The bacterial ATPase SecA functions as a monomer in protein translocation

Protein translocation across the eukaryotic endoplasmic reticulum and bacterial plasma membranes

A decisive step in the biosynthesis of many proteins is their partial or complete translocation across the eukaryotic endoplasmic reticulum membrane or the prokaryotic plasma membrane. Most of these proteins are translocated through a protein-conducting channel that is formed by a conserved, heterotrimeric membrane-protein complex, the Sec61 or SecY complex. Depending on channel binding partners, polypeptides are moved by different mechanisms: the polypeptide chain is transferred directly into the channel by the translating ribosome, a ratcheting mechanism is used by the endoplasmic reticulum chaperone BiP, and a pushing mechanism is used by the bacterial ATPase SecA. Structural, genetic and biochemical data show how the channel opens across the membrane, releases hydrophobic segments of membrane proteins laterally into lipid, and maintains the membrane barrier for small molecules.

Additional in vitro and in vivo evidence for SecA functioning as dimers in the membrane: dissociation into monomers is not essential for protein translocation in Escherichia coli

SecA is an essential component in the Sec-dependent protein translocation pathway and, together with ATP, provides the driving force for the transport of secretory proteins across the cytoplasmic membrane of Escherichia coli. Previous studies established that SecA undergoes monomer-dimer equilibrium in solution. However, the oligomeric state of functional SecA during the protein translocation process is controversial. In this study, we provide additional evidence that SecA functions as a dimer in the membrane by (i) demonstration of the capability of the presumably monomeric SecA derivative to be cross-linked as dimers in vitro and in vivo, (ii) complementation of the growth of a secA(Ts) mutant with another nonfunctional SecA or (iii) in vivo complementation and in vitro function of a genetically tandem SecA dimer that does not dissociate into monomers, and (iv) formation of similar ring-like structures by the tandem SecA dimer and SecA in the presence of lipid bilayers. We conclude that SecA functions as a dimer in the membrane and dissociation into monomers is not necessary during protein translocation.

Structure and function of the bacterial Sec translocon

Bacteria and archaea possess a protein complex in the plasma membrane that governs protein secretion and membrane protein insertion. Eukaryotes carry homologues in the endoplasmic reticulum (ER) where they direct the same reaction. A combination of experiments conducted on the systems found in all three domains of life has revealed a great deal about protein translocation. The channel provides a route for proteins to pass through the hydrophobic barrier of the membrane, assisted by various partner proteins which maintain an unfolded state of the substrate, target it to the channel and provide the energy and mechanical drive required for transport. In bacteria, the post-translational reaction utilizes an ATPase that couples the free energy of ATP binding and hydrolysis to move the substrate through the protein pore. This review will draw on genetic, biochemical and structural findings in an account of our current understanding of this mechanism.

A large conformational change couples the ATP binding site of SecA to the SecY protein channel

In bacteria, the SecYEG protein translocation complex employs the cytosolic ATPase SecA to couple the energy of ATP binding and hydrolysis to the mechanical force required to push polypeptides through the membrane. The molecular basis of this energy transducing reaction is not well understood. A peptide-binding array has been employed to identify sites on SecYEG that interact with SecA. These results along with fluorescence spectroscopy have been exploited to characterise a long-distance conformational change that connects the nucleotide-binding fold of SecA to the transmembrane polypeptide channel in SecY. These movements are driven by binding of non-hydrolysable ATP analogues to a monomer of SecA in association with the SecYEG complex. We also determine that interaction with SecYEG simultaneously decreases the affinity of SecA for ATP and inhibitory magnesium, favouring a previously identified active state of the ATPase. Mutants of SecA capable of binding but not hydrolysing ATP do not elicit this conformationally active state, implicating residues of the Walker B motif in the early chain of events that couple ATP binding to the mobility of the channel.

Topology inversion of SecG is essential for cytosolic SecA-dependent stimulation of protein translocation

SecG, a subunit of the protein translocon, undergoes a cycle of topology inversion. To further examine the role of this topology inversion, we analyzed the activity of membrane vesicles carrying a SecG-PhoA fusion protein (SecG-PhoA inverted membrane vesicles (IMVs)). In the absence of externally added SecA, SecG-PhoA IMVs were as active in protein translocation as SecG(+) IMVs per SecA. Consistent with this observation, insertion of membrane-bound SecA into SecG-PhoA IMVs was normally observed. On the other hand, externally added SecA did not affect the activity of SecG-PhoA IMVs, but it caused >10-fold stimulation of the translocation activity of SecG(+) IMVs, indicating that the topology inversion of SecG, which cannot occur in SecG-PhoA IMVs, is essential for cytosolic SecA-dependent stimulation of protein translocation. SecG-PhoA IMVs generated a 46-kDa fragment of SecA upon trypsin treatment. The accumulation of this membrane-inserted SecA in the SecG-PhoA IMVs was responsible for the loss of the soluble SecA-dependent stimulation. Moreover, fixation of the inverted SecG topology was found to be dependent on soluble SecA. The dual functions of SecG in protein translocation will be discussed.

Sec- and Tat-mediated protein secretion across the bacterial cytoplasmic membrane--distinct translocases and mechanisms

In bacteria, two major pathways exist to secrete proteins across the cytoplasmic membrane. The general Secretion route, termed Sec-pathway, catalyzes the transmembrane translocation of proteins in their unfolded conformation, whereupon they fold into their native structure at the trans-side of the membrane. The Twin-arginine translocation pathway, termed Tat-pathway, catalyses the translocation of secretory proteins in their folded state. Although the targeting signals that direct secretory proteins to these pathways show a high degree of similarity, the translocation mechanisms and translocases involved are vastly different.

Protein translocation is mediated by oligomers of the SecY complex with one SecY copy forming the channel

Many proteins are translocated across the bacterial plasma membrane by the interplay of the cytoplasmic ATPase SecA with a protein-conducting channel, formed from the evolutionarily conserved heterotrimeric SecY complex. Here, we have used purified E. coli components to address the mechanism of translocation. Disulfide bridge crosslinking demonstrates that SecA transfers both the signal sequence and the mature region of a secreted substrate into a single SecY molecule. However, protein translocation involves oligomers of the SecY complex, because a SecY molecule defective in translocation can be rescued by linking it covalently with a wild-type SecY copy. SecA interacts through one of its domains with a nontranslocating SecY copy and moves the polypeptide chain through a neighboring SecY copy. Oligomeric channels with only one active pore likely mediate protein translocation in all organisms.

Cross-linked SecA dimers are not functional in protein translocation

The ATPase SecA is involved in post-translational protein translocation through the SecY channel across the bacterial inner membrane. SecA is a dimer that can dissociate into monomers with translocation activity. Here, we have addressed whether dissociation of the SecA dimer is required for translocation. We show that a dimer in which the two subunits are cross-linked by disulfide bridges is inactive in protein translocation, translocation ATPase, and binding to a lipid bilayer. In contrast, upon reduction of the disulfide bridges, the resulting monomers regain these activities. These data support the notion that dissociation of SecA dimers into monomers occurs during protein translocation.

Pushing, pulling and trapping--modes of motor protein supported protein translocation

Protein translocation across the cellular membranes is an ubiquitous and crucial activity of cells. This process is mediated by translocases that consist of a protein conducting channel and an associated motor protein. Motor proteins interact with protein substrates and utilize the free energy of ATP binding and hydrolysis for protein unfolding, translocation and unbinding. Since motor proteins are found either at the cis- or trans-side of the membrane, different mechanisms for translocation have been proposed. In the Power stroke model, cis-acting motors are thought to push, while trans-motors pull on the substrate protein during translocation. In the Brownian ratchet model, translocation occurs by diffusion of the unfolded polypeptide through the translocation pore while directionality is achieved by trapping and refolding. Recent insights in the structure and function of the molecular motors suggest that different mechanisms can be employed simultaneously.

Allosteric regulation of SecA: magnesium-mediated control of conformation and activity

In bacteria, the SecA protein associates with a ubiquitous protein channel SecYEG where it drives the post-translational secretion of pre-proteins across the plasma membrane. The high-resolution structures of both proteins have been determined in their resting states; however, the mechanism that couples ATP hydrolysis to active transport of substrate proteins through the membrane is not well understood. An analysis of the steady-state ATPase activity of the enzyme reveals that there is an allosteric binding site for magnesium distinct from that associated with hydrolysis of ATP. We have demonstrated that this regulation involves a large conformational change to the SecA dimer, which exerts a strong influence on the turnover and affinity for ATP, as well as the affinity for ADP. The strong inhibitory influence of magnesium on the ATPase activity can be countered by cardiolipin and conditions that promote protein translocation.

Nanodiscs unravel the interaction between the SecYEG channel and its cytosolic partner SecA

The translocon is a membrane-embedded protein assembly that catalyzes protein movement across membranes. The core translocon, the SecYEG complex, forms oligomers, but the protein-conducting channel is at the center of the monomer. Defining the properties of the SecYEG protomer is thus crucial to understand the underlying function of oligomerization. We report here the reconstitution of a single SecYEG complex into nano-scale lipid bilayers, termed Nanodiscs. These water-soluble particles allow one to probe the interactions of the SecYEG complex with its cytosolic partner, the SecA dimer, in a membrane-like environment. The results show that the SecYEG complex triggers dissociation of the SecA dimer, associates only with the SecA monomer and suffices to (pre)-activate the SecA ATPase. Acidic lipids surrounding the SecYEG complex also contribute to the binding affinity and activation of SecA, whereas mutations in the largest cytosolic loop of the SecY subunit, known to abolish the translocation reaction, disrupt both the binding and activation of SecA. Altogether, the results define the fundamental contribution of the SecYEG protomer in the translocation subreactions and illustrate the power of nanoscale lipid bilayers in analyzing the dynamics occurring at the membrane.

Secretion by numbers: Protein traffic in prokaryotes

Almost all aspects of protein traffic in bacteria were covered at the ASM-FEMS meeting on the topic in Iraklio, Crete in May 2006. The studies presented ranged from mechanistic analysis of specific events leading proteins to their final destinations to the physiological roles of the targeted proteins. Among the highlights from the meeting that are reviewed here are the molecular dynamics of SecA protein, membrane protein insertion, type III secretion needles and chaperones, type IV secretion, the two partner and autosecretion systems, the 'secretion competent state', and the recently discovered type VI secretion system.

Structure of dimeric SecA, the Escherichia coli preprotein translocase motor

SecA is the preprotein translocase ATPase subunit and a superfamily 2 (SF2) RNA helicase. Here we present the 2 A crystal structures of the Escherichia coli SecA homodimer in the apo form and in complex with ATP, ADP and adenosine 5'-[beta,gamma-imido]triphosphate (AMP-PNP). Each monomer contains the SF2 ATPase core (DEAD motor) built of two domains (nucleotide binding domain, NBD and intramolecular regulator of ATPase 2, IRA2), the preprotein binding domain (PBD), which is inserted in NBD and a carboxy-terminal domain (C-domain) linked to IRA2. The structures of the nucleotide complexes of SecA identify an interfacial nucleotide-binding cleft located between the two DEAD motor domains and residues critical for ATP catalysis. The dimer comprises two virtually identical protomers associating in an antiparallel fashion. Dimerization is mediated solely through extensive contacts of the DEAD motor domains leaving the C-domain facing outwards from the dimerization core. This dimerization mode explains the effect of functionally important mutations and is completely different from the dimerization models proposed for other SecA structures. The repercussion of these findings on translocase assembly and catalysis is discussed.

In vivo membrane topology of Escherichia coli SecA ATPase reveals extensive periplasmic exposure of multiple functionally important domains clustering on one face of SecA

The Sec-dependent protein translocation pathway promotes the transport of proteins into or across the bacterial plasma membrane. SecA ATPase has been shown to be a nanomotor that associates with its protein cargo as well as the SecYEG channel complex and to undergo ATP-driven cycles of membrane insertion and retraction that promote stepwise protein translocation. Previous studies have shown that both the 65-kDa N-domain and 30-kDa C-domain of SecA appear to undergo such membrane cycling. In the present study we performed in vivo sulfhydryl labeling of an extensive collection of monocysteine secA mutants under topologically specific conditions to identify regions of SecA that are accessible to the trans side of the membrane in its membrane-integrated state. Our results show that distinct regions of five of six SecA domains were labeled under these conditions, and such labeling clusters to a single face of the SecA structure. Our results demarcate an extensive face of SecA that interacts with SecYEG and is in fluid contact with the protein-conducting channel. The observed domain-specific labeling patterns should also provide important constraints on model building efforts in this dynamic system.

The Long α-Helix of SecA Is Important for the ATPase Coupling of Translocation

SecA contains two ATPase folds (NBF1 and NBF2) and other interaction/regulatory domains, all of which are connected by a long helical scaffold domain (HSD) running along the molecule. Here we identified a functionally important and spatially adjacent pair of SecA residues, Arg-642 on HSD and Glu-400 on NBF1. A charge-reversing substitution at either position as well as disulfide tethering of these positions inactivated the translocation activity. Interestingly, however, the translocation-inactive SecA variants fully retained the ability to up-regulate the ATPase in response to a preprotein and the SecYEG translocon. The translocation defect was suppressible by second site alterations at the hinge-forming boundary of NBF2 and HSD. Based on these results, we propose that the motor function of SecA is realized by ligand-activated ATPase engine and its HSD-mediated conversion into the mechanical work of preprotein translocation.

Different modes of SecY-SecA interactions revealed by site-directed in vivo photo-cross-linking

While the SecA ATPase drives protein translocation across the bacterial cytoplasmic membrane by interacting with the SecYEG translocon, molecular details of SecA-SecY interaction remain poorly understood. Here, we studied SecY-SecA interaction by using an in vivo site-directed cross-linking technique developed by Schultz and coworkers [Chin, J. W., Martin, A. B., King, D. S., Wang, L., Schultz, P. G. (2002) Proc. Natl. Acad. Sci. USA 99:11020-11024 and Chin, J. W., Schultz, P. G. (2002) ChemBioChem 3:1135-1137]. Benzoyl-phenylalanine introduced into specific SecY positions at the second, fourth, fifth, and sixth cytoplasmic domains allowed UV cross-linking with SecA. Cross-linked products exhibited two distinct electrophoretic mobilities. SecA cross-linking at the most C-terminal cytoplasmic region (C6) was specifically enhanced in the presence of NaN(3), which arrests the ATPase cycle, and this enhancement was canceled by cis placement of some secY mutations that affect SecY-SecA cooperation. In vitro experiments showed directly that SecA approaches C6 when it is engaging in ATP-dependent preprotein translocation. On the basis of these findings, we propose that the C6 tail of SecY interacts with the working form of SecA, whereas C4-C5 loops may offer constitutive SecA-binding sites.

Crystal structure of the translocation ATPase SecA from Thermus thermophilus reveals a parallel, head-to-head dimer

The mechanism of pre-protein export through the bacterial cytoplasmic membrane, in which the SecA ATPase plays a crucial role as an "energy supplier", is poorly understood. In particular, biochemical and structural studies provide contradictory data as to the oligomeric state of SecA when it is integrated into the active trans-membrane translocase. Here, we report the 2.8 A resolution crystal structure of the Thermus thermophilus SecA protein (TtSecA). Whereas the structure of the TtSecA monomer closely resembles that from other bacteria, the oligomeric state of TtSecA is strikingly distinct. In contrast to the antiparallel (head-to-tail) dimerization reported previously for the other bacterial systems, TtSecA forms parallel (head-to-head) dimers that are reminiscent of open scissors. The dimer interface is abundant in bulky Arg and Lys side-chains from both subunits, which stack on one another to form an unusual "basic zipper" that is highly conserved, as revealed by homology modeling and sequence analysis. The basic zipper is sealed on both ends by two pairs of the salt bridges formed between the basic side-chains from the zipper and two invariant acidic residues. The organization of the dimers, in which the two pre-protein binding domains are located proximal to each other at the tip of the "scissors", might allow a concerted mode of substrate recognition while the opening/closing of the scissors might facilitate translocation.

Oligomeric states of the SecA and SecYEG core components of the bacterial Sec translocon

Many proteins synthesized in the cytoplasm ultimately function in non-cytoplasmic locations. In Escherichia coli, the general secretory (Sec) pathway transports the vast majority of these proteins. Two fundamental components of the Sec transport pathway are the SecYEG heterotrimeric complex that forms the channel through the cytoplasmic membrane, and SecA, the ATPase that drives the preprotein to and across the membrane. This review focuses on what is known about the oligomeric states of these core Sec components and how the oligomeric state might change during the course of the translocation of a preprotein.

A novel dimer interface and conformational changes revealed by an X-ray structure of B. subtilis SecA

The SecA ATPase moves polypeptides post-translationally across the plasma membrane of eubacteria, but the mechanism of transport is still unclear. We describe the crystal structure of a novel dimeric form of Bacillus subtilis SecA. Dimerization of SecA occurs at the prominent groove formed by the nucleotide binding domain 2 (nbd2) and the preprotein cross-linking (ppx) domain. The dimer interface is very large, burying approximately 5400 A(2) of solvent accessible surface per monomer. Single cysteine disulfide cross-linking shows the presence of this novel SecA dimer in solution. In addition, other dimers also exist in solution, arguing that they all are in equilibrium with monomeric SecA and supporting the idea that the monomer may be the functional species. Dimerization of SecA causes an alpha-helix of one subunit to convert to a short beta-strand that participates in beta-sheet formation with strands in the other subunit. This conversion of secondary structure elements occurs close to the connection between the nbd1 and ppx domains, a potential site of interaction with translocation substrate. Comparing the different X-ray structures of B. subtilis SecA suggests that small changes in the nucleotide binding domains could be amplified via helix 1 of the helical scaffold domain (hsd) to generate larger movements of the domains involved in polypeptide binding.

Characterization of three areas of interactions stabilizing complexes between SecA and SecB, two proteins involved in protein export

The general secretory, Sec, system translocates precursor polypeptides from the cytosol across the cytoplasmic membrane in Escherichia coli. SecB, a small cytosolic chaperone, captures the precursor polypeptides before they fold and delivers them to the membrane translocon through interactions with SecA. Both SecB and SecA display twofold symmetry and yet the complex between the two is stabilized by contacts that are distributed asymmetrically. Two distinct regions of interaction have been defined previously and here we identify a third. Calorimetric studies of complexes stabilized by different subsets of these interactions were carried out to determine the binding affinities and the thermodynamic parameters that underlie them. We show here that there is no change in affinity when either one of two contact areas out of the three is lacking. This fact and the asymmetry of the binding contacts may be important to the function of the complex in protein export.

Cloning, purification, crystallization and preliminary crystallographic analysis of SecA from Enterococcus faecalis

The gene coding for SecA from Enterococcus faecalis was cloned and overexpressed in Escherichia coli. In this protein, the lysine at position 6 was replaced by an asparagine in order to reduce sensitivity towards proteases. The modified protein was purified and crystallized. Crystals diffracting to 2.4 A resolution were obtained using the vapour-diffusion technique. The crystals belong to the monoclinic space group C2, with unit-cell parameters a = 203.4, b = 49.8, c = 100.8 A, alpha = gamma = 90.0, beta = 119.1 degrees. A selenomethionine derivative was prepared and is currently being tested in crystallization trials.

Probing the affinity of SecA for signal peptide in different environments

SecA, the peripheral subunit of the Escherichia coli preprotein translocase, interacts with a number of ligands during export, including signal peptides, membrane phospholipids, and nucleotides. Using fluorescence resonance energy transfer (FRET), we studied the interactions of wild-type (WT) and mutant SecAs with IAEDANS-labeled signal peptide, and how these interactions are modified in the presence of other transport ligands. We find that residues on the third alpha-helix in the preprotein cross-linking domain (PPXD) are important for the interaction of SecA and signal peptide. For SecA in aqueous solution, saturation binding data using FRET analysis fit a single-site binding model and yielded a Kd of 2.4 microM. FRET is inhibited for SecA in lipid vesicles relative to that in aqueous solution at a low signal peptide concentration. The sigmoidal nature of the binding curve suggests that SecA in lipids has two conformational states; our results do not support different oligomeric states of SecA. Using native gel electrophoresis, we establish signal peptide-induced SecA monomerization in both aqueous solution and lipid vesicles. Whereas the affinity of SecA for signal peptide in an aqueous environment is unaffected by temperature or the presence of nucleotides, in lipids the affinity decreases in the presence of ADP or AMP-PCP but increases at higher temperature. The latter finding is consistent with SecA existing in an elongated form while inserting the signal peptide into membranes.

SecA dimer cross-linked at its subunit interface is functional for protein translocation

SecA facilitates protein transport across the eubacterial plasma membrane by its association with cargo proteins and the SecYEG translocon, followed by ATP-driven conformational changes that promote protein translocation in a stepwise manner. Whether SecA functions as a monomer or a dimer during this process has been the subject of considerable controversy. Here we utilize cysteine-directed mutagenesis along with the crystal structure of the SecA dimer to create a cross-linked dimer at its subunit interface, which was normally active for in vitro protein translocation.

Protein translocation by the Sec61/SecY channel

The conserved protein-conducting channel, referred to as the Sec61 channel in eukaryotes or the SecY channel in eubacteria and archaea, translocates proteins across cellular membranes and integrates proteins containing hydrophobic transmembrane segments into lipid bilayers. Structural studies illustrate how the protein-conducting channel accomplishes these tasks. Three different mechanisms, each requiring a different set of channel binding partners, are employed to move polypeptide substrates: The ribosome feeds the polypeptide chain directly into the channel, a ratcheting mechanism is used by the eukaryotic endoplasmic reticulum chaperone BiP, and a pushing mechanism is utilized by the bacterial ATPase SecA. We review these translocation mechanisms, relating biochemical and genetic observations to the structures of the protein-conducting channel and its binding partners.

Biogenesis of inner membrane proteins in Escherichia coli

Gram-negative bacteria such as Escherichia coli are surrounded by two membranes, the inner membrane and the outer membrane. The biogenesis of most inner membrane proteins (IMPs), typical alpha-helical proteins, appears to follow a partly conserved cotranslational pathway. Targeting involves a relatively simple signal recognition particle (SRP) and SRP-receptor. Insertion of most IMPs into the membrane occurs via the Sec-translocon, which is also used for the vectorial transport of secretory proteins. Similar to eukaryotic systems, little is known about the later stages of biogenesis of IMPs, the folding and assembly in the lipid bilayer. Recently, YidC has been identified as a factor that assists in the integration, folding, and assembly of IMPs both in association with the Sec-translocon and separately. This review deals mainly with recent structural and biochemical data from various experimental systems that offer new insight into the different stages of biogenesis of E. coli IMPs.

The oligomeric distribution of SecYEG is altered by SecA and translocation ligands

The multimeric membrane protein complex translocase mediates the transport of preproteins across and integration of membrane proteins into the inner membrane of Escherichia coli. The translocase consists of the peripheral membrane-associated ATPase SecA and the heterotrimeric channel-forming complex consisting of SecY, SecE and SecG. We have investigated the quaternary structure of the SecYEG complex in proteoliposomes. Fluorescence resonance energy transfer demonstrates that SecYEG forms oligomers when embedded in the membrane. Freeze-fracture techniques were used to examine the oligomeric composition under non-translocating and translocating conditions. Our data show that membrane-embedded SecYEG exists in a concentration-dependent equilibrium between monomers, dimers and tetramers, and that dynamic exchange of subunits between oligomers can occur. Remarkably, the formation of dimers and tetramers in the lipid environment is stimulated significantly by membrane insertion of SecA and by the interaction with translocation ligands SecA, preprotein and ATP, suggesting that the active translocation channel consists of multiple SecYEG complexes.

Protein Complexes of the Escherichia coli Cell Envelope

Protein complexes are an intrinsic aspect of life in the membrane. Knowing which proteins are assembled in these complexes is therefore essential to understanding protein function(s). Unfortunately, recent high throughput protein interaction studies have failed to deliver any significant information on proteins embedded in the membrane, and many membrane protein complexes remain ill defined. In this study, we have optimized the blue native-PAGE technique for the study of membrane protein complexes in the inner and outer membranes of Escherichia coli. In combination with second dimension SDS-PAGE and mass spectrometry, we have been able to identify 43 distinct protein complexes. In addition to a number of well characterized complexes, we have identified known and orphan proteins in novel oligomeric states. For two orphan proteins, YhcB and YjdB, our findings enable a tentative functional assignment. We propose that YhcB is a hitherto unidentified additional subunit of the cytochrome bd oxidase and that YjdB, which co-localizes with the ZipA protein, is involved in cell division. Our reference two-dimensional blue native-SDS-polyacrylamide gels will facilitate future studies of the assembly and composition of E. coli membrane protein complexes during different growth conditions and in different mutant backgrounds.

The structural view of bacterial translocation-specific chaperone SecB: implications for function

SecB is a molecular chaperone that functions in bacterial post-translational protein translocation pathway. It maintains newly synthesized precursor polypeptide chains in a translocation-competent state and guides them to the translocon via its high-affinity binding to the ligand as well as to the membrane-embedded ATPase SecA. Recent advances in elucidating the structures of SecB have enabled the examination of protein function in the structural context. Structures of SecB from both Haemophilus influenzae and Escherichia coli support the early two-subsite polypeptide-binding model. In addition, the detailed molecular interaction between SecB and SecA was revealed by a structure of SecB in complex with the C-terminal zinc-containing domain of SecA. These observations explain the dual role of SecB plays in the translocation pathway, as a molecular chaperone and a specific targeting factor. A model of SecB-SecA complex suggests that the binding of SecA to SecB changes the conformation of the polypeptide binding sites in the chaperone, enabling transfer of precursor polypeptides from SecB to SecA. Recent studies also show the presence of a second zinc-independent SecB binding site in SecA and the new interaction might contribute to the function of SecB.

Covalently dimerized SecA is functional in protein translocation

The ATPase SecA provides the driving force for the transport of secretory proteins across the cytoplasmic membrane of Escherichia coli. SecA exists as a dimer in solution, but the exact oligomeric state of SecA during membrane binding and preprotein translocation is a topic of debate. To study the requirements of oligomeric changes in SecA during protein translocation, a non-dissociable SecA dimer was formed by oxidation of the carboxyl-terminal cysteines. The cross-linked SecA dimer interacts with the SecYEG complex with a similar stoichiometry as non-cross-linked SecA. Cross-linking reversibly disrupts the SecB binding site on SecA. However, in the absence of SecB, the activity of the disulfide-bonded SecA dimer is indistinguishable from wild-type SecA. Moreover, SecYEG binding stabilizes a cold sodium dodecylsulfate-resistant dimeric state of SecA. The results demonstrate that dissociation of the SecA dimer is not an essential feature of the protein translocation reaction.

Dimeric SecA is essential for protein translocation

SecA facilitates bacterial protein translocation by its association with presecretory or membrane proteins and the SecYEG translocon channel. Once assembled, SecA ATPase undergoes cycles of membrane insertion and retraction at SecYEG that drive protein translocation in a stepwise fashion. SecA exists in equilibrium between a monomer and dimer, and association with its translocation ligands shifts this equilibrium dramatically. Here, we examined the proposal that protein translocation can occur by means of a SecA monomer. We produced a mutant SecA protein lacking residues 2-11, which was found to exist mostly as a monomer, and it was unable to complement a conditional-lethal secA mutant, was inactive for in vitro protein translocation, and was poorly active for translocation ATPase activity. Furthermore, we developed a technique termed membrane trapping, where wild-type SecA subunits became trapped within the membrane by overproduction of membrane-stuck mutant SecA proteins, and, in one case, a membrane-associated SecA heterodimer was demonstrated. Finally, we examined both endogenous and reconstituted membrane-bound SecA and found a significant level of SecA dimer in both cases, as assessed by chemical crosslinking. Collectively, our results strongly suggest that membrane-bound SecA dimer is critical for the protein translocation cycle, although these results cannot exclude participation of SecA monomer at some stage in the translocation process. Our findings have important implications regarding SecA motor function and translocon assembly and activation.