificial transmembrane domain had no effect on palmitoylation. However, an inverse correlation was found between palmitoylation of proSP-C and the number of amino acids present between the cysteines and the transmembrane domain. Moreover, substitution by alanines of amino acids localized on the N-terminal side of the cysteines had drastic effects on palmitoylation, probably as a result of the removal of hydrophobic amino acids. These data, together with the observation that substitution by alanines of the amino acids localized between the cysteines and the transmembrane domain had no effect on palmitoylation, suggest that the palmitoylation of proSP-C depends not on specific sequence motifs, but more on the probability that the cysteine is in the vicinity of the membrane surface. This is probably determined not only by the number of amino acids between the cysteines and the transmembrane domain, but also by the hydrophobic interaction of the N-terminus with the membrane. This may also be the case for the palmitoylation of other transmembrane proteins.