Transmembrane orientation of signal-anchor proteins is affected by the folding state but not the size of the N-terminal domain.

Upon insertion of a signal-anchor protein into the endoplasmic reticulum membrane, either the C-terminal or the N-terminal domain is translocated across the membrane. Charged residues flanking the transmembrane domain are important determinants for this decision, but are not necessarily sufficient to generate a unique topology. Using a model protein that is inserted into the membrane to an equal extent in either orientation, we have tested the influence of the size and the folding state of the N-terminal domain on the insertion process. A small zinc finger domain or the full coding sequence of dihydrofolate reductase were fused to the N-terminus. These stably folding domains hindered or even prevented their translocation. Disruption of their structure by destabilizing mutations largely restored transport across the membrane. Translocation efficiency, however, did not depend on the size of the N-terminal domain within a range of 40-237 amino acids. The folding behavior of the N-terminal domain is thus an important factor in the topogenesis of signal-anchor proteins.     

Chaperoning of insertion of membrane proteins into lipid bilayers by hemifluorinated surfactants: application to diphtheria toxin

Hemifluorinated compounds, such as HF-TAC, make up a novel class of nondetergent surfactants designed to keep membrane proteins soluble under nondissociating conditions [Breyton, C., et al. (2004) FEBS Lett. 564, 312]. Because fluorinated and hydrogenated chains do not mix well, supramicellar concentrations of these surfactants can coexist with intact lipid vesicles. To test the ability of HF-TAC to assist proper membrane insertion of proteins, we examined its effect on the pH-triggered insertion of the diphtheria toxin T-domain. The function of the T-domain is to translocate the catalytic domain across the lipid bilayer in response to acidification of the endosome. This translocation is accompanied by the formation of a pore, which we used as a measure of activity in a vesicle leakage assay. We have also used F?rster resonance energy transfer to follow the effect of HF-TAC on aggregation of aqueous and membrane-bound T-domain. Our data indicate that the pore-forming activity of the T-domain is affected by the dynamic interplay of two principal processes: productive pH-triggered membrane insertion and nonproductive aggregation of the aqueous T-domain at low pH. The presence of HF-TAC in the buffer is demonstrated to suppress aggregation in solution and ensure correct insertion and folding of the T-domain into the lipid vesicles, without solubilizing the latter. Thus, hemifluorinated surfactants stabilize the low-pH conformation of the T-domain as a water-soluble monomer while acting as low-molecular weight chaperones for its insertion into preformed lipid bilayers.     

Concerted protonation of key histidines triggers membrane interaction of the diphtheria toxin T domain

The translocation domain (T domain) of the diphtheria toxin contributes to the transfer of the catalytic domain from the cell endosome to the cytosol, where it blocks protein synthesis. Translocation is initiated when endosome acidification induces the interaction of the T domain with the membrane of the compartment. We found that the protonation of histidine side chains triggers the conformational changes required for membrane interaction. All histidines are involved in a concerted manner, but none is indispensable. However, the preponderance of each histidine varies according to the transition observed. The pair His(223)-His(257) and His(251) are the most sensitive triggers for the formation of the molten globule state in solution, whereas His(322)-His(323) and His(251) are the most sensitive triggers for membrane binding. Interestingly, the histidines are located at key positions throughout the structure of the protein, in hinges and at the interface between each of the three layers of helices forming the domain. Their protonation induces local destabilizations, disrupting the tertiary structure and favoring membrane interaction. We propose that the selection of histidine residues as triggers of membrane interaction enables the T domain to initiate translocation at the rather mild pH found in the endosome, contributing to toxin efficacy.     

Conformation of the diphtheria toxin T domain in membranes: a site-directed spin-labeling study of the TH8 helix and TL5 loop.

The isolated T domain of diphtheria toxin was mutated by cysteine-scanning mutagenesis at 28 consecutive sites (residues 328-355) that comprise the TH8 helix and the TL5 interhelical loop in the native toxin. After derivatizing the mutant proteins with a sulfhydryl-selective nitroxide reagent, we examined the mobility of each nitroxide and its accessibility to polar and nonpolar paramagnetic reagents, before and after insertion into phospholipid bilayers. The data obtained with the proteins in solution at pH 8 are generally consistent with predictions from the crystal structure of the toxin. Upon membrane binding at pH 4.6, a major structural reorganization of the domain was seen, which dramatically reduced the accessibility of most residues in this region to the polar reagent nickel(II)-ethylenediaminediacetate complex (NiEDDA). Many of these residues also showed reduced accessibility to the nonpolar reagent O(2). Periodic accessibility of the nitroxide side chains along the sequence to these reagents shows that TH8 remains largely helical in the membrane-bound state, with one surface associated with protein and the other facing the hydrophobic interior of the bilayer. In addition, the TL5 loop also appears to become alpha-helical in the membrane, with one surface in contact with protein and the other in contact with the bilayer interior. These findings provide a structural framework for understanding how the T domain forms a transmembrane channel and mediates translocation of diphtheria toxin's enzymic moiety across a membrane.     

Interaction of Escherichia coli hemolysin with biological membranes. A study using cysteine scanning mutagenesis.

Escherichia coli hemolysin (HlyA) is a membrane-permeabilizing protein belonging to the family of RTX-toxins. Lytic activity depends on binding of Ca2(+) to the C-terminus of the molecule. The N-terminus of HlyA harbors hydrophobic sequences that are believed to constitute the membrane-inserting domain. In this study, 13 HlyA cysteine-replacement mutants were constructed and labeled with the polarity-sensitive fluorescent probe 6-bromoacetyl-2-dimethylaminonaphthalene (badan). The fluorescence emission of the label was examined in soluble and membrane-bound toxin. Binding effected a major blue shift in the emission of six residues within the N-terminal hydrophobic domain, indicating insertion of this domain into the lipid bilayer. The emission shifts occurred both in the presence and absence of Ca2(+), suggesting that Ca2(+) is not required for the toxin to enter membranes. However, binding of Ca2(+) to HlyA in solution effected conformational changes in both the C-terminal and N-terminal domain that paralleled activation. Our data indicate that binding of Ca2(+) to the toxin in solution effects a conformational change that is relayed to the N-terminal domain, rendering it capable of adopting the structure of a functional pore upon membrane binding.     

Roles of Glu 349 and Asp 352 in membrane insertion and translocation by diphtheria toxin.

Acidic conditions within the endosomal lumen induce the T domain of receptor-bound diphtheria toxin (DT) to insert into the endosomal membrane and mediate translocation of the toxin's catalytic domain to the cytosol. A conformational rearrangement in the toxin occurring near pH5 allows a buried apolar helical hairpin of the native T domain (helices TH8 and TH9) to undergo membrane insertion. If the inserted hairpin spans the bilayer, as hypothesized, then the two acidic residues within the TL5 interhelical loop, Glu 349 and Asp 352, should become exposed at the neutral cytosolic face of the membrane and reionize. To investigate the roles of these residues in toxin action, we characterized mutant toxins in which one or both acidic residues had been replaced with nonionizable ones. Each of two double mutants examined showed a several-fold reduction in cytotoxicity in 24-h Vero cell assays (sixfold for E349A + D352A and fourfold for E349Q + D352N), whereas the individual E349Q and D352N mutations caused smaller reductions in toxicity. The single and double mutations also attenuated the toxin's ability to permeabilize Vero cells to Rb+ at low pH and decreased channel formation by the toxin in artificial planar bilayers. Neither of the double mutations affected the pH-dependence profile of the toxin's conformational rearrangement in solution, as measured by binding of the hydrophobic fluorophore, 2-p-toluidinyl-naphthalene 6-sulfonate. The results demonstrate that, although there is no absolute requirement for an acidic residue within the TL5 loop for toxicity, Glu 349 and Asp 352 do significantly enhance the biological activity of the protein. The data are consistent with a model in which ionization of these residues at the cytosolic face of the endosomal membrane stabilizes the TH8/TH9 hairpin in a transmembrane configuration, thereby facilitating channel formation and translocation of the toxin's catalytic chain.     

Accessibility changes within diphtheria toxin T domain upon membrane penetration probed by hydrogen exchange and mass spectrometry

The translocation domain of diphtheria toxin inserts in membrane and becomes functional when the pH inside endosomes is acid. At that stage, the domain is in a partially folded state; this prevents the use of high-resolution methods for the characterization of its functional structure. On that purpose, we report here the use of hydrogen/deuterium exchange experiments coupled to mass spectrometry. The conformation changes during the different steps of insertion into lipid bilayer are monitored with a resolution of few residues. Three parts of the translocation domain can be distinguished. With a high protection against exchange, the C-terminal hydrophobic helical hairpin is embedded in the membrane. Despite a lower protection, a significant effect in the presence of lipid vesicles shows that the N-terminal part is in interaction with the membrane interface. The sensitivity to the ionic strength indicates that electrostatic interactions are important for the binding. The middle part of the domain has an intermediate protection; this suggests that this part of the domain can be embedded within the membrane but remains quite dynamic. These results provide unprecedented insight into the structure reorganization of the protein to go from a soluble state to a membrane-inserted one.     

His1205 and His1223 are essential for the activity of the mitogenic Pasteurella multocida toxin

Pasteurella multocida produces a 146-kDa protein toxin (PMT), which activates multiple cellular signal-transduction pathways, resulting in the activation of PLCbeta, Rho, JNK, and ERK. In addition to an essential cysteine residue at position 1165, PMT contains several histidine residues in the catalytically important C-terminal part of the protein. To elucidate the role of the histidine residues, we treated PMT with the histidine-modifying substance diethyl pyrocarbonate (DEPC). DEPC inhibited PMT in a time- and concentration-dependent manner, suggesting that one or several histidine residues are essential for the biological activity of PMT. In experiments in which PMT was directly delivered into the cytosol of EBL cells by electroporation, we show that DEPC treatment inhibits the catalytically important histidine residues. Leucine substitutions of eight individual histidine residues in the C-terminal catalytic domain of PMT were constructed, and the effect on the biological activity of PMT was analyzed by determining PLCbeta, Rho, and ERK activation. Substitution of two histidine residues, H1205 and H1223, led to inactivation of the resulting PMT proteins, indicating that H1205 and H1223 play an important role in biological activity of the toxin. In addition, we show that the mutant toxins appear to be correctly folded, as judged by protease digestion. The precise function of H1205 and H1223 is not yet known. However, treatment of PMT with the cation chelating substance 1,10-phenantroline led to inactivation of the toxin, indicating that the essential histidine residues and cysteine 1165 might be involved in metal ion binding.     

M13 procoat protein insertion into YidC and SecYEG proteoliposomes and liposomes

M13 procoat protein was one of the first model proteins used to study bacterial membrane protein insertion. It contains a signal peptide of 23 amino acid residues and is not membrane targeted by the signal recognition particle. The translocation of its periplasmic domain is independent of the preprotein translocase (SecAYEG) but requires electrochemical membrane potential and the membrane insertase YidC of Escherichia coli. We show here that YidC is sufficient for efficient membrane insertion of the purified M13 procoat protein into energized YidC proteoliposomes. When no membrane potential is applied, the insertion is substantially reduced. Only in the presence of YidC, membrane insertion occurs if bilayer integrity is preserved and membrane potential is stable for more than 20 min. A mutant of the M13 procoat protein, H5EE, with two additional negatively charged residues in the periplasmic domain inserted into YidC proteoliposomes and SecYEG proteoliposomes with equal efficiencies. We conclude that the protein can use both the YidC-only pathway and the Sec pathway. This poses the questions of how procoat H5EE is inserted in vivo and how insertion pathways are selected in the cell.     

Membrane protein insertion regulated by bringing electrostatic and hydrophobic interactions into play. A case study with the translocation domain of diphtheria toxin

The study of the membrane insertion of the translocation domain of diphtheria toxin deepens our insight into the interactions between proteins and membranes. During cell intoxication, this domain undergoes a change from a soluble and folded state at alkaline pH to a functional membrane-inserted state at acid pH. We found that hydrophobic and electrostatic interactions occur in a sequential manner between the domain and the membrane during the insertion. The first step involves hydrophobic interactions by the C-terminal region. This is because of the pH-induced formation of a molten globule specialized for binding to the membrane. Accumulation of this molten globule follows a precise molecular mechanism adapted to the toxin function. The second step, as the pH decreases, leads to the functional inserted state. It arises from the changes in the balance of electrostatic attractions and repulsions between the N-terminal part and the membrane. Our study shows how the structural changes and the interaction with membranes of the translocation domain are finely tuned by pH changes to take advantage of the cellular uptake system.     

Oligomerization of membrane-bound diphtheria toxin (CRM197) facilitates a transition to the open form and deep insertion

Diphtheria toxin (DT) contains separate domains for receptor-specific binding, translocation, and enzymatic activity. After binding to cells, DT is taken up into endosome-like acidic compartments where the translocation domain inserts into the endosomal membrane and releases the catalytic domain into the cytosol. The process by which the catalytic domain is translocated across the endosomal membrane is known to involve pH-induced conformational changes; however, the molecular mechanisms are not yet understood, in large part due to the challenge of probing the conformation of the membrane-bound protein. In this work neutron reflection provided detailed conformational information for membrane-bound DT (CRM197) in situ. The data revealed that the bound toxin oligomerizes with increasing DT concentration and that the oligomeric form (and only the oligomeric form) undergoes a large extension into solution with decreasing pH that coincides with deep insertion of residues into the membrane. We interpret the large extension as a transition to the open form. These results thus indicate that as a function of bulk DT concentration, adsorbed DT passes from an inactive state with a monomeric dimension normal to the plane of the membrane to an active state with a dimeric dimension normal to the plane of the membrane.     

Reversible refolding of the diphtheria toxin T-domain on lipid membranes

The catalytic domain of diphtheria toxin (DT) is translocated across endosomal membranes by the T-domain (DTT) in response to acidification. Understanding the energetics of translocation, besides clarifying the mechanism of translocation, should provide insights into general principles of membrane protein stability and assembly. As a first step, we have evaluated the energetics of DTT binding to lipid vesicles using three single-cysteine mutants (L350C, Q369C, and Y280C) labeled with a 7-nitrobenz-2-oxa-1,3-diazol-4-yl (NBD) fluorophore sensitive to polarity changes. Remarkably strong association with the vesicles was detected for all mutants, even at pH 7 at which DTT is believed to be in a fully folded membrane-incompetent state. Lowering the pH in the presence of anionic membranes resulted in a strong but reversible increase in emission of NBD-labeled mutants, consistent with reversible membrane insertion. This reversibility permitted free energies of DTT interactions with vesicles to be determined for the first time. Free energy values for the three mutants ranged from -8 to -10 kcal mol(-1) at pH 4.3 and from -7 to -8 kcal mol(-1) at pH 7. Insights into the disposition of DTT on membranes were obtained using a novel hydropathy analysis that considers the relative free energies of transmembrane and interfacial interactions as a function of pH. This analysis suggests that interactions at the membrane interface dominate pH-triggered insertion of DTT, implying that the folding pathway involves interfacial intermediates.     

Identification and characterization of the Pasteurella multocida toxin translocation domain

The Pasteurella multocida toxin (PMT) is a potent mitogen which enters the cytosol of eukaryotic cells via a low pH membrane translocation event. In common with the Escherichia coli cytotoxic necrotizing factor 1 (CNF1), the core of the PMT translocation domain is composed of two predicted hydrophobic helices (H1 - residues 402-423, H2 - 437-457) linked by a hydrophilic loop (PMT-TL - 424-436). The peptide loop contains three acidic residues (D425, D431 and E434), which may play a role equivalent to D373, D379 and E382/383 in CNF1. To test this hypothesis, a series of point mutants was generated in which acidic residues were mutated into the permanently charged positive residue lysine. Individual mutation of D425, D431 and E434 each caused a four- to sixfold reduction in toxin activity. Interestingly, mutation of D401 located immediately outside the predicted helix-loop-helix motif completely abolished toxin activity. Individual mutations did not affect cell binding nor greatly altered toxin structure, but did prevent translocation of the surface-bound proteins into the cytosol after a low pH pulse. Moreover, we demonstrate using an in vitro assay that PMT undergoes a pH-dependent membrane insertion.     

Roles of the C-terminal end of SecY in protein translocation and viability of Escherichia coli

SecY, a central component of the membrane-embedded sector of protein translocase, contains six cytosolic domains. Here, we examined the importance of the C-terminal cytosolic region of SecY by systematically shortening the C-terminal end and examining the functional consequences of these mutations in vivo and in vitro. It was indicated that the C-terminal five residues are dispensable without any appreciable functional defects in SecY. Mutants missing the C-terminal six to seven residues were partially compromised, especially at low temperature or in the absence of SecG. In vitro analyses indicated that the initial phase of the translocation reaction, in which the signal sequence region of the preprotein is inserted into the membrane, was affected by the lack of the C-terminal residues. SecA binding was normal, but SecA insertion in response to ATP and a preprotein was impaired. It is suggested that the C-terminal SecY residues are required for SecA-dependent translocation initiation.     

Analyzing topography of membrane-inserted diphtheria toxin T domain using BODIPY-streptavidin: at low pH, helices 8 and 9 form a transmembrane hairpin but helices 5-7 form stable nonclassical inserted segments on the cis side of the bilayer

Low pH-induced membrane insertion by diphtheria toxin T domain is crucial for A chain translocation into the cytoplasm. To define the membrane topography of the T domain, the exposure of biotinylated Cys residues to the cis and trans bilayer surfaces was examined using model membrane vesicles containing a deeply inserted T domain. To do this, the reactivity of biotin with external and vesicle-entrapped BODIPY-labeled streptavidin was measured. The T domain was found to insert with roughly 70-80% of the molecules in the physiologically relevant orientation. In this orientation, residue 349, located in the loop between hydrophobic helices 8 and 9, was exposed to the trans side of the bilayer, while other solution-exposed residues along the hydrophobic helices 5-9 region of the T domain located near the cis surface. A protocol developed to detect the movement of residues back and forth across the membranes demonstrated that T domain sequences did not rapidly equilibrate between the cis and the trans sides of the bilayer. Binding streptavidin to biotinylated residues prior to membrane insertion only inhibited T domain pore formation for residues in the loop between helices 8 and 9. Pore formation experiments used an approach avoiding interference from transient membrane defects/leakage that may occur upon the initial insertion of protein. Combined, these results indicate that at low pH hydrophobic helices 8 and 9 form a transmembrane hairpin, while hydrophobic helices 5-7 form a nonclassical deeply inserted nontransmembraneous state. We propose that this represents a novel pre-translocation state that is distinct from a previously defined post-translocation state.     

Biogenesis of Tim23 and Tim17, integral components of the TIM machinery for matrix-targeted preproteins.

We analysed the import pathway of Tim23 and of Tim17, components of the mitochondrial import machinery for matrix-targeted preproteins. Tim23 contains two independent import signals. One is located within the first 62 amino acid residues of the hydrophilic domain that, in the assembled protein, is exposed to the intermembrane space. This signal mediates translocation of Tim23 across the outer membrane independently of the membrane potential, DeltaPsi. A second import signal is located in the C-terminal membrane-integrated portion of Tim23. It mediates translocation across the outer membrane and insertion into the inner membrane in a strictly DeltaPsi-dependent fashion. Structurally, Tim17 is related to Tim23 but lacks a hydrophilic domain. It contains an import signal in the C-terminal half and its import requires DeltaPsi. The DeltaPsi-dependent import signals of Tim23 and Tim17 are located at corresponding sites in these two homologous proteins. They exhibit features reminiscent of the positively charged N-terminal presequences of matrix-targeted precursors. Import of Tim23 and its insertion into the inner membrane requires Tim22 but not functional Tim23. Thus, biogenesis of the Tim23.17 complex depends on the Tim22 complex, which is the translocase identified as mediating the import of carrier proteins.     

Defining the regions of Escherichia coli YidC that contribute to activity

The YidC/Oxa1/Alb3 family of proteins catalyzes membrane protein insertion in bacteria, mitochondria, and chloroplasts. In this study, we investigated which regions of the bacterial YidC protein are important for its function in membrane protein biogenesis. In Escherichia coli, YidC spans the membrane six times, with a large 319-residue periplasmic domain following the first transmembrane domain. We found that this large periplasmic domain is not required for YidC function and that the residues in the exposed hydrophilic loops or C-terminal tail are not critical for YidC activity. Rather, the five C-terminal transmembrane segments that contain the three consensus sequences in the YidC/Oxa1/Alb3 family are important for its function. However, by systematically replacing all the residues in transmembrane segment (TM) 2, TM3, and TM6 with serine and by swapping TM4 and TM5 with unrelated transmembrane segments, we show that the precise sequence of these transmembrane regions is not essential for in vivo YidC activity. Single serine mutations in TM2, TM3, and TM6 impaired the membrane insertion of the Sec-independent procoat-leader peptidase protein. We propose that the five C-terminal transmembrane segments of YidC function as a platform for the translocating substrate protein to support its insertion into the membrane.     

Acid-triggered membrane insertion of Pseudomonas exotoxin A involves an original mechanism based on pH-regulated tryptophan exposure

Exposure to low endosomal pH during internalization of Pseudomonas exotoxin A (PE) triggers membrane insertion of its translocation domain. This process is a prerequisite for PE translocation to the cytosol where it inactivates protein synthesis. Although hydrophobic helices enable membrane insertion of related bacterial toxins such as diphtheria toxin, the PE translocation domain is devoid of hydrophobic stretches and the structural features triggering acid-induced membrane insertion of PE are not known. Here we have identified a molecular device that enables PE membrane insertion. This process is promoted by exposure of a key tryptophan residue. At neutral pH, this Trp is buried in a hydrophobic pocket closed by the smallest alpha-helix of the translocation domain. Upon acidification, protonation of the Asp that is the N-cap residue of the helix leads to its destabilization, enabling Trp side chain insertion into the endosome membrane. This tryptophan-based membrane insertion system is surprisingly similar to the membrane-anchoring mechanism of human annexin-V and could be used by other proteins as well.     

Biophysical and biological studies of end-group-modified derivatives of Pep-1

Pep-1 is a tryptophane-rich cell-penetrating peptide (CPP) that has been previously proposed to bind protein cargoes by hydrophobic assembly and translocate them across cellular membranes. To date, however, the molecular mechanisms responsible for cargo binding and translocation have not been clearly identified. This study was conducted to gain insight into the interaction between Pep-1 with its cargo and the biological membrane to identify the thereby involved structural elements crucial for translocation. We studied three peptides differing in their N- and C-termini: (i) Pep-1, carrying an acetylated N-terminus and a C-terminal cysteamine elongation, (ii) AcPepWAmide, with an acetylated N-terminus and an amidated C-terminus, and (iii) PepW, with two free termini. Thioredoxin (TRX) and beta-galactosidase were used as protein cargoes. To study CPP-membrane interactions, we performed biophysical as well as biological assays. To mimic biological membranes, we used phospholipid liposomes in a dye leakage assay and surfactant micelles for high-resolution NMR studies. In addition, membrane integrity, cell viability, and translocation efficiency were analyzed in HeLa cells. An alpha-helical structure was found for all peptides in the hydrophobic N-terminal region encompassing residues 4-13, whereas the hydrophilic region remained unstructured in the presence of micelles. Our results show that the investigated peptides interacted with the micelles as well as with the protein cargo via their tryptophan-rich domain. All peptides displayed an orientation parallel to the micelle surface. The C-terminal cysteamine group formed an additional membrane anchor, leading to more efficient translocation properties in cells. No membrane permeabilization was observed, and our data were largely compatible with an endocytic pathway for cellular uptake.     

Ion selectivity of the anthrax toxin channel and its effect on protein translocation

Anthrax toxin consists of three ∼85-kD proteins: lethal factor (LF), edema factor (EF), and protective antigen (PA). PA₆₃ (the 63-kD, C-terminal portion of PA) forms heptameric channels ((PA₆₃)₇) in planar phospholipid bilayer membranes that enable the translocation of LF and EF across the membrane. These mushroom-shaped channels consist of a globular cap domain and a 14-stranded β-barrel stem domain, with six anionic residues lining the interior of the stem to form rings of negative charges. (PA₆₃)₇ channels are highly cation selective, and, here, we investigate the effects on both cation selectivity and protein translocation of mutating each of these anionic residues to a serine. We find that although some of these mutations reduce cation selectivity, selectivity alone does not directly predict the rate of protein translocation; local changes in electrostatic forces must be considered as well.