Energy-transducing thylakoid membranes remain highly impermeable to ions during protein translocation

The polar amino acid in the TatA transmembrane helix is not strictly necessary for protein function

The twin-arginine translocation (Tat) pathway utilizes the proton-motive force (pmf) to transport folded proteins across cytoplasmic membranes in bacteria and archaea, as well as across the thylakoid membrane in plants and the inner membrane in mitochondria. In most species, the minimal components required for Tat activity consist of three subunits, TatA, TatB, and TatC. Previous studies have shown that a polar amino acid is present at the N-terminus of the TatA transmembrane helix (TMH) across many different species. In order to systematically assess the functional importance of this polar amino acid in the TatA TMH in Escherichia coli, we examined a complete set of 19-amino-acid substitutions. Unexpectedly, although being preferred overall, our experiments suggest that the polar amino acid is not necessary for a functional TatA. Hydrophilicity and helix-stabilizing properties of this polar amino acid were found to be highly correlated with the Tat activity. Specifically, change in charge status of the amino acid side chain due to pH resulted in a shift in hydrophilicity, which was demonstrated to impact the Tat transport activity. Furthermore, we identified a four-residue motif at the N-terminus of the TatA TMH by sequence alignment. Using a biochemical approach, we found that the N-terminal motif was functionally significant, with evidence indicating a potential role in the preference for utilizing different pmf components. Taken together, these findings yield new insights into the functionality of TatA and its potential role in the Tat transport mechanism.

New insights into the Tat protein transport cycle from characterizing the assembled Tat translocon

The twin-arginine protein translocation (Tat) system transports folded proteins across the bacterial cytoplasmic membrane and the thylakoid membrane of chloroplasts. The Tat translocation site is transiently assembled by the recruitment of multiple TatA proteins to a substrate-activated TatBC receptor complex in a process requiring the protonmotive force. The ephemeral nature of the Tat translocation site has so far precluded its isolation. We now report that detergent solubilization of membranes during active transport allows the recovery of receptor complexes that are associated with elevated levels of TatA. We apply this biochemical analysis in combination with live cell fluorescence imaging to Tat systems trapped in the assembled state. We resolve sub-steps in the Tat translocation cycle and infer that TatA assembly precedes the functional interaction of TatA with a polar cluster site on TatC. We observe that dissipation of the protonmotive force releases TatA oligomers from the assembled translocation site demonstrating that the stability of the TatA oligomer does not depend on binding to the receptor complex and implying that the TatA oligomer is assembled at the periphery of the receptor complex. This work provides new insight into the Tat transport cycle and advances efforts to isolate the active Tat translocon.

Oligomerization state of the functional bacterial twin-arginine translocation (Tat) receptor complex

The twin-arginine translocation (Tat) system transports folded proteins across bacterial and plastid energy transducing membranes. Ion leaks are generally considered to be mitigated by the creation and destruction of the translocation conduit in a cargo-dependent manner, a mechanism that enables tight sealing around a wide range of cargo shapes and sizes. In contrast to the variable stoichiometry of the active translocon, the oligomerization state of the receptor complex is considered more consistently stable but has proved stubbornly difficult to establish. Here, using a single molecule photobleaching analysis of individual inverted membrane vesicles, we demonstrate that Tat receptor complexes are tetrameric in native membranes with respect to both TatB and TatC. This establishes a maximal diameter for a resting state closed pore. A large percentage of Tat-deficient vesicles explains the typically low transport efficiencies observed. This individual reaction chamber approach will facilitate examination of the effects of stochastically distributed molecules.

Membrane Translocation of Folded Proteins

An ever-increasing number of proteins have been shown to translocate across various membranes of bacterial as well as eukaryotic cells in their folded states as a part of physiological and/or pathophysiological processes. Herein we provide an overview of the systems/processes that are established or likely to involve the membrane translocation of folded proteins, such as protein export by the twin-arginine translocation (TAT) system in bacteria and chloroplasts, unconventional protein secretion (UPS) and protein import into the peroxisome in eukaryotes, and the cytosolic entry of proteins (e.g., bacterial toxins) and viruses into eukaryotes. We also discuss the various mechanistic models that have previously been proposed for the membrane translocation of folded proteins including pore/channel formation, local membrane disruption, membrane thinning, and transport by membrane vesicles. Finally, we introduce a newly discovered vesicular transport mechanism, vesicle budding and collapse (VBC), and present evidence that VBC may represent a unifying mechanism that drives some (and potentially all) of folded protein translocation processes.

Hydrophobic mismatch is a key factor in protein transport across lipid bilayer membranes via the Tat pathway

The twin-arginine translocation (Tat) pathway transports folded proteins across membranes in bacteria, thylakoids, plant mitochondria, and archaea. In most species, the active Tat machinery consists of three independent subunits: TatA, TatB, and TatC. TatA and TatB possess short transmembrane alpha helices (TMHs), both of which are only 15 residues long in Escherichia coli. Such short TMHs cause a hydrophobic mismatch between Tat subunits and the membrane bilayer, although the functional significance of this mismatch is unclear. Here, we sought to address the functional importance of the hydrophobic mismatch in the Tat transport mechanism in E. coli. We conducted three different assays to evaluate the effect of TMH length mutants on Tat activity and observed that the TMHs of TatA and TatB appear to be evolutionarily tuned to 15 amino acids, with activity dropping off following any modification of this length. Surprisingly, TatA and TatB with as few as 11 residues in their TMHs can still insert into the membrane bilayer, albeit with a decline in membrane integrity. These findings support a model of Tat transport utilizing localized toroidal pores that form when the membrane bilayer is thinned to a critical threshold. In this context, we conclude that the 15-residue length of the TatA and TatB TMHs can be seen as a compromise between the need for some hydrophobic mismatch to allow the membrane to reversibly reach the threshold thinness required for toroidal pore formation and the permanently destabilizing effect of placing even shorter helices into these energy-transducing membranes.

Electrochromic shift supports the membrane destabilization model of Tat-mediated transport and shows ion leakage during Sec transport

The mechanism and pore architecture of the Tat complex during transport of folded substrates remain a mystery, partly due to rapid dissociation after translocation. In contrast, the proteinaceous SecY pore is a persistent structure that needs only to undergo conformational shifts between "closed" and "opened" states when translocating unfolded substrate chains. Where the proteinaceous pore model describes the SecY pore well, the toroidal pore model better accounts for the high-energy barrier that must be overcome when transporting a folded substrate through the hydrophobic bilayer in Tat transport. Membrane conductance behavior can, in principle, be used to distinguish between toroidal and proteinaceous pores, as illustrated in the examination of many antimicrobial peptides as well as mitochondrial Bax and Bid. Here, we measure the electrochromic shift (ECS) decay as a proxy for conductance in isolated thylakoids, both during protein transport and with constitutively assembled translocons. We find that membranes with the constitutively assembled Tat complex and those undergoing Tat transport display conductance characteristics similar to those of resting membranes. Membranes undergoing Sec transport and those with the substrate-engaged SecY pore result in significantly more rapid electric field decay. The responsiveness of the ECS signal in membranes with active SecY recalls the steep relationship between applied voltage and conductance in a proteinaceous pore, while the nonaccelerated electric field decay with both Tat transport and the constitutive Tat complex under the same electric field is consistent with the behavior of a toroidal pore.

Structural considerations of folded protein import through the chloroplast TOC/TIC translocons

Protein import into chloroplasts is carried out by the protein translocons at the outer and inner envelope membranes (TOC and TIC). Detailed structures for these translocons are lacking, with only a low-resolution TOC complex structure available. Recently, we showed that the TOC/TIC translocons can import folded proteins, a rather unique feat for a coupled double membrane system. We also determined the maximum functional TOC/TIC pore size to be 30-35 Å. Here, we discuss how such large pores could form and compare the structural dynamics of the pore-forming Toc75 subunit to its bacterial/mitochondrial Omp85 family homologs. We put forward structural models that can be empirically tested and also briefly review the pore dynamics of other protein translocons with known structures.

Evaluating the Functional Pore Size of Chloroplast TOC and TIC Protein Translocons: Import of Folded Proteins

The degree of residual structure retained by proteins while passing through biological membranes is a fundamental mechanistic question of protein translocation. Proteins are generally thought to be unfolded while transported through canonical proteinaceous translocons, including the translocons of the outer and inner chloroplast envelope membranes (TOC and TIC). Here, we readdressed the issue and found that the TOC/TIC translocons accommodated the tightly folded dihydrofolate reductase (DHFR) protein in complex with its stabilizing ligand, methotrexate (MTX). We employed a fluorescein-conjugated methotrexate (FMTX), which has slow membrane transport rates relative to unconjugated MTX, to show that the rate of ligand accumulation inside chloroplasts is faster when bound to DHFR that is actively being imported. Stromal accumulation of FMTX is ATP dependent when DHFR is actively being imported but is otherwise ATP independent, again indicating DHFR/FMTX complex import. Furthermore, the TOC/TIC pore size was probed with fixed-diameter particles and found to be greater than 25.6 Å, large enough to support folded DHFR import and also larger than mitochondrial and bacterial protein translocons that have a requirement for protein unfolding. This unique pore size and the ability to import folded proteins have critical implications regarding the structure and mechanism of the TOC/TIC translocons.

Dual Protein Localization to the Envelope and Thylakoid Membranes Within the Chloroplast

The chloroplast houses various metabolic processes essential for plant viability. This organelle originated from an ancestral cyanobacterium via endosymbiosis and maintains the three membranes of its progenitor. Among them, the outer envelope membrane functions mainly in communication with cytoplasmic components while the inner envelope membrane houses selective transport of various metabolites and the biosynthesis of several compounds, including membrane lipids. These two envelope membranes also play essential roles in import of nuclear-encoded proteins and in organelle division. The third membrane, the internal membrane system known as the thylakoid, houses photosynthetic electron transport and chemiosmotic phosphorylation. The inner envelope and thylakoid membranes share similar lipid composition. Specific targeting pathways determine their defined proteomes and, thus, their distinct functions. Nonetheless, several proteins have been shown to exist in both the envelope and thylakoid membranes. These proteins include those that play roles in protein transport, tetrapyrrole biosynthesis, membrane dynamics, or transport of nucleotides or inorganic phosphate. In this review, we summarize the current knowledge about proteins localized to both the envelope and thylakoid membranes in the chloroplast, discussing their roles in each membrane and potential mechanisms of their dual localization. Addressing the unanswered questions about these dual-localized proteins should help advance our understanding of chloroplast development, protein transport, and metabolic regulation.

Role of vesicle-inducing protein in plastids 1 in cpTat transport at the thylakoid

VIPP1 has been shown to be required for the proper formation of thylakoid membranes. However, studies on VIPP1 itself, as well as on PspA, its bacterial homolog, suggests that this protein may be involved in a number of additional functions, including protein translocation. The role of VIPP1 in protein translocation in the chloroplast has not been investigated. To this end, we conducted in vitro thylakoid protein transport assays to look at the effect of VIPP1 on the cpTat pathway, which is one of three translocation pathways found in both the chloroplast and its bacterial progenitor. We found that VIPP1 does indeed enhance protein transport through the cpTat pathway by up to 100%. The VIPP1 effect on cpTat activity occurs without interacting with the substrates or components of the translocon, and does not alter the energy potentials driving this translocation pathway. Instead, VIPP1 greatly enhances the amount of substrate bound productively to the thylakoids. Moreover, the presence of increasing VIPP1 concentrations in the reactions resulted in greater interactions between thylakoid membranes. Taken together, these results demonstrate a stimulatory role for VIPP1 in cpTat transport by enhancement of substrate binding, probably to the membrane lipid regions of the thylakoid. We propose a model in which VIPP1 facilitates reorganization of the thylakoid structure to increase substrate access to productive binding regions of the membrane as an early step in the cpTat pathway.

Measurement of the ΔpH and electric field developed across Arabidopsis thylakoids in the light

Measurement of the different components of the proton motive force (pmf) gives information about the coupling of proton movement within thylakoids to chemiosmotic processes such as photophosphorylation and protein transport, as well as that relating to the overall quality of a thylakoid preparation. The techniques to assess the pmf have been known for many years, as they have been applied to the most popular model plants for photosynthetic research. The emergence of Arabidopsis thaliana as the prominent model plant in developmental and genetics research prompted us to apply these techniques to thylakoids isolated from Arabidopsis chloroplasts. We describe here two spectroscopic techniques to measure the transmembrane pH gradient and electric field developed in the light in Arabidopsis thylakoids.

Clustering of C-terminal stromal domains of Tha4 homo-oligomers during translocation by the Tat protein transport system

The chloroplast Twin arginine translocation (Tat) pathway uses three membrane proteins and the proton gradient to transport folded proteins across sealed membranes. Precursor proteins bind to the cpTatC-Hcf106 receptor complex, triggering Tha4 assembly and protein translocation. Tha4 is required only for the translocation step and is thought to be the protein-conducting component. The organization of Tha4 oligomers was examined by substituting pairs of cysteine residues into Tha4 and inducing disulfide cross-links under varying stages of protein translocation. Tha4 formed tetramers via its transmembrane domain in unstimulated membranes and octamers in membranes stimulated by precursor and the proton gradient. Tha4 formed larger oligomers of at least 16 protomers via its carboxy tail, but such C-tail clustering only occurred in stimulated membranes. Mutational studies showed that transmembrane domain directed octamers as well as C-tail clusters require Tha4's transmembrane glutamate residue and its amphipathic helix, both of which are necessary for Tha4 function. A novel double cross-linking strategy demonstrated that both transmembrane domain directed- and C-tail directed oligomerization occur in the translocase. These results support a model in which Tha4 oligomers dock with a precursor-receptor complex and undergo a conformational switch that results in activation for protein transport. This possibly involves accretion of additional Tha4 into a larger transport-active homo-oligomer.

The chloroplast Tat pathway transports substrates in the dark

Photosynthetic electron transport pumps protons into the thylakoid lumen, creating an electrochemical potential called the protonmotive force (PMF). The energy of the thylakoid PMF is utilized by such machinery as the chloroplast F(0)F(1)-ATPase as well as the chloroplast Tat (cpTat) pathway (a protein transporter) to do work. The bulk phase thylakoid PMF decays rapidly after the termination of actinic illumination, and it has been well established via potentiometric measurements that there is no detectable electrical or chemical potential in the thylakoid after a brief time in the dark. Yet, we report herein that cpTat transport can occur for long periods in the dark. We show that the thylakoid PMF is actually present long after actinic illumination of the thylakoids ceases and that this energy is present in physiologically useful quantities. Consistent with previous studies, the dark-persisting thylakoid potential is not detectable by established indicators. We propose that cpTat transport in the dark is dependent on a pool of protons in the thylakoid held out of equilibrium with those in the bulk aqueous phase.

The chloroplast Tat pathway utilizes the transmembrane electric potential as an energy source

The thylakoid membrane, located inside the chloroplast, requires proteins transported across it for plastid biogenesis and functional photosynthetic electron transport. The chloroplast Tat translocator found on thylakoids transports proteins from the plastid stroma to the thylakoid lumen. Previous studies have shown that the chloroplast Tat pathway is independent of NTP hydrolysis as an energy source and instead depends on the thylakoid transmembrane proton gradient to power protein translocation. Because of its localization on the same membrane as the proton motive force-dependent F(0)F(1) ATPase, we believed that the chloroplast Tat pathway also made use of the thylakoid electric potential for transporting substrates. By adjusting the rate of photosynthetic proton pumping and by utilizing ionophores, we show that the chloroplast Tat pathway can also utilize the transmembrane electric potential for protein transport. Our findings indicate that the chloroplast Tat pathway is likely dependent on the total protonmotive force (PMF) as an energy source. As a protonmotive-dependent device, certain predictions can be made about structural features expected to be found in the Tat translocon, specifically, the presence of a proton well, a device in the membrane that converts electrical potential into chemical potential.

Evidence for a dynamic and transient pathway through the TAT protein transport machinery

Tat systems transport completely folded proteins across ion-tight membranes. Three membrane proteins comprise the Tat machinery in most systems. In thylakoids, cpTatC and Hcf106 mediate precursor recognition, whereas Tha4 facilitates translocation. We used chimeric precursor proteins with unstructured peptides and folded domains to test predictions of competing translocation models. Two models invoke protein-conducting channels, whereas another model proposes that cpTatC pulls substrates through a patch of Tha4 on the lipid bilayer. The thylakoid system transported unstructured peptide substrates alone or when fused to folded domains. However, larger substrates stalled before completion, some with amino- and carboxyl-folded domains on opposite sides of the membrane. The length of the precursor that resulted in translocation arrest (20 to 30 nm) exceeded that expected for a single 'pull' mechanism, suggesting that a sustained driving force rather than a single pull moves the protein across the bilayer. Three different methods showed that stalled substrates were not stuck in a channel or even associated with Tat machinery. This finding favors the Tha4 patch model for translocation.

Current concepts in nuclear pore electrophysiology

Over 4 decades ago, microelectrode studies of in situ nuclei showed that, under certain conditions, the nuclear envelope (NE) behaves as a barrier opposing the nucleocytoplasmic flow of physiological ions. As the nuclear pore complexes (NPCs) of the NE are the only pathways for direct nucleocytoplasmic flow, those experiments implied that the NPCs are capable of restricting ion flow. These early studies validated electrophysiology as a useful approach to quantify some of the mechanisms by which NPCs mediate gene activity and expression. Since electron microscopy (EM) and other non-electrophysiological investigations, showed that the NPC lumen is a nanochannel, the opinion prevailed that the NPC could not oppose the flow of ions and, therefore, that electrophysiological observations resulted from technical artifacts. Consequently, the initial enthusiasm with nuclear electrophysiology faded out in less than a decade. In 1990, nuclear electrophysiology was revisited with patch-clamp, the most powerful electrophysiological technique to date. Patch-clamp has consistently demonstrated that the NE has intrinsic ion channel activity. Direct demonstrations of the NPC on-off ion channel gating behavior were published for artificial conditions in 1995 and for intact living nuclei in 2002. This on-off switching/gating behavior can be interpreted in terms of a metastable energy barrier. In the hope of advancing nuclear electrophysiology, and to complement the other papers contained in this special issue of the journal, here I review some of the main technical, experimental, and theoretical issues of the field, with special focus on NPCs.

Oligomers of Tha4 organize at the thylakoid Tat translocase during protein transport

The Tat (twin arginine translocation) systems of thylakoids and bacteria transport fully folded protein substrates without breaching the permeability barrier of the membrane. Two components of the thylakoid system, cpTatC and Hcf106, compose a precursor-bound receptor complex. The third component, Tha4, assembles with the precursor-bound receptor complex for the translocation step and is thought to compose at least part of the protein-conducting channel. Here, we used two different cross-linking approaches to explore the organization of Tha4 in the translocase. These cross-linking techniques showed that transition to an active protein transport state resulted in an alignment of the Tha4 amphipathic helix and C-terminal tail domains to form Tha4 oligomers. Oligomerization required functional Tha4, a twin arginine signal peptide, and an active cpTatC-Hcf106 receptor complex. The spectrum of oligomers obtained was independent of the mature folded domain of the precursor. We propose a trapdoor mechanism for translocation whereby aligned oligomers of Tha4 amphipathic helices fold into the membrane to allow formfitting passage of precursor proteins.

An alternative model of the twin arginine translocation system

The twin arginine translocation (Tat) system is a machinery which can translocate folded proteins across energy transducing membranes. Currently it is supposed that Tat substrates bind directly to Tat translocon components before a ApH-driven translocation occurs. In this review, an alternative model is presented which proposes that membrane integration could precede Tat-dependent translocation. This idea is mainly supported by the recent observations of Tat-independent membrane insertion of Tat substrates in vivo and in vitro. Membrane insertion may allow i) a quality control of the folded state by membrane bound proteases like FtsH, ii) the recognition of the membrane spanning signal peptide by Tat system components, and iii) a pulling mechanism of translocation. In some cases of folded Tat substrates, the membrane targeting process may require ATP-dependent N-terminal unfolding-steps.

Protein transport via the cpTat pathway displays cooperativity and is stimulated by transport-incompetent substrate

Kinetic analyses of cpTat-mediated protein transport across the thylakoid membrane were conducted, revealing three important characteristics of this translocation pathway. First, transport via the cpTAT system displays a non-Michaelis-Menten, sigmoidal rate-substrate relationship with an apparent Hill coefficient of 1.80, indicative of positive homotropic cooperativity. Second, the presence of transport-incompetent substrates was found not to competitively inhibit the translocation of transport-competent substrates. However, the presence of low concentrations of transport-incompetent protein enhances the transport of wild type substrate. Together, these findings suggest that interaction between Tat machinery components and both transport-competent and transport-incompetent protein may elicit a cooperative effect on the translocation rate.

Thylakoid targeting of Tat passenger proteins shows no delta pH dependence in vivo

The Tat pathway is a major route for protein export in prokaryotes and for protein targeting to thylakoids in chloroplasts. Based on in vitro studies, protein translocation through this pathway is thought to be strictly dependent on a transmembrane delta pH. In this paper, we assess the delta pH sensitivity of the Tat pathway in vivo. Using Chlamydomonas reinhardtii, we observed changes in the efficiency of thylakoid targeting in vivo by mutating the Tat signal of the Rieske protein. We then employed two endogenous pH probes located on the lumen side of the thylakoid membranes to estimate spectroscopically the delta pH in vivo. Using experimental conditions in which the trans-thylakoid delta pH was almost zero, we found no evidence for a delta pH dependence of the Tat pathway in vivo. We confirmed this observation in higher plants using attached barley leaves. We conclude that the Tat pathway does not require a delta pH under physiological conditions, but becomes delta pH sensitive when probed in vitro/in organello because of the loss of some critical intracellular factors.

Energetics of protein transport across biological membranes. a study of the thylakoid DeltapH-dependent/cpTat pathway

Among the pathways for protein translocation across biological membranes, the DeltapH-dependent/Tat system is unusual in its sole reliance upon the transmembrane pH gradient to drive protein transport. The free energy cost of protein translocation via the chloro-plast DeltapH-dependent/Tat pathway was measured by conducting in vitro transport assays with isolated thylakoids while concurrently monitoring energetic parameters. These experiments revealed a substrate-specific energetic barrier to cpTat-mediated transport as well as direct utilization of protons from the gradient, consistent with a H+/protein antiporter mechanism. The magnitude of proton flux was assayed by four independent approaches and averaged 7.9 x 10(4) protons released from the gradient per transported protein. This corresponds to a DeltaG transport of 6.9 x 10(5) kJ.mol protein translocated(-1), representing the utilization of an energetic equivalent of 10(4) molecules of ATP. At this cost, we estimate that the DeltapH-dependent/cpTat pathway utilizes approximately 3% of the total energy output of the chloroplast.

A twin arginine signal peptide and the pH gradient trigger reversible assembly of the thylakoid [Delta]pH/Tat translocase

The thylakoid DeltapH-dependent/Tat pathway is a novel system with the remarkable ability to transport tightly folded precursor proteins using a transmembrane DeltapH as the sole energy source. Three known components of the transport machinery exist in two distinct subcomplexes. A cpTatC-Hcf106 complex serves as precursor receptor and a Tha4 complex is required after precursor recognition. Here we report that Tha4 assembles with cpTatC-Hcf106 during the translocation step. Interactions among components were examined by chemical cross-linking of intact thylakoids followed by immunoprecipitation and immunoblotting. cpTatC and Hcf106 were consistently associated under all conditions tested. In contrast, Tha4 was only associated with cpTatC and Hcf106 in the presence of a functional precursor and the DeltapH. Interestingly, a synthetic signal peptide could replace intact precursor in triggering assembly. The association of all three components was transient and dissipated upon the completion of protein translocation. Such an assembly-disassembly cycle could explain how the DeltapH/Tat system can assemble translocases to accommodate folded proteins of varied size. It also explains in part how the system can exist in the membrane without compromising its ion and proton permeability barrier.

Acyl-CoA oxidase is imported as a heteropentameric, cofactor-containing complex into peroxisomes of Yarrowia lipolytica

Five isoforms of acyl-CoA oxidase (Aox), designated Aox1p to Aox5p, constitute a 443-kD heteropentameric complex containing one polypeptide chain of each isoform within the peroxisomal matrix of the yeast Yarrowia lipolytica. Assembly of the Aox complex occurs in the cytosol and precedes its import into peroxisomes. Peroxisomal targeting of the Aox complex is abolished in a mutant lacking the peroxin Pex5p, a component of the matrix protein targeting machinery. Import of the Aox complex into peroxisomes does not involve the cytosolic chaperone Pex20p, which mediates the oligomerization and import of peroxisomal thiolase. Aox2p and Aox3p play a pivotal role in the formation of the Aox complex in the cytosol and can substitute for one another in promoting assembly of the complex. In vitro, these subunits retard disassembly of the Aox complex and increase the efficiency of its reassembly. Neither Aox2p nor Aox3p is required for acquisition of the cofactor FAD by other components of the complex. We provide evidence that the Aox2p- and Aox3p-assisted assembly of the Aox complex in the cytosol is mandatory for its import into peroxisomes and that no component of the complex can penetrate the peroxisomal matrix as a monomer.

Post-translational protein translocation into thylakoids by the Sec and DeltapH-dependent pathways

Two distinct protein translocation pathways that employ hydrophobic signal peptides function in the plant thylakoid membrane. These two systems are precursor specific and distinguished by their energy and component requirements. Recent studies have shown that one pathway is homologous to the bacterial general export system called Sec. The other one, called the DeltapH-dependent pathway, was originally considered to be unique to plant thylakoids. However, it is now known that homologous transport systems are widely present in prokaryotes and even present in archaea. Here we review these protein transport pathways and discuss their capabilities and mechanisms of operation.

Functional reconstitution of bacterial Tat translocation in vitro

The Tat (twin-arginine translocation) pathway is a Sec-independent mechanism for translocating folded preproteins across or into the inner membrane of Escherichia coli. To study Tat translocation, we sought an in vitro translocation assay using purified inner membrane vesicles and in vitro synthesized substrate protein. While membrane vesicles derived from wild-type cells translocate the Sec-dependent substrate proOmpA, translocation of a Tat-dependent substrate, SufI, was not detected. We established that in vivo overexpression of SufI can saturate the Tat translocase, and that simultaneous overexpression of TatA, B and C relieves this SufI saturation. Using membrane vesicles derived from cells overexpressing TatABC, in vitro translocation of SufI was detected. Like translocation in vivo, translocation of SufI in vitro requires TatABC, an intact membrane potential and the twin-arginine targeting motif within the signal peptide of SUFI: In contrast to Sec translocase, we find that Tat translocase does not require ATP. The development of an in vitro translocation assay is a prerequisite for further biochemical investigations of the mechanism of translocation, substrate recognition and translocase structure.

Protein targeting by the twin-arginine translocation pathway

The twin-arginine translocation pathway operates in the thylakoid membrane of chloroplasts and in the plasma membrane of most free-living bacteria. Its main function is to transport fully folded proteins across the membrane. Three important tat genes have been identified and the sequences of the encoded proteins, together with the unusual properties of the pathway, indicate that the Tat system is completely different from other protein translocases.

Characterization of the early steps of OE17 precursor transport by the thylakoid DeltapH/Tat machinery

In order to probe the structure and protein translocation function of the thylakoid Tat machinery, a 25-residue C-terminal extension containing a 13-residue in vivo biotinylation tag and a 6x His tag was added to a mutant precursor of the 17-kDa subunit of the oxygen-evolving complex to form pOE17(C)-BioHis. When avidin was attached to biotinylated precursor in situ, the precursor-avidin complex was neither imported nor did it form a membrane-spanning translocation intermediate. It did, however, competitively inhibit the translocation of unbiotinylated precursor with an apparent KI unaffected by avidin. It is shown that the precursor protein achieves a stable folded structure upon dilution from urea, suggesting that the avidin-induced inhibition of transport results from a folding-induced proximity of N-terminal and C-terminal domains. It is further demonstrated that the majority of precursor rapidly binds to the thylakoid membrane, remaining import competent and yet undissociable by high salt or high pH treatment at ice temperature. The membrane binding event is unaffected by avidin. Import kinetics reveal that nonproton motive force-driven transport steps make up a major fraction of the transport time. These observations suggest that the N-terminal presequence on the avidin-bound precursor is available for membrane binding and initial recognition by the transport machinery, but the attached avidin signals the machinery that the precursor is an incorrectly configured substrate and thus import is aborted. Consequently, the DeltapH/Tat machinery's proofreading mechanism must operate after precursor recognition but before the committed step in transport.

The twin-arginine translocation system: a novel means of transporting folded proteins in chloroplasts and bacteria

Protein translocases have been characterised in several membrane systems and the translocation mechanisms have been shown to differ in critical respects. Nevertheless, the majority were believed to transport proteins only in a largely unfolded state, and this widespread characteristic was viewed as a likely evolutionary effort to minimise the diameter of translocation pore required. Within the last few years, however, studies on the chloroplast thylakoid membrane have revealed a novel class of protein translocase which possesses the apparently unique ability to transport fully-folded proteins across a tightly sealed energy-transducing membrane. A related system, (the twin-arginine translocation, or Tat system) has now been characterised in the Escherichia coli plasma membrane and considerations of its substrate specificity again point to its involvement in the transport of folded proteins. The emerging data suggest a critical involvement in many membranes for the biogenesis of two types of globular protein: those that are obliged to fold prior to translocation, and those that fold too tightly or rapidly for other types of protein translocase to handle.

The Tat protein export pathway

The Tat (twin-arginine translocation) system is a bacterial protein export pathway with the remarkable ability to transport folded proteins across the cytoplasmic membrane. Preproteins are directed to the Tat pathway by signal peptides that bear a characteristic sequence motif, which includes consecutive arginine residues. Here, we review recent progress on the characterization of the Tat system and critically discuss the structure and operation of this major new bacterial protein export pathway.

How to get a folded protein across a membrane

Several protein-targeting fields have recently converged in their observations of what once was thought to be a rare phenomenon: the transport of folded and oligomerized proteins across membranes. Three of the newly characterized pathways that are known to accommodate folded substrates are the peroxisomal targeting machinery for matrix proteins, the twin-arginine translocation (Tat) of bacteria and the related DeltapH-dependent pathway of plant plastids, and the cytoplasm-to-vacuole targeting (Cvt) pathway in Saccharomyces cerevisiae. Current work strives to understand the molecular mechanisms that accomplish transport of folded substrates. The aim of this commentary is to highlight our knowledge of transport mechanisms, point out areas for future research and address how paradigms of classical protein translocation have shaped current views.

The maize tha4 gene functions in sec-independent protein transport in chloroplasts and is related to hcf106, tatA, and tatB

Proteins are translocated across the chloroplast thylakoid membrane by a variety of mechanisms. Some proteins engage a translocation machinery that is derived from the bacterial Sec export system and require an interaction with a chloroplast-localized SecA homologue. Other proteins engage a machinery that is SecA-independent, but requires a transmembrane pH gradient. Recently, a counterpart to this Delta pH mechanism was discovered in bacteria. Genetic studies revealed that one maize protein involved in this mechanism, HCF106, is related in both structure and function to the bacterial tatA and tatB gene products. We describe here the mutant phenotype and molecular cloning of a second maize gene that functions in the Delta pH mechanism. This gene, thylakoid assembly 4 (tha4), is required specifically for the translocation of proteins that engage the Delta pH pathway. The sequence of the tha4 gene product resembles those of the maize hcf106 gene and the bacterial tatA and tatB genes. Sequence comparisons suggest that tha4 more closely resembles tatA, and hcf106 more closely resembles tatB. These findings support the notion that this sec-independent translocation mechanism has been highly conserved during the evolution of eucaryotic organelles from bacterial endosymbionts.

Sec-dependent pathway and DeltapH-dependent pathway do not share a common translocation pore in thylakoidal protein transport

Thylakoidal proteins of plant chloroplasts are transported to thylakoids via several different pathways, including the DeltapH-dependent and the Sec-dependent pathways. In this study, we asked if these two pathways utilize a common translocation pore. A fusion protein consisting of a 23-kDa subunit of the oxygen evolving complex and Escherichia coli biotin carboxyl carrier protein was biotinylated in E. coli cells and purified. When incubated with isolated pea thylakoids in the absence of avidin, the purified fusion protein was imported into the thylakoids via the DeltapH-dependent pathway. However in the presence of avidin, the fusion protein became lodged in the thylakoid membranes, with its N terminus reaching the thylakoidal lumen, while its C-terminal segment complexed with avidin exposed on the thylakoidal surface. The translocation intermediate of the fusion protein inhibited the import of authentic 23-kDa subunit, suggesting that it occupies a putative translocation pore for the DeltapH-dependent pathway. However the intermediate did not block import of the 33-kDa subunit of the oxygen evolving complex, which is a substrate for the Sec-dependent pathway. These results provide evidence against the possibility of a common translocation pore shared by the Sec-dependent pathway and the DeltapH-dependent pathway.

Component specificity for the thylakoidal Sec and Delta pH-dependent protein transport pathways

Prokaryotes and prokaryote-derived thylakoid membranes of chloroplasts share multiple, evolutionarily conserved pathways for protein export. These include the Sec, signal recognition particle (SRP), and Delta pH/Tat systems. Little is known regarding the thylakoid membrane components involved in these pathways. We isolated a cDNA clone to a novel component of the Delta pH pathway, Tha4, and prepared antibodies against pea Tha4, against maize Hcf106, a protein implicated in Delta pH pathway transport by genetic studies, and against cpSecY, the thylakoid homologue of the bacterial SecY translocon protein. These components were localized to the nonappressed thylakoid membranes. Tha4 and Hcf106 were present in approximately 10-fold excess over active translocation sites. Antibodies to either Tha4 or Hcf106 inhibited translocation of four known Delta pH pathway substrate proteins, but not of Sec pathway or SRP pathway substrates. This suggests that Tha4 and Hcf106 operate either in series or as subunits of a heteromultimeric complex. cpSecY antibodies inhibited translocation of Sec pathway substrates but not of Delta pH or SRP pathway substrates. These studies provide the first biochemical evidence that Tha4 and Hcf106 are specific components of the Delta pH pathway and provide one line of evidence that cpSecY is used specifically by the Sec pathway.

Protein translocation into and across the bacterial plasma membrane and the plant thylakoid membrane

Over the past decade, some familiar themes have emerged on how proteins are inserted into or translocated across the plant chloroplast thylakoid membrane and bacterial inner membranes. In the SecA and signal recognition particle (SRP) pathways, nucleotides and soluble factors are used to translocate proteins across the membrane bilayer in the unfolded state. However, the delta pH-dependent pathway in thylakoids uses a radically different mechanism: transport of proteins across the membrane is driven by the transmembrane pH gradient, and neither stromal factors nor nucleotide triphosphates are needed. In addition, this pathway, which requires the membrane-bound protein Hcf106, appears to translocate proteins in a tightly folded form. Recently, a similar pathway has been shown to operate in eubacteria, and several of its components have been identified.

Old and new pathways of protein export in chloroplasts and bacteria

Targeting of chloroplast proteins to the thylakoid membrane is analogous to bacterial secretion, and much of what we know has been learned from secretory mechanisms in Escherichia coli. However, chloroplasts also use a delta pH-dependent pathway to target thylakoid proteins, at least some of which are folded before transport. Previously, this pathway seemed to have no cognate in bacteria, but recent results have shown that the HCF106 gene in maize encodes a component of this pathway and has bacterial homologues. This delta pH-dependent pathway might be an ancient conserved mechanism for protein translocation that evolved before the endosymbiotic origin of plastids and mitochondria.

Protein secretion: getting folded proteins across membranes

Work on metalloprotein export in bacteria, and protein import into chloroplasts, has converged in the recognition of a novel membrane translocation system with two fascinating properties: it is driven energetically by the transmembrane pH gradient, and it is capable of translocating folded proteins.