Bacterial Sec protein transport is rate-limited by precursor length: a single turnover study

Membrane protein chaperone and sodium chloride modulate the kinetics and morphology of amyloid beta aggregation

Protein aggregation is a biological phenomenon caused by the accumulation of misfolded proteins. Amyloid beta (Aβ) peptides are derived from the cleavage of a larger membrane protein molecule and accumulate to form plaques extracellularly. According to the amyloid hypothesis, accumulation of Aβ aggregates in the brain is primarily responsible for the pathogenesis of Alzheimer's disease (AD). Therefore, the disassembly of Aβ aggregates may provide opportunities for alleviating or treating AD. Here, we show that the novel protein targeting machinery from chloroplast, chloroplast signal recognition particle 43 (cpSRP43), is an ATP-independent membrane protein chaperone that can both prevent and reverse Aβ aggregation effectively. Using of thioflavin T dye, we obtained the aggregation kinetics of Aβ aggregation and determined that the chaperone prevents Aβ aggregation in a concentration-dependent manner. Size exclusion chromatography and sedimentation assays showed that 10-fold excess of cpSRP43 can keep Aβ in the soluble monomeric form. Electron microscopy showed that the fibril structure was disrupted in the presence of this chaperone. Importantly, cpSRP43 utilizes the binding energy to actively remodel the preformed Aβ aggregates without assistance by a co-chaperone and ATP, emphasizing its unique function among protein chaperones. Moreover, when sodium chloride concentration is higher than 25 mm, the Aβ aggregation rate increases drastically to form tightly associated aggregates and generate more oligomers. Our results demonstrate that the presence of cpSRP43 and low NaCl levels inhibit or retard Aβ peptide aggregation, potentially opening new avenues to strategically develop an effective treatment for AD.

Preparation of Uniformly Oriented Inverted Inner (Cytoplasmic) Membrane Vesicles from Gram-Negative Bacterial Cells

The complex double-membrane organization of the envelope in Gram-negative bacteria places unique biosynthetic and topological constraints that can affect translocation of lipids and proteins synthesized on cytoplasm facing leaflet of cytoplasmic (inner) membrane (IM), across IM and between IM and outer membrane (OM). Uniformly oriented inside-out (ISO) vesicles became functional requisite for many biochemical reconstitution functional assays, vectorial proteomics, and vectorial lipidomics. Due to these demands, it is necessary to develop simple and reliable approaches for preparation of uniformly oriented IM membrane vesicles and validation of their sidedness. The uniformly ISO oriented membrane vesicles which have the cytoplasmic face of the membrane on the outside and the periplasmic side facing the sealed lumen can be obtained following intact cell disruption by a single passage through a French pressure cell (French press) at desired total pressure. Although high-pressure lysis leads to the formation of mostly inverted membrane vesicles (designated and abbreviated usually as ISO vesicles, everted or inverted membrane vesicles (IMVs)), inconclusive results are quite common. This uncertainty is due mainly by applying a different pressures, using either intact cells or spheroplasts and presence or absence of sucrose during rupture procedure. Many E. coli envelope fractionation techniques result in heterogeneity among isolated IM membrane vesicles. In part, this is due to difficulties in simple validation of sidedness of oriented membrane preparations of unknown sidedness. The sidedness of various preparations of membrane vesicles can be inferred from the orientation of residing uniformly oriented transmembrane protein. We outline the method in which the orientation of membrane vesicles can be verified by mapping of uniform or mixed topologies of essential protein E. coli protein leader peptidase (LepB) by advanced SCAM™. Although the protocol discussed in this chapter has been developed using Escherichia coli and Yersinia pseudotuberculosis, it can be directly adapted to other Gram-negative bacteria including pathogens.

A unifying mechanism for protein transport through the core bacterial Sec machinery

Encapsulation and compartmentalization are fundamental to the evolution of cellular life, but they also pose a challenge: how to partition the molecules that perform biological functions-the proteins-across impermeable barriers into sub-cellular organelles, and to the outside. The solution lies in the evolution of specialized machines, translocons, found in every biological membrane, which act both as gate and gatekeeper across and into membrane bilayers. Understanding how these translocons operate at the molecular level has been a long-standing ambition of cell biology, and one that is approaching its denouement; particularly in the case of the ubiquitous Sec system. In this review, we highlight the fruits of recent game-changing technical innovations in structural biology, biophysics and biochemistry to present a largely complete mechanism for the bacterial version of the core Sec machinery. We discuss the merits of our model over alternative proposals and identify the remaining open questions. The template laid out by the study of the Sec system will be of immense value for probing the many other translocons found in diverse biological membranes, towards the ultimate goal of altering or impeding their functions for pharmaceutical or biotechnological purposes.

Characterizing the Locus of a Peripheral Membrane Protein-Lipid Bilayer Interaction Underlying Protein Export Activity in E. coli

Quantitative characterization of the strength of peripheral membrane protein-lipid bilayer interactions is fundamental in the understanding of many protein targeting pathways. SecA is a peripheral membrane protein that plays a central role in translocating precursor proteins across the inner membrane of E. coli. The membrane binding activity of the extreme N-terminus of SecA is critical for translocase function. Yet, the mechanical strength of the interaction and the kinetic pathways that this segment of SecA experiences when in proximity of an E. coli polar lipid bilayer has not been characterized. We directly measured the N-terminal SecA-lipid bilayer interaction using precision single molecule atomic force microscope (AFM)-based dynamic force spectroscopy. To provide conformational data inaccessible to AFM, we also performed all-atom molecular dynamics simulations and circular dichroism measurements. The N-terminal 10 amino acids of SecA have little secondary structure when bound to zwitterionic lipid head groups, but secondary structure, which rigidifies the lipid-bound protein segment, emerges when negatively charged lipids are present. Analysis of the single molecule protein-lipid dissociation data converged to a well-defined lipid-bound-state lifetime in the absence of force, τ₀^lipid = 0.9 s, which is well separated from and longer than the fundamental time scale of the secretion process, defined as the time required to translocate a single amino acid residue (∼50 ms). This value of τ₀^lipid is likely to represent a lower limit of the in vivo membrane-bound lifetime due to factors including the minimal system employed here.

Direct visualization of the E. coli Sec translocase engaging precursor proteins in lipid bilayers

Escherichia coli exports proteins via a translocase comprising SecA and the translocon, SecYEG. Structural changes of active translocases underlie general secretory system function, yet directly visualizing dynamics has been challenging. We imaged active translocases in lipid bilayers as a function of precursor protein species, nucleotide species, and stage of translocation using atomic force microscopy (AFM). Starting from nearly identical initial states, SecA more readily dissociated from SecYEG when engaged with the precursor of outer membrane protein A as compared to the precursor of galactose-binding protein. For the SecA that remained bound to the translocon, the quaternary structure varied with nucleotide, populating SecA₂ primarily with adenosine diphosphate (ADP) and adenosine triphosphate, and the SecA monomer with the transition state analog ADP-AlF₃. Conformations of translocases exhibited precursor-dependent differences on the AFM imaging time scale. The data, acquired under near-native conditions, suggest that the translocation process varies with precursor species.

Translocation of non-interacting heteropolymer protein chains in terms of single helical propensity and size

In this work we present an analytical framework to calculate the average translocation time τ required for an ideal proteinogenic polypeptide chain to cross over a small pore on a membrane. Translocation is considered to proceed as a chain of non-interacting amino acid residues of sequence {X_j} diffuses through the pore against an energy barrier Δℱ, set by chain entropy and unfolding-folding energetics. We analyze the effect of sequence heterogeneity on the dynamics of translocation by means of helical propensity of amino acid residues. In our calculations we use sequences of fifteen well-known proteins that are translocated which span two orders of magnitude in size according to the number of residues N. Results show non-symmetric free energy barriers as a consequence of sequence heterogeneity, such asymmetry in energy may be useful in differentiated directions of translocation. For the fifteen polypeptide chains considered we found conditions when sequence heterogeneity has not a significant effect on the time scale of translocation leading to a scaling law τ ∝ N^ν, where ν ∼ 1.6 is an exponent that holds for most ground state energies. We also identify conditions when sequence heterogeneity has a great impact on the time scale of translocation, in consequence, no more scaling laws for τ there exist.

A High-Resolution Luminescent Assay for Rapid and Continuous Monitoring of Protein Translocation across Biological Membranes

Protein translocation is a fundamental process in biology. Major gaps in our understanding of this process arise due the poor sensitivity, low time resolution and irreproducibility of translocation assays. To address this, we applied NanoLuc split-luciferase to produce a new strategy for measuring protein transport. The system reduces the timescale of data collection from days to minutes and allows for continuous acquisition with a time resolution in the order of seconds, yielding kinetics parameters suitable for mechanistic elucidation and mathematical fitting. To demonstrate its versatility, we implemented and validated the assay in vitro and in vivo for the bacterial Sec system and the mitochondrial protein import apparatus. Overall, this technology represents a major step forward, providing a powerful new tool for fundamental mechanistic enquiry of protein translocation and for inhibitor (drug) screening, with an intensity and rigor unattainable through classical methods.

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Conventional methods for monitoring protein translocation are laborious and discontinuous and lack kinetic detail.

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A split NanoLuc system was adapted for real-time monitoring of protein translocation through the bacterial Sec system and the mitochondrial Tim23 complex.

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The new assay reduces the timescale of data acquisition from days to minutes.

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It produces high-quality results suitable for kinetic fitting and model derivation.

ATP-induced asymmetric pre-protein folding as a driver of protein translocation through the Sec machinery

Transport of proteins across membranes is a fundamental process, achieved in every cell by the 'Sec' translocon. In prokaryotes, SecYEG associates with the motor ATPase SecA to carry out translocation for pre-protein secretion. Previously, we proposed a Brownian ratchet model for transport, whereby the free energy of ATP-turnover favours the directional diffusion of the polypeptide (Allen et al., 2016). Here, we show that ATP enhances this process by modulating secondary structure formation within the translocating protein. A combination of molecular simulation with hydrogendeuterium-exchange mass spectrometry and electron paramagnetic resonance spectroscopy reveal an asymmetry across the membrane: ATP-induced conformational changes in the cytosolic cavity promote unfolded pre-protein structure, while the exterior cavity favours its formation. This ability to exploit structure within a pre-protein is an unexplored area of protein transport, which may apply to other protein transporters, such as those of the endoplasmic reticulum and mitochondria.

Single-molecule observation of nucleotide induced conformational changes in basal SecA-ATP hydrolysis

SecA is the critical adenosine triphosphatase that drives preprotein transport through the translocon, SecYEG, in Escherichia coli. This process is thought to be regulated by conformational changes of specific domains of SecA, but real-time, real-space measurement of these changes is lacking. We use single-molecule atomic force microscopy (AFM) to visualize nucleotide-dependent conformations and conformational dynamics of SecA. Distinct topographical populations were observed in the presence of specific nucleotides. AFM investigations during basal adenosine triphosphate (ATP) hydrolysis revealed rapid, reversible transitions between a compact and an extended state at the ~100-ms time scale. A SecA mutant lacking the precursor-binding domain (PBD) aided interpretation. Further, the biochemical activity of SecA prepared for AFM was confirmed by tracking inorganic phosphate release. We conclude that ATP-driven dynamics are largely due to PBD motion but that other segments of SecA contribute to this motion during the transition state of the ATP hydrolysis cycle.

Dynamic action of the Sec machinery during initiation, protein translocation and termination

Protein translocation across cell membranes is a ubiquitous process required for protein secretion and membrane protein insertion. In bacteria, this is mostly mediated by the conserved SecYEG complex, driven through rounds of ATP hydrolysis by the cytoplasmic SecA, and the trans-membrane proton motive force. We have used single molecule techniques to explore SecY pore dynamics on multiple timescales in order to dissect the complex reaction pathway. The results show that SecA, both the signal sequence and mature components of the pre-protein, and ATP hydrolysis each have important and specific roles in channel unlocking, opening and priming for transport. After channel opening, translocation proceeds in two phases: a slow phase independent of substrate length, and a length-dependent transport phase with an intrinsic translocation rate of ~40 amino acids per second for the proOmpA substrate. Broad translocation rate distributions reflect the stochastic nature of polypeptide transport.

Driving Forces of Translocation Through Bacterial Translocon SecYEG

This review focusses on the energetics of protein translocation via the Sec translocation machinery. First we complement structural data about SecYEG's conformational rearrangements by insight obtained from functional assays. These include measurements of SecYEG permeability that allow assessment of channel gating by ligand binding and membrane voltage. Second we will discuss the power stroke and Brownian ratcheting models of substrate translocation and the role that the two models assign to the putative driving forces: (i) ATP (SecA) and GTP (ribosome) hydrolysis, (ii) interaction with accessory proteins, (iii) membrane partitioning and folding, (iv) proton motive force (PMF), and (v) entropic contributions. Our analysis underlines how important energized membranes are for unravelling the translocation mechanism in future experiments.

The Sec System: Protein Export in Escherichia coli

In Escherichia coli, proteins found in the periplasm or the outer membrane are exported from the cytoplasm by the general secretory, Sec, system before they acquire stably folded structure. This dynamic process involves intricate interactions among cytoplasmic and membrane proteins, both peripheral and integral, as well as lipids. In vivo, both ATP hydrolysis and proton motive force are required. Here, we review the Sec system from the inception of the field through early 2016, including biochemical, genetic, and structural data.

Channel crossing: how are proteins shipped across the bacterial plasma membrane?

The structure of the first protein-conducting channel was determined more than a decade ago. Today, we are still puzzled by the outstanding problem of protein translocation—the dynamic mechanism underlying the consignment of proteins across and into membranes. This review is an attempt to summarize and understand the energy transducing capabilities of protein-translocating machines, with emphasis on bacterial systems: how polypeptides make headway against the lipid bilayer and how the process is coupled to the free energy associated with ATP hydrolysis and the transmembrane protein motive force. In order to explore how cargo is driven across the membrane, the known structures of the protein-translocation machines are set out against the background of the historic literature, and in the light of experiments conducted in their wake. The paper will focus on the bacterial general secretory (Sec) pathway (SecY-complex), and its eukaryotic counterpart (Sec61-complex), which ferry proteins across the membrane in an unfolded state, as well as the unrelated Tat system that assembles bespoke channels for the export of folded proteins.

Dynamic Organization of SecA and SecY Secretion Complexes in the B. subtilis Membrane

In prokaryotes, about one third of cellular proteins are translocated across the plasma membrane or inserted into it by concerted action of the cytoplasmic ATPase SecA and the universally conserved SecYEG heterotrimeric polypeptide-translocating pore. Secretion complexes have been reported to localize in specific subcellular sites in Bacillus subtilis. In this work, we used a combination of total internal reflection microscopy, scanning fluorescence correlation spectroscopy, and pair correlation function to study the localization and dynamics of SecA and SecY in growing Bacillus subtilis cells. Both SecA and SecY localized in transient and dynamic foci in the cytoplasmic membrane, which displayed no higher-level organization in helices. Foci of SecA and SecY were in constant flux with freely diffusing SecA and SecY molecules. Scanning FCS confirmed the existence of populations of cellular SecA and SecY molecules with a wide range of diffusion coefficients. Diffusion of SecY as an uncomplexed molecular species was short-lived and only local while SecY complexed with its protein partners traversed distances of over half a micrometer in the cell.

Two-way communication between SecY and SecA suggests a Brownian ratchet mechanism for protein translocation

The essential process of protein secretion is achieved by the ubiquitous Sec machinery. In prokaryotes, the drive for translocation comes from ATP hydrolysis by the cytosolic motor-protein SecA, in concert with the proton motive force (PMF). However, the mechanism through which ATP hydrolysis by SecA is coupled to directional movement through SecYEG is unclear. Here, we combine all-atom molecular dynamics (MD) simulations with single molecule FRET and biochemical assays. We show that ATP binding by SecA causes opening of the SecY-channel at long range, while substrates at the SecY-channel entrance feed back to regulate nucleotide exchange by SecA. This two-way communication suggests a new, unifying 'Brownian ratchet' mechanism, whereby ATP binding and hydrolysis bias the direction of polypeptide diffusion. The model represents a solution to the problem of transporting inherently variable substrates such as polypeptides, and may underlie mechanisms of other motors that translocate proteins and nucleic acids.

High Throughput Screen for Escherichia coli Twin Arginine Translocation (Tat) Inhibitors

The twin arginine translocation (Tat) pathway transports fully-folded and assembled proteins in bacteria, archaea and plant thylakoids. The Tat pathway contributes to the virulence of numerous bacterial pathogens that cause disease in humans, cattle and poultry. Thus, the Tat pathway has the potential to be a novel therapeutic target. Deciphering the Tat protein transport mechanism has been challenging since the active translocon only assembles transiently in the presence of substrate and a proton motive force. To identify inhibitors of Tat transport that could be used as biochemical tools and possibly as drug development leads, we developed a high throughput screen (HTS) to assay the effects of compounds in chemical libraries against protein export by the Escherichia coli Tat pathway. The primary screen is a live cell assay based on a fluorescent Tat substrate that becomes degraded in the cytoplasm when Tat transport is inhibited. Consequently, low fluorescence in the presence of a putative Tat inhibitor was scored as a hit. Two diverse chemical libraries were screened, yielding average Z'-factors of 0.74 and 0.44, and hit rates of ~0.5% and 0.04%, respectively. Hits were evaluated by a series of secondary screens. Electric field gradient (Δψ) measurements were particularly important since the bacterial Tat transport requires a Δψ. Seven low IC₅₀ hits were eliminated by Δψ assays, suggesting ionophore activity. As Δψ collapse is generally toxic to animal cells and efficient membrane permeability is generally favored during the selection of library compounds, these results suggest that secondary screening of hits against electrochemical effects should be done early during hit validation. Though none of the short-listed compounds inhibited Tat transport directly, the screening and follow-up assays developed provide a roadmap to pursue Tat transport inhibitors.

Steric contribution of macromolecular crowding to the time and activation energy for preprotein translocation across the endoplasmic reticulum membrane

Protein translocation from the cytosol to the endoplasmic reticulum (ER) or vice versa, an essential process for cell function, includes the transport of preproteins destined to become secretory, luminal, or integral membrane proteins (translocation) or misfolded proteins returned to the cytoplasm to be degraded (retrotranslocation). An important aspect in this process that has not been fully studied is the molecular crowding at both sides of the ER membrane. By using models of polymers crossing a membrane through a pore, in an environment crowded by either static or dynamic spherical agents, we computed the following transport properties: the free energy, the activation energy, the force, and the transport times for translocation and retrotranslocation. Using experimental protein crowding data for the cytoplasm and ER sides, we showed that dynamic crowding, which resembles biological environments where proteins are translocated or retrotranslocated, increases markedly all the physical properties of translocation and retrotranslocation as compared with translocation in a diluted system. By contrast, transport properties in static crowded systems were similar to those in diluted conditions. In the dynamic regime, the effects of crowding were more notorious in the transport times, leading to a huge difference for large chains. We indicate that this difference is the result of the synergy between the free energy and the diffusivity of the translocating chain. That synergy leads to translocation rates similar to experimental measures in diluted systems, which indicates that the effects of crowding can be measured. Our data also indicate that effects of crowding cannot be neglected when studying translocation because protein dynamic crowding has a relevant steric contribution, which changes the properties of translocation.

Biogenesis of β-barrel integral proteins of bacterial outer membrane

Gram-negative bacteria are enveloped by two membranes, the inner (cytoplasmic) (CM) and the outer (OM). The majority of integral outer membrane proteins are arranged in β-barrels of cylindrical shape composed of amphipathic antiparallel β-strands. In bacteria, β-barrel proteins function as water-filled pores, active transporters, enzymes, receptors, and structural proteins. Proteins of bacterial OM are synthesized in the cytoplasm as unfolded polypeptides with an N-terminal sequence that marks them for transport across the CM. Precursors of membrane proteins move through the aqueous medium of the cytosol and periplasm under the protection of chaperones (SecB, Skp, SurA, and DegP), then cross the CM via the Sec system composed of a polypeptide-conducting channel (SecYEG) and ATPase (SecA), the latter providing the energy for the translocation of the pre-protein. Pre-protein folding and incorporation in the OM require the participation of the Bam-complex, probably without the use of energy. This review summarizes current data on the biogenesis of the β-barrel proteins of bacterial OM. Data on the structure of the proteins included in the multicomponent system for delivery of the OM proteins to their destination in the cell and on their complexes with partners, including pre-proteins, are presented. Molecular models constructed on the basis of structural, genetic, and biochemical studies that describe the mechanisms of β-barrel protein assembly by this molecular transport machinery are also considered.

Direct simulation of early-stage Sec-facilitated protein translocation

Direct simulations reveal key mechanistic features of early-stage protein translocation and membrane integration via the Sec-translocon channel. We present a novel computational protocol that combines non-equilibrium growth of the nascent protein with microsecond timescale molecular dynamics trajectories. Analysis of multiple, long timescale simulations elucidates molecular features of protein insertion into the translocon, including signal-peptide docking at the translocon lateral gate (LG), large lengthscale conformational rearrangement of the translocon LG helices, and partial membrane integration of hydrophobic nascent-protein sequences. Furthermore, the simulations demonstrate the role of specific molecular interactions in the regulation of protein secretion, membrane integration, and integral membrane protein topology. Salt-bridge contacts between the nascent-protein N-terminus, cytosolic translocon residues, and phospholipid head groups are shown to favor conformations of the nascent protein upon early-stage insertion that are consistent with the Type II (N(cyt)/C(exo)) integral membrane protein topology, and extended hydrophobic contacts between the nascent protein and the membrane lipid bilayer are shown to stabilize configurations that are consistent with the Type III (N(exo)/C(cyt)) topology. These results provide a detailed, mechanistic basis for understanding experimentally observed correlations between integral membrane protein topology, translocon mutagenesis, and nascent-protein sequence.

Position-dependent effects of polylysine on Sec protein transport

The bacterial Sec protein translocation system catalyzes the transport of unfolded precursor proteins across the cytoplasmic membrane. Using a recently developed real time fluorescence-based transport assay, the effects of the number and distribution of positive charges on the transport time and transport efficiency of proOmpA were examined. As expected, an increase in the number of lysine residues generally increased transport time and decreased transport efficiency. However, the observed effects were highly dependent on the polylysine position in the mature domain. In addition, a string of consecutive positive charges generally had a more significant effect on transport time and efficiency than separating the charges into two or more charged segments. Thirty positive charges distributed throughout the mature domain resulted in effects similar to 10 consecutive charges near the N terminus of the mature domain. These data support a model in which the local effects of positive charge on the translocation kinetics dominate over total thermodynamic constraints. The rapid translocation kinetics of some highly charged proOmpA mutants suggest that the charge is partially shielded from the electric field gradient during transport, possibly by the co-migration of counter ions. The transport times of precursors with multiple positively charged sequences, or "pause sites," were fairly well predicted by a local effect model. However, the kinetic profile predicted by this local effect model was not observed. Instead, the transport kinetics observed for precursors with multiple polylysine segments support a model in which translocation through the SecYEG pore is not the rate-limiting step of transport.

SecA, a remarkable nanomachine

Biological cells harbor a variety of molecular machines that carry out mechanical work at the nanoscale. One of these nanomachines is the bacterial motor protein SecA which translocates secretory proteins through the protein-conducting membrane channel SecYEG. SecA converts chemically stored energy in the form of ATP into a mechanical force to drive polypeptide transport through SecYEG and across the cytoplasmic membrane. In order to accommodate a translocating polypeptide chain and to release transmembrane segments of membrane proteins into the lipid bilayer, SecYEG needs to open its central channel and the lateral gate. Recent crystal structures provide a detailed insight into the rearrangements required for channel opening. Here, we review our current understanding of the mode of operation of the SecA motor protein in concert with the dynamic SecYEG channel. We conclude with a new model for SecA-mediated protein translocation that unifies previous conflicting data.

The oligomeric state and arrangement of the active bacterial translocon

Protein secretion in bacteria is driven through the ubiquitous SecYEG complex by the ATPase SecA. The structure of SecYEG alone or as a complex with SecA in detergent reveal a monomeric heterotrimer enclosing a central protein channel, yet in membranes it is dimeric. We have addressed the functional significance of the oligomeric status of SecYEG in protein translocation using single molecule and ensemble methods. The results show that while monomers are sufficient for the SecA- and ATP-dependent association of SecYEG with pre-protein, active transport requires SecYEG dimers arranged in the back-to-back conformation. Molecular modeling of this dimeric structure, in conjunction with the new functional data, provides a rationale for the presence of both active and passive copies of SecYEG in the functional translocon.

Interconvertibility of lipid- and translocon-bound forms of the bacterial Tat precursor pre-SufI

Signal peptides target protein cargos for secretion from the bacterial cytoplasm. These signal peptides contain a tri-partite structure consisting of a central hydrophobic domain (h-domain), and two flanking polar domains. Using a recently developed in vitro transport assay, we report here that a central h-domain position (C17) of the twin arginine translocation (Tat) substrate pre-SufI is especially sensitive to amino acid hydrophobicity. The C17I mutant is transported more efficiently than wild type, whereas charged substitutions completely block transport. Transport efficiency is well-correlated with Tat translocon binding efficiency. The precursor protein also binds to non-Tat components of the membrane, presumably to the lipids. This lipid-bound precursor can be chased through the Tat translocons under conditions of high proton motive force. Thus, the non-Tat bound form of the precursor is a functional intermediate in the transport cycle. This intermediate appears to directly equilibrate with the translocon-bound form of the precursor.