The membrane insertion of colicins.

Pore-forming toxins, such as colicin A, are water-soluble proteins that insert into lipid bilayers. The water-soluble structure of Colicin A is known at a high resolution and this review describes the kinetic and structural steps involved in its soluble-to-membrane bound transformation.     

Insights into membrane insertion based on studies of colicins

The recently determined three-dimensional structure of the pore-forming domain of colicin A has led to a hypothetical model for membrane insertion and channel formation. Certain features of this model have implications for understanding the mechanism of membrane insertion by other toxins and may have a broader relevance to protein transport in general.     

Dicarboxylate transport across the inner membrane of the chloroplast envelope.

The uptake of radioactively labeled dicarboxylates into the sorbitol-impermeable 3H2O space (the space surrounded by the inner envelope membrane) of spinach chloroplasts has been studied by means of silicone layer filtering centrifugation. 1. Malate, aspartate and a number of other dicarboxylates are rapidly transported across the envelope leading to an accumulation in the chloroplasts. This uptake proceeds mainly by a counterexchange with the dicarboxylates present there. 2. The dicarboxylate transport shows saturation characteristics allowing the determination of Km and V. 3. All dicarboxylates transported act as competitive inhibitors of the transport. 4. The activation energy of the transport as determined from the temperature dependency is evaluated to be 7 kcal/mol. 5. The rate of dicarboxylate transport is influenced by illumination, the countertransported molecules and the pH in the medium. These changes effect the transport velocity, whereas the corresponding Km values are not altered. 6. It is discussed whether there is more than one carrier involved in dicarboxylate transport in spinach chloroplasts.     

Protein insertion into the inner membrane of mitochondria

The inner membrane of mitochondria harbours a large number of polypeptides, many of which have evolved from proteins of the prokaryotic progenitors of mitochondria. The sorting routes on which these proteins are integrated into the mitochondrial inner membrane reflect their phylogenetic origin: Proteins of eukaryotic descent typically reach their destination following arrest of import at the level of the inner membrane. In contrast, many proteins inherited from the prokaryotic progenitor cell are inserted into the inner membrane in an export step following translocation into the matrix. Recently, three different insertion pathways from the matrix into the inner membrane were identified which show considerable parallels to the protein insertion processes in bacteria and chloroplasts. Two of these pathways depend on the related inner membrane proteins Oxa1 and Cox18. A third route is less well defined and depends on the membrane-associated matrix protein Mba1.     

SecA protein: autoregulated ATPase catalysing preprotein insertion and translocation across the Escherichia coli inner membrane

Recent insight into the biochemical mechanism of protein translocation in Escherichia coli indicates that SecA ATPase is required both for the initial binding of preproteins to the inner membrane as well as subsequent translocation across this structure. SecA appears to promote these events by direct recognition of the preprotein or preprotein-SecB complex, binding to inner-membrane anionic phospholipids, insertion into the membrane biiayer and association with the preprotein translocator, SecY/SecE. ATP binding appears to control the affinity of SecA for the various components of the system and ATP hydrolysis promotes cycling between its different biochemical states. As a component likely to catalyse a rate-determining step in protein secretion, SecA synthesis is co-ordinated with the activity of the protein export pathway. This form of negative reguiation appears to rely on SecA protein binding to its mRNA and repressing translation if conditions of rapid protein secretion prevail within the cell. A precise biochemical scheme for SecA-dependent catalysis of protein export and the details of secA regulation appear to be close at hand. The evolutionary conservation of SecA protein among eubacteria as well as the general requirement for translocation ATPases in other protein secretion systems argues for a mechanistic commonality of all prokaryotic protein export pathways.     

Import of preproteins into the chloroplast inner envelope membrane

The chloroplast inner envelope membrane contains many integral proteins which differ in the number of alpha-helices that anchor the protein into the bilayer. For most of these proteins it is not known which pathway they engage to reach their final localisation within the membrane. In yeast mitochondria, two distinct sorting/insertion pathways have been described for integral inner membrane proteins, involving the Tim22 and Tim23 translocases. These routes involve on the one hand a conservative sorting, on the other hand a stop-transfer pathway. In this study we performed a systematic characterisation of the import behaviour of seven inner envelope proteins representing different numbers of predicted alpha-helices. We investigated their energy dependence, import rate, involvement of components of the chloroplast general import pathway and distribution between soluble and membrane fractions. Our results show the existence of at least two different families of inner envelope proteins that can be classified due to the occurrence of an intermediate processing form. Each of the proteins we investigated seems to use a stop-transfer pathway for insertion into the inner envelope.     

Targeting and insertion of heterologous membrane proteins in E. coli

Membrane targeting and insertion of the archaeal Halobacter halobium proton pump bacterioopsin (Bop) and the human melanocortin 4 receptor (MC(4)R) were studied in vitro, using E. coli components for protein synthesis, membrane targeting and insertion. These heterologous proteins are targeted to E. coli membranes in a signal recognition particle (SRP) dependent manner and inserted into the membrane co-translationally. Furthermore, we demonstrate that nascent chains of Bop and MC(4)R first interact with SecY and then with YidC as they move through the translocon. Our results suggest that the initial stages of membrane targeting and insertion of heterologous proteins in E. coli proceed by the pathway used for native E. coli membrane proteins. No significant pausing of protein elongation was observed in the presence of E. coli SRP, in line with the suggestion that translational arrest requires an Alu domain, which is absent in SRP from E. coli.     

A synergistic role for two predicted inner membrane proteins of Escherichia coli in cell envelope integrity

The bacterial cytoplasmic membrane is a principal site of protein translocation, lipid and peptidoglycan biogenesis, signal transduction, transporters and energy generating components of the respiratory chain. Although 25–30% of bacterial proteomes consist of membrane proteins, a comprehensive understanding of their influence on fundamental cellular processes is incomplete. Here, we show that YciB and DcrB, two small cytoplasmic membrane proteins of previously unknown functions, play an essential synergistic role in maintaining cell envelope integrity of Escherichia coli. Lack of both YciB and DcrB results in pleiotropic cell defects including increased levels of lipopolysaccharide, membrane vesiculation, dynamic shrinking and extension of the cytoplasmic membrane accompanied by lysis and cell death. The stalling of an abundant outer membrane lipoprotein, Lpp, at the periplasmic face of the inner membrane leads to lethal inner membrane–peptidoglycan linkages. Additionally, the periplasmic chaperone Skp contributes to yciB dcrB mutant cell death by possibly mistargeting stalled porins into the inner membrane. Consistent with the idea of a compromised envelope in the yciB dcrB mutant, multiple envelope stress response systems are induced, with Cpx signal transduction being required for growth. Taken together, our results suggest a fundamental role for YciB and DcrB in cell envelope biogenesis.     

Molecular chaperones and protein translocation across the Escherichia coli inner membrane

Proteins that are able to translocate across biological membranes assume a loosely folded structure. In this review it is suggested that the loosely folded structure, referred to here as the 'pre-folded conformation', is a particular structure that interacts favourably with components of the export apparatus. Two soluble factors, SecB and GroEL, have been implicated in maintenance of the pre-folded conformation and have been termed 'molecular chaperones'. Results suggest that SecB may be a chaperone that is specialized for binding to exported protein precursors, while GroEL may be a general folding modulator that binds to many intracellular proteins.     

Membrane-spanning peptides induce phospholipid flop: a model for phospholipid translocation across the inner membrane of E. coli

The mechanism by which phospholipids translocate (flop) across the E. coli inner membrane remains to be elucidated. We tested the hypothesis that the membrane-spanning domains of proteins catalyze phospholipid flop by their mere presence in the membrane. As a model, peptides mimicking the transmembrane stretches of proteins, with the amino acid sequence GXXL(AL)(n)XXA (with X = K, H, or W and n = 8 or 12), were incorporated in large unilamellar vesicles composed of E. coli phospholipids. Phospholipid flop was measured by assaying the increase in accessibility to dithionite of a 2,6-(7-nitro-2,1,3-benzoxadiazol-4-yl)aminocaproyl (C(6)NBD)-labeled phospholipid analogue, initially exclusively present in the inner leaflet of the vesicle membrane. Fast flop of C(6)NBD-phosphatidylglycerol (C(6)NBD-PG) was observed in vesicles in which GKKL(AL)(12)KKA was incorporated, with the apparent first-order flop rate constant (K(flop)) linearly increasing with peptide:phospholipid molar ratios, reaching a translocation half-time of approximately 10 min at a 1:250 peptide:phospholipid molar ratio at 25 degrees C. The peptides of the series GXXL(AL)(8)XXA also induced flop of C(6)NBD-PG, supporting the hypothesis that transmembrane parts of proteins mediate phospholipid translocation. In this series, K(flop) decreased in the order X = K > H > W, indicating that peptide-lipid interactions in the interfacial region of the membrane modulate the efficiency of a peptide to cause flop. For the peptides tested, flop of C(6)NBD-phosphatidylethanolamine (C(6)NBD-PE) was substantially slower than that of C(6)NBD-PG. In vesicles without peptide, flop was negligible both for C(6)NBD-PG and for C(6)NBD-PE. A model for peptide-induced flop is proposed, which takes into account the observed peptide and lipid specificity.     

A patch-clamp study of ion channels of inner and outer membranes and of contact zones of E. coli, fused into giant liposomes. Pressure-activated channels are localized in the inner membrane

Inner and outer membranes of Escherichia coli and contact zones were isolated and fused separately with giant liposomes amenable to patch-clamp recording. Different types of large pressure-activated channels were localized in the inner membrane fraction which also contained smaller, pressure-insensitive channels. The outer membrane contained pressure-insensitive channels with large conductances and long opening and closing times which are likely to be porins. Large channels were also observed in contact zones.     

Import of colicins across the outer membrane of Escherichia coli involves multiple protein interactions in the periplasm

Several proteins of the Tol/Pal system are required for group A colicin import into Escherichia coli. Colicin A interacts with TolA and TolB via distinct regions of its N-terminal domain. Both interactions are required for colicin translocation. Using in vivo and in vitro approaches, we show in this study that colicin A also interacts with a third component of the Tol/Pal system required for colicin import, TolR. This interaction is specific to colicins dependent on TolR for their translocation, strongly suggesting a direct involvement of the interaction in the colicin translocation step. TolR is anchored to the inner membrane by a single transmembrane segment and protrudes into the periplasm. The interaction involves part of the periplasmic domain of TolR and a small region of the colicin A N-terminal domain. This region and the other regions responsible for the interaction with TolA and TolB have been mapped precisely within the colicin A N-terminal domain and appear to be arranged linearly in the colicin sequence. Multiple contacts with periplasmic-exposed Tol proteins are therefore a general principle required for group A colicin translocation.     

MsbA-dependent translocation of lipids across the inner membrane of Escherichia coli

MsbA is an essential ABC transporter in Escherichia coli required for exporting newly synthesized lipids from the inner to the outer membrane. It remains uncertain whether or not MsbA catalyzes trans-bilayer lipid movement (i.e. flip-flop) within the inner membrane. We now show that newly synthesized lipid A accumulates on the cytoplasmic side of the inner membrane after shifting an E. coli msbA missense mutant to the non-permissive temperature. This conclusion is based on the selective inhibition of periplasmic, but not cytoplasmic, covalent modifications of lipid A that occur in polymyxin-resistant strains of E. coli. The accessibility of newly synthesized phosphatidylethanolamine to membrane impermeable reagents, like 2,4,6-trinitrobenzene sulfonic acid, is also reduced severalfold. Our data showed that MsbA facilitates the rapid translocation of some lipids from the cytoplasmic to the periplasmic side of the inner membrane in living cells.     

Conserved mechanism of Oxa1 insertion into the mitochondrial inner membrane

Oxa1 is the mitochondrial representative of a family of related proteins that mediate the insertion of substrate proteins into the membranes of bacteria, chloroplasts, and mitochondria. Several studies have demonstrated that the bacterial homologue YidC participates both in the direct uptake of proteins from the bacterial cytosol, and in the uptake of nascent proteins from the Sec translocase. Studies on the biogenesis of membrane proteins in mitochondria established that Oxa1 has the capability to receive substrates at the inner surface of the inner membrane. In this study, we asked if Oxa1 may similarly cooperate with a protein translocase within the membrane. Since Oxa1 is involved in its own biogenesis, we used the precursor of Oxa1 as a model protein and investigated its import pathway. We found that immediately after import into mitochondria, Oxa1 initially accumulates at Tim23 that forms the inner membrane protein translocase. Cleavage of the Oxa1 presequence is dependent on mtHsp70, a heat shock protein of the mitochondrial matrix. However, mutant mtHsp70 showing a defect in the release of bound substrate proteins does not interfere with subsequent membrane insertion, indicating that membrane insertion of the mature protein is essentially mtHsp70-independent. We conclude that Oxa1 has the ability to accept preproteins within the membrane.     

Experimentally based topology models for E. coli inner membrane proteins

Membrane protein topology predictions can be markedly improved by the inclusion of even very limited experimental information. We have recently introduced an approach for the production of reliable topology models based on a combination of experimental determination of the location (cytoplasmic or periplasmic) of a protein's C terminus and topology prediction. Here, we show that determination of the location of a protein's C terminus, rather than some internal loop, is the best strategy for large-scale topology mapping studies. We further report experimentally based topology models for 31 Escherichia coli inner membrane proteins, using methodology suitable for genome-scale studies.     

Energy requirements for protein translocation across the Escherichia coli inner membrane

Both ATP and an electrochemical potential play roles in translocating proteins across the inner membrane of Escherichia coli. Recent discoveries have dissected the overall transmembrane movement into separate subreactions with different energy requirements, identified a translocation ATPase, and reconstituted both energy-requiring steps of the reaction from purified components. A more refined understanding of the energetics of this fundamental process is beginning to provide answers about the basic issues of how proteins move across the hydrophobic membrane barrier.     

Mode of insertion of lipopolysaccharide into the outer membrane of escherichia coli.

A mutant of Escherichia coli that lacks uridine 5'-diphosphate galactose-4-epimerase makes lipopolysaccharide with less carbohydrate than the parent, unless galactose is present during growth. Carbohydrate is dense, and the outer membrane, which contains lipopolysaccharide, was found to be denser when isolated from cells grown with galactose then when galactose was omitted. Cells given galactose after growth in its absence rapidly formed dense regions within the outer membrane that disappeared when galactose was removed. These results indicate that lipopolysaccharide enters the outer membrane nonrandomly at a minimum of 10 to 22 discrete "insertion points." Isopycnic centrifugation provides a method for isolating these regions.