Anionic phospholipids are involved in membrane association of FtsY and stimulate its GTPase activity

Phospholipid dependency of membrane protein insertion by the Sec translocon

Membrane protein insertion into and translocation across the bacterial cytoplasmic membrane are essential processes facilitated by the Sec translocon. Membrane insertion occurs co-translationally whereby the ribosome nascent chain is targeted to the translocon via signal recognition particle and its receptor FtsY. The phospholipid dependence of membrane protein insertion has remained mostly unknown. Here we assessed in vitro the dependence of the SecA independent insertion of the mannitol permease MtlA into the membrane on the main phospholipid species present in Escherichia coli. We observed that insertion depends on the presence of phosphatidylglycerol and is due to the anionic nature of the polar headgroup, while insertion is stimulated by the zwitterionic phosphatidylethanolamine. We found an optimal insertion efficiency at about 30 mol% DOPG and 50 mol% DOPE which approaches the bulk membrane phospholipid composition of E. coli.

Coping with stress: How bacteria fine-tune protein synthesis and protein transport

Maintaining a functional proteome under different environmental conditions is challenging for every organism, in particular for unicellular organisms, such as bacteria. In order to cope with changing environments and stress conditions, bacteria depend on strictly coordinated proteostasis networks that control protein production, folding, trafficking, and degradation. Regulation of ribosome biogenesis and protein synthesis are cornerstones of this cellular adaptation in all domains of life, which is rationalized by the high energy demand of both processes and the increased resistance of translationally silent cells against internal or external poisons. Reduced protein synthesis ultimately also reduces the substrate load for protein transport systems, which are required for maintaining the periplasmic, inner, and outer membrane subproteomes. Consequences of impaired protein transport have been analyzed in several studies and generally induce a multifaceted response that includes the upregulation of chaperones and proteases and the simultaneous downregulation of protein synthesis. In contrast, generally less is known on how bacteria adjust the protein targeting and transport machineries to reduced protein synthesis, e.g., when cells encounter stress conditions or face nutrient deprivation. In the current review, which is mainly focused on studies using Escherichia coli as a model organism, we summarize basic concepts on how ribosome biogenesis and activity are regulated under stress conditions. In addition, we highlight some recent developments on how stress conditions directly impair protein targeting to the bacterial membrane. Finally, we describe mechanisms that allow bacteria to maintain the transport of stress-responsive proteins under conditions when the canonical protein targeting pathways are impaired.

mRNA targeting eliminates the need for the signal recognition particle during membrane protein insertion in bacteria

Signal-sequence-dependent protein targeting is essential for the spatiotemporal organization of eukaryotic and prokaryotic cells and is facilitated by dedicated protein targeting factors such as the signal recognition particle (SRP). However, targeting signals are not exclusively contained within proteins but can also be present within mRNAs. By in vivo and in vitro assays, we show that mRNA targeting is controlled by the nucleotide content and by secondary structures within mRNAs. mRNA binding to bacterial membranes occurs independently of soluble targeting factors but is dependent on the SecYEG translocon and YidC. Importantly, membrane insertion of proteins translated from membrane-bound mRNAs occurs independently of the SRP pathway, while the latter is strictly required for proteins translated from cytosolic mRNAs. In summary, our data indicate that mRNA targeting acts in parallel to the canonical SRP-dependent protein targeting and serves as an alternative strategy for safeguarding membrane protein insertion when the SRP pathway is compromised.

A multifactorial assessment of the SRP pathway constituent FtsY as a vital mycobacterial constituent

The signal recognition particle (SRP) system plays an imperative role in transporting the secretory protein to its intended location. The SRP pathway running in Mycobacterium tuberculosis constitutes FtsY (signal receptor), FfH (SRP), and 4.5S RNA in which signal receptor acts in the GTP-dependent manner. In this study, we are rendering the essential facts of FtsY with respect to mycobacterial growth. The growth study experiment showed that downexpressed FtsY slowed the growth of Mycobacterium smegmatis mc² 155 from the initial lag phase to stationary phase. Previously, we have showed that GTPase activity of FtsY is metal ion dependent and showed the maximum activity with 10 mM magnesium. The effect of Mg²⁺ and Mn²⁺ on mycobacterial growth showed that Mg²⁺ did not affect the growth, whereas higher concentration of Mn²⁺ decreases the bacterial growth. After searching the inhibitor database, 14 GTPase and ATPase inhibitors, Mac0182344, ML141, ITX3, NAV_2729, Br-GTP, Rhosin_HCl, Mac0182099, CCG_50014, CID_1067700, Mac0174809, Nsc_23766, Berberine, Nexinhib20, and EHT1864, were found to interact with FtsY. Further, ML141 and NAV2729 found to decrease the enzymatic activity of FtsY as well as the mycobacterial growth. Therefore, the conclusive statement of the present study can be stated as that the FtsY plays major role in mycobacterial cell survival and ML141 and NAV2729 can be used to constrain the SRP pathway.

Cotranslational Biogenesis of Membrane Proteins in Bacteria

Nascent polypeptides emerging from the ribosome during translation are rapidly scanned and processed by ribosome-associated protein biogenesis factors (RPBs). RPBs cleave the N-terminal formyl and methionine groups, assist cotranslational protein folding, and sort the proteins according to their cellular destination. Ribosomes translating inner-membrane proteins are recognized and targeted to the translocon with the help of the signal recognition particle, SRP, and SRP receptor, FtsY. The growing nascent peptide is then inserted into the phospholipid bilayer at the translocon, an inner-membrane protein complex consisting of SecY, SecE, and SecG. Folding of membrane proteins requires that transmembrane helices (TMs) attain their correct topology, the soluble domains are inserted at the correct (cytoplasmic or periplasmic) side of the membrane, and – for polytopic membrane proteins – the TMs find their interaction partner TMs in the phospholipid bilayer. This review describes the recent progress in understanding how growing nascent peptides are processed and how inner-membrane proteins are targeted to the translocon and find their correct orientation at the membrane, with the focus on biophysical approaches revealing the dynamics of the process. We describe how spontaneous fluctuations of the translocon allow diffusion of TMs into the phospholipid bilayer and argue that the ribosome orchestrates cotranslational targeting not only by providing the binding platform for the RPBs or the translocon, but also by helping the nascent chains to find their correct orientation in the membrane. Finally, we present the auxiliary role of YidC as a chaperone for inner-membrane proteins. We show how biophysical approaches provide new insights into the dynamics of membrane protein biogenesis and raise new questions as to how translation modulates protein folding.

Fidelity of Cotranslational Protein Targeting to the Endoplasmic Reticulum

Fidelity of protein targeting is essential for the proper biogenesis and functioning of organelles. Unlike replication, transcription and translation processes, in which multiple mechanisms to recognize and reject noncognate substrates are established in energetic and molecular detail, the mechanisms by which cells achieve a high fidelity in protein localization remain incompletely understood. Signal recognition particle (SRP), a conserved pathway to mediate the localization of membrane and secretory proteins to the appropriate cellular membrane, provides a paradigm to understand the molecular basis of protein localization in the cell. In this chapter, we review recent progress in deciphering the molecular mechanisms and substrate selection of the mammalian SRP pathway, with an emphasis on the key role of the cotranslational chaperone NAC in preventing protein mistargeting to the ER and in ensuring the organelle specificity of protein localization.

Archaeal SRP RNA and SRP19 facilitate the assembly of SRP54-FtsY targeting complex

The signal recognition particle (SRP) plays an essential role in protein translocation across biological membranes. Stable complexation of two GTPases in the signal recognition particle (SRP) and its receptor (SR) control the delivery of nascent polypeptide to the membrane translocon. In archaea, protein targeting is mediated by the SRP54/SRP19/7S RNA ribonucleoprotein complex (SRP) and the FtsY protein (SR). In the present study, using fluorescence resonance energy transfer (FRET), we demonstrate that archaeal 7S RNA stabilizes the SRP54·FtsY targeting complex (TC). Moreover, we show that archaeal SRP19 further assists 7S RNA in stabilizing the targeting complex (TC). These results suggest that archaeal 7S RNA and SRP19 modulate the conformation of the targeting complex and thereby reinforce TC to execute protein translocation via concomitant GTP hydrolysis.

Receptor compaction and GTPase rearrangement drive SRP-mediated cotranslational protein translocation into the ER

The conserved signal recognition particle (SRP) cotranslationally delivers ~30% of the proteome to the eukaryotic endoplasmic reticulum (ER). The molecular mechanism by which eukaryotic SRP transitions from cargo recognition in the cytosol to protein translocation at the ER is not understood. Here, structural, biochemical, and single-molecule studies show that this transition requires multiple sequential conformational rearrangements in the targeting complex initiated by guanosine triphosphatase (GTPase)-driven compaction of the SRP receptor (SR). Disruption of these rearrangements, particularly in mutant SRP54G226E linked to severe congenital neutropenia, uncouples the SRP/SR GTPase cycle from protein translocation. Structures of targeting intermediates reveal the molecular basis of early SRP-SR recognition and emphasize the role of eukaryote-specific elements in regulating targeting. Our results provide a molecular model for the structural and functional transitions of SRP throughout the targeting cycle and show that these transitions provide important points for biological regulation that can be perturbed in genetic diseases.

The Dynamic SecYEG Translocon

The spatial and temporal coordination of protein transport is an essential cornerstone of the bacterial adaptation to different environmental conditions. By adjusting the protein composition of extra-cytosolic compartments, like the inner and outer membranes or the periplasmic space, protein transport mechanisms help shaping protein homeostasis in response to various metabolic cues. The universally conserved SecYEG translocon acts at the center of bacterial protein transport and mediates the translocation of newly synthesized proteins into and across the cytoplasmic membrane. The ability of the SecYEG translocon to transport an enormous variety of different substrates is in part determined by its ability to interact with multiple targeting factors, chaperones and accessory proteins. These interactions are crucial for the assisted passage of newly synthesized proteins from the cytosol into the different bacterial compartments. In this review, we summarize the current knowledge about SecYEG-mediated protein transport, primarily in the model organism Escherichia coli, and describe the dynamic interaction of the SecYEG translocon with its multiple partner proteins. We furthermore highlight how protein transport is regulated and explore recent developments in using the SecYEG translocon as an antimicrobial target.

GTP-Dependent FlhF Homodimer Supports Secretion of a Hemolysin in Bacillus cereus

The multidomain (B-NG) protein FlhF, a flagellar biogenesis regulator in several bacteria, is the third paralog of the signal recognition particle (SRP)-GTPases Ffh and FtsY, which are known to drive protein-delivery to the plasma membrane. Previously, we showed that FlhF is required for Bacillus cereus pathogenicity in an insect model of infection, being essential for physiological peritrichous flagellation, for motility, and for the secretion of virulence proteins. Among these proteins, we found that the L₂ component of hemolysin BL, one of the most powerful toxins B. cereus produces, was drastically reduced by the FlhF depletion. Herein, we demonstrate that B. cereus FlhF forms GTP-dependent homodimers in vivo since the replacement of residues critical for their GTP-dependent homodimerization alters this ability. The protein directly or indirectly controls flagellation by affecting flagellin-gene transcription and its overproduction leads to a hyperflagellated phenotype. On the other hand, FlhF does not affect the expression of the L₂-encoding gene (hblC), but physically binds L₂ when in its homodimeric form, recruiting the protein to the plasma membrane for secretion. We additionally show that FlhF overproduction increases L₂ secretion and that the FlhF/L₂ interaction requires the NG domain of FlhF. Our findings demonstrate the peculiar behavior of B. cereus FlhF, which is required for the correct flagellar pattern and acts as SRP-GTPase in the secretion of a bacterial toxin subunit.

Co-translational protein targeting in bacteria

About 30% of all bacterial proteins execute their function outside of the cytosol and have to be transported into or across the cytoplasmic membrane. Bacteria use multiple protein transport systems in parallel, but the majority of proteins engage two distinct targeting systems. One is the co-translational targeting by two universally conserved GTPases, the signal recognition particle (SRP) and its receptor FtsY, which deliver inner membrane proteins to either the SecYEG translocon or the YidC insertase for membrane insertion. The other targeting system depends on the ATPase SecA, which targets secretory proteins, i.e. periplasmic and outer membrane proteins, to SecYEG for their subsequent ATP-dependent translocation. While SRP selects its substrates already very early during their synthesis, the recognition of secretory proteins by SecA is believed to occur primarily after translation termination, i.e. post-translationally. In this review we highlight recent progress on how SRP recognizes its substrates at the ribosome and how the fidelity of the targeting reaction to SecYEG is maintained. We furthermore discuss similarities and differences in the SRP-dependent targeting to either SecYEG or YidC and summarize recent results that suggest that some membrane proteins are co-translationally targeted by SecA.

The Escherichia coli SRP Receptor Forms a Homodimer at the Membrane

The Escherichia coli signal recognition particle (SRP) receptor, FtsY, plays a fundamental role in co-translational targeting of membrane proteins via the SRP pathway. Efficient targeting relies on membrane interaction of FtsY and heterodimerization with the SRP protein Ffh, which is driven by detachment of α helix (αN1) in FtsY. Here we show that apart from the heterodimer, FtsY forms a nucleotide-dependent homodimer on the membrane, and upon αN1 removal also in solution. Homodimerization triggers reciprocal stimulation of GTP hydrolysis and occurs in vivo. Biochemical characterization together with integrative modeling suggests that the homodimer employs the same interface as the heterodimer. Structure determination of FtsY NG+1 with GMPPNP shows that a dimerization-induced conformational switch of the γ-phosphate is conserved in Escherichia coli, filling an important gap in SRP GTPase activation. Our findings add to the current understanding of SRP GTPases and may challenge previous studies that did not consider homodimerization of FtsY.

Co-translational Folding Intermediate Dictates Membrane Targeting of the Signal Recognition Particle Receptor

Much of our knowledge on the function of proteins is deduced from their mature, folded states. However, it is unknown whether partially synthesized nascent protein segments can execute biological functions during translation and whether their premature folding states matter. A recent observation that a nascent chain performs a distinct function, co-translational targeting in vivo, has been made with the Escherichia coli signal recognition particle receptor FtsY, a major player in the conserved pathway of membrane protein biogenesis. FtsY functions as a membrane-associated entity, but very little is known about the mode of its targeting to the membrane. Here we investigated the underlying structural mechanism of the co-translational FtsY targeting to the membrane. Our results show that helices N_2-4, which mediate membrane targeting, form a stable folding intermediate co-translationally that greatly differs from its fold in the mature FtsY. These results thus resolve a long-standing mystery of how the receptor targets the membrane even when deleted of its alleged membrane targeting sequence. The structurally distinct targeting determinant of FtsY exists only co-translationally. Our studies will facilitate further efforts to seek cellular factors required for proper targeting and association of FtsY with the membrane. Moreover, the results offer a hallmark example for how co-translational nascent intermediates may dictate biological functions.

Activation of ROP6 GTPase by Phosphatidylglycerol in Arabidopsis

Plant Rho-like GTPases (ROPs) are switch-like proteins which play essential roles in controlling cell polarity development and cellular activities. ROPs are regulated by many factors, such as auxin, light, and RopGEFs and RopGAPs proteins. However, it has not been reported yet whether small molecules play a role in the regulation of ROP activity. Here, we showed that AtROP6 specially bound to a phospholipid, phosphatidylglycerol (PG), by the protein-lipid overlay and liposome sedimentation assays, and further MST assay gave a dissociation constant (Kd) of 4.8 ± 0.4 μM for binding of PG to His-AtROP6. PG profile analysis in Arabidopsis revealed that PG existed both in leaves and roots but with distinctive fatty acyl chain patterns. By evaluating AtROP6 activity using RIC1 effector binding-based assay, we found that PG stimulated AtROP6 activity. In the FM4-64 uptake experiment, PG inhibited AtROP6-mediated endocytosis process. By evaluating internalization of PIN2, PG was shown to regulate endocytosis process coordinately with NAA. Further root gravitropism experiment revealed that PG enhanced the AtROP6-mediated root gravity response. These results suggest that the phospholipid PG physically binds AtROP6, stimulates its activity and influences AtROP6-mediated root gravity response in Arabidopsis.

Two-step membrane binding by the bacterial SRP receptor enable efficient and accurate Co-translational protein targeting

The signal recognition particle (SRP) delivers ~30% of the proteome to the eukaryotic endoplasmic reticulum, or the bacterial plasma membrane. The precise mechanism by which the bacterial SRP receptor, FtsY, interacts with and is regulated at the target membrane remain unclear. Here, quantitative analysis of FtsY-lipid interactions at single-molecule resolution revealed a two-step mechanism in which FtsY initially contacts membrane via a Dynamic mode, followed by an SRP-induced conformational transition to a Stable mode that activates FtsY for downstream steps. Importantly, mutational analyses revealed extensive auto-inhibitory mechanisms that prevent free FtsY from engaging membrane in the Stable mode; an engineered FtsY pre-organized into the Stable mode led to indiscriminate targeting in vitro and disrupted FtsY function in vivo. Our results show that the two-step lipid-binding mechanism uncouples the membrane association of FtsY from its conformational activation, thus optimizing the balance between the efficiency and fidelity of co-translational protein targeting.

DOI:http://dx.doi.org/10.7554/eLife.25885.001

Targeting and Insertion of Membrane Proteins

The insertion and assembly of proteins into the inner membrane of bacteria are crucial for many cellular processes, including cellular respiration, signal transduction, and ion and pH homeostasis. This process requires efficient membrane targeting and insertion of proteins into the lipid bilayer in their correct orientation and proper conformation. Playing center stage in these events are the targeting components, signal recognition particle (SRP) and the SRP receptor FtsY, as well as the insertion components, the Sec translocon and the YidC insertase. Here, we will discuss new insights provided from the recent high-resolution structures of these proteins. In addition, we will review the mechanism by which a variety of proteins with different topologies are inserted into the inner membrane of Gram-negative bacteria. Finally, we report on the energetics of this process and provide information on how membrane insertion occurs in Gram-positive bacteria and Archaea. It should be noted that most of what we know about membrane protein assembly in bacteria is based on studies conducted in Escherichia coli.

Anionic Phospholipids and the Albino3 Translocase Activate Signal Recognition Particle-Receptor Interaction during Light-harvesting Chlorophyll a/b-binding Protein Targeting

The universally conserved signal recognition particle (SRP) co-translationally delivers newly synthesized membrane and secretory proteins to the target cellular membrane. The only exception is found in the chloroplast of green plants, where the chloroplast SRP (cpSRP) post-translationally targets light-harvesting chlorophyll a/b-binding proteins (LHCP) to the thylakoid membrane. The mechanism and regulation of this post-translational mode of targeting by cpSRP remain unclear. Using biochemical and biophysical methods, here we show that anionic phospholipids activate the cpSRP receptor cpFtsY to promote rapid and stable cpSRP54·cpFtsY complex assembly. Furthermore, the stromal domain of the Alb3 translocase binds with high affinity to and regulates GTP hydrolysis in the cpSRP54·cpFtsY complex, suggesting that cpFtsY is primarily responsible for initial recruitment of the targeting complex to Alb3. These results suggest a new model for the sequential recruitment, remodeling, and unloading of the targeting complex at membrane translocase sites in the post-translational cpSRP pathway.

ATPase and GTPase Tangos Drive Intracellular Protein Transport

The GTPase superfamily of proteins provides molecular switches to regulate numerous cellular processes. The 'GTPase switch' paradigm, in which external regulatory factors control the switch of a GTPase between 'on' and 'off' states, has been used to interpret the regulatory mechanism of many GTPases. However, recent work unveiled a class of nucleotide hydrolases that do not adhere to this classical paradigm. Instead, they use nucleotide-dependent dimerization cycles to regulate key cellular processes. In this review article, recent studies of dimeric GTPases and ATPases involved in intracellular protein targeting are summarized. It is suggested that these proteins can use the conformational plasticity at their dimer interface to generate multiple points of regulation, thereby providing the driving force and spatiotemporal coordination of complex cellular pathways.

Electrostatics and Intrinsic Disorder Drive Translocon Binding of the SRP Receptor FtsY

Integral membrane proteins in bacteria are co‐translationally targeted to the SecYEG translocon for membrane insertion via the signal recognition particle (SRP) pathway. The SRP receptor FtsY and its N‐terminal A domain, which is lacking in any structural model of FtsY, were studied using NMR and fluorescence spectroscopy. The A domain is mainly disordered and highly flexible; it binds to lipids via its N terminus and the C‐terminal membrane targeting sequence. The central A domain binds to the translocon non‐specifically and maintains disorder. Translocon targeting and binding of the A domain is driven by electrostatic interactions. The intrinsically disordered A domain tethers FtsY to the translocon, and because of its flexibility, allows the FtsY NG domain to scan a large area for binding to the NG domain of ribosome‐bound SRP, thereby promoting the formation of the quaternary transfer complex at the membrane.

Ribosome binding induces repositioning of the signal recognition particle receptor on the translocon

The cotranslational transfer of nascent membrane proteins to the SecYEG translocon is facilitated by a reorientation of the SecY-bound signal recognition particle (SRP) receptor, FtsY, which accompanies the formation of a quaternary targeting complex consisting of SecYEG, FtsY, SRP, and the ribosome.

Cotranslational protein targeting delivers proteins to the bacterial cytoplasmic membrane or to the eukaryotic endoplasmic reticulum membrane. The signal recognition particle (SRP) binds to signal sequences emerging from the ribosomal tunnel and targets the ribosome-nascent-chain complex (RNC) to the SRP receptor, termed FtsY in bacteria. FtsY interacts with the fifth cytosolic loop of SecY in the SecYEG translocon, but the functional role of the interaction is unclear. By using photo-cross-linking and fluorescence resonance energy transfer measurements, we show that FtsY–SecY complex formation is guanosine triphosphate independent but requires a phospholipid environment. Binding of an SRP–RNC complex exposing a hydrophobic transmembrane segment induces a rearrangement of the SecY–FtsY complex, which allows the subsequent contact between SecY and ribosomal protein uL23. These results suggest that direct RNC transfer to the translocon is guided by the interaction between SRP and translocon-bound FtsY in a quaternary targeting complex.

Dynamics of co-translational protein targeting

Most membrane and secretory proteins are delivered co-translationally to protein translocation channels in their destination membrane by the signal recognition particle (SRP) and its receptor. This co-translational molecular machinery is conserved across all kingdoms of life, though it varies in composition and function. Here we report recent progress towards understanding the mechanism of SRP function, focusing on findings about Escherichia coli SRP's conformational dynamics throughout the targeting process. These insights shed light on a key checkpoint in the targeting cycle: how SRP regulates engagement of an actively translating ribosome with the translocation machinery at the membrane.

In vitro reconstitution of co-translational D1 insertion reveals a role of the cpSec-Alb3 translocase and Vipp1 in photosystem II biogenesis

Photosystem II (PS II) is a multi-subunit complex localized in the thylakoid membrane that performs the light-dependent photosynthetic charge separation. The PS II reaction centre comprises, among others, the D1 protein. De novo synthesis and repair of PS II require efficient mechanisms for transport and insertion of plastid encoded D1 into the thylakoid membrane. To elucidate the process of D1 insertion, we used an in vitro translation system derived from pea chloroplasts to reconstitute the D1 insertion. Thereby, truncated D1 encoding psbA mRNAs lacking a stop codon were translated in the presence of thylakoid membranes and the translation was stalled by addition of chloramphenicol. The generated ribosome nascent chain complexes (RNCs) were tightly associated with the thylakoids. Subsequently, these D1 insertion intermediates were enriched from solubilized thylakoids by sucrose cushion centrifugation. Immunological analyses demonstrated the presence of the cpSec translocase, Alb3, cpFtsY, cpSRP54 and Vipp1 (vesicle-inducing protein in plastids 1) in the enriched D1 insertion intermediates. A complex formation between cpSecY, Alb3, cpFtsY and Vipp1 in thylakoid membranes was shown by gel filtration chromatography, BN (Blue Native)/SDS-PAGE and co-immunoprecipitation experiments. Furthermore, a stimulating effect of recombinant Vipp1 on the formation of a D1 insertion intermediate was observed in vitro. These results suggest a co-operative function of these proteins in D1 insertion.

Impact of glycerol-3-phosphate dehydrogenase on virulence factor production by Pseudomonas aeruginosa

Pseudomonas aeruginosa establishes life-long chronic infections in the cystic fibrosis (CF) lung by utilizing various adaptation strategies. Some of these strategies include altering metabolic pathways to utilize readily available nutrients present in the host environment. The airway sputum contains various host-derived nutrients that can be utilized by P. aeruginosa, including phosphatidylcholine, a major component of lung surfactant. Pseudomonas aeruginosa can degrade phosphatidylcholine to glycerol and fatty acids to increase the availability of usable carbon sources in the CF lung. In this study, we show that some CF-adapted P. aeruginosa isolates utilize glycerol more efficiently as a carbon source than nonadapted isolates. Furthermore, a mutation in a gene required for glycerol utilization impacts the production of several virulence factors in both acute and chronic isolates of P. aeruginosa. Taken together, the results suggest that interference with this metabolic pathway may have potential therapeutic benefits.

Co-translational protein targeting to the bacterial membrane

Co-translational protein targeting by the Signal Recognition Particle (SRP) is an essential cellular pathway that couples the synthesis of nascent proteins to their proper cellular localization. The bacterial SRP, which contains the minimal ribonucleoprotein core of this universally conserved targeting machine, has served as a paradigm for understanding the molecular basis of protein localization in all cells. In this review, we highlight recent biochemical and structural insights into the molecular mechanisms by which fundamental challenges faced by protein targeting machineries are met in the SRP pathway. Collectively, these studies elucidate how an essential SRP RNA and two regulatory GTPases in the SRP and SRP receptor (SR) enable this targeting machinery to recognize, sense and respond to its biological effectors, i.e. the cargo protein, the target membrane and the translocation machinery, thus driving efficient and faithful co-translational protein targeting. This article is part of a Special Issue entitled: Protein trafficking and secretion in bacteria. Guest Editors: Anastassios Economou and Ross Dalbey.

Signal recognition particle: an essential protein-targeting machine

The signal recognition particle (SRP) and its receptor compose a universally conserved and essential cellular machinery that couples the synthesis of nascent proteins to their proper membrane localization. The past decade has witnessed an explosion in in-depth mechanistic investigations of this targeting machine at increasingly higher resolutions. In this review, we summarize recent work that elucidates how the SRP and SRP receptor interact with the cargo protein and the target membrane, respectively, and how these interactions are coupled to a novel GTPase cycle in the SRP·SRP receptor complex to provide the driving force and enhance the fidelity of this fundamental cellular pathway. We also discuss emerging frontiers in which important questions remain to be addressed.

SecYEG activates GTPases to drive the completion of cotranslational protein targeting

SecYEG drives conformational changes in the cotranslational targeting complex to activate it for GTP hydrolysis and the handover of the translating ribosome.

Signal recognition particle (SRP) and its receptor (SR) comprise a highly conserved cellular machine that cotranslationally targets proteins to a protein-conducting channel, the bacterial SecYEG or eukaryotic Sec61p complex, at the target membrane. Whether SecYEG is a passive recipient of the translating ribosome or actively regulates this targeting machinery remains unclear. Here we show that SecYEG drives conformational changes in the cargo-loaded SRP–SR targeting complex that activate it for GTP hydrolysis and for handover of the translating ribosome. These results provide the first evidence that SecYEG actively drives the efficient delivery and unloading of translating ribosomes at the target membrane.

Intra-plastid protein trafficking: how plant cells adapted prokaryotic mechanisms to the eukaryotic condition

Protein trafficking and localization in plastids involve a complex interplay between ancient (prokaryotic) and novel (eukaryotic) translocases and targeting machineries. During evolution, ancient systems acquired new functions and novel translocation machineries were developed to facilitate the correct localization of nuclear encoded proteins targeted to the chloroplast. Because of its post-translational nature, targeting and integration of membrane proteins posed the biggest challenge to the organelle to avoid aggregation in the aqueous compartments. Soluble proteins faced a different kind of problem since some had to be transported across three membranes to reach their destination. Early studies suggested that chloroplasts addressed these issues by adapting ancient-prokaryotic machineries and integrating them with novel-eukaryotic systems, a process called 'conservative sorting'. In the last decade, detailed biochemical, genetic, and structural studies have unraveled the mechanisms of protein targeting and localization in chloroplasts, suggesting a highly integrated scheme where ancient and novel systems collaborate at different stages of the process. In this review we focus on the differences and similarities between chloroplast ancestral translocases and their prokaryotic relatives to highlight known modifications that adapted them to the eukaryotic situation. This article is part of a Special Issue entitled: Protein Import and Quality Control in Mitochondria and Plastids.

Isolation, characterization and lipid-binding properties of the recalcitrant FtsA division protein from Escherichia coli

We have obtained milligram amounts of highly pure Escherichia coli division protein FtsA from inclusion bodies with an optimized purification method that, by overcoming the reluctance of FtsA to be purified, surmounts a bottleneck for the analysis of the molecular basis of FtsA function. Purified FtsA is folded, mostly monomeric and interacts with lipids. The apparent affinity of FtsA binding to the inner membrane is ten-fold higher than to phospholipids, suggesting that inner membrane proteins could modulate FtsA-membrane interactions. Binding of FtsA to lipids and membranes is insensitive to ionic strength, indicating that a net contribution of hydrophobic interactions is involved in the association of FtsA to lipid/membrane structures.

Biogenesis of inner membrane proteins in Escherichia coli

The inner membrane proteome of the model organism Escherichia coli is composed of inner membrane proteins, lipoproteins and peripherally attached soluble proteins. Our knowledge of the biogenesis of inner membrane proteins is rapidly increasing. This is in particular true for the early steps of biogenesis - protein targeting to and insertion into the membrane. However, our knowledge of inner membrane protein folding and quality control is still fragmentary. Furthering our knowledge in these areas will bring us closer to understand the biogenesis of individual inner membrane proteins in the context of the biogenesis of the inner membrane proteome of Escherichia coli as a whole. This article is part of a Special Issue entitled: Biogenesis/Assembly of Respiratory Enzyme Complexes.

Structural basis for the molecular evolution of SRP-GTPase activation by protein

Small G proteins have key roles in signal transduction pathways. They are switched from the signaling 'on' to the non-signaling 'off' state when GTPase-activating proteins (GAPs) provide a catalytic residue. The ancient signal recognition particle (SRP)-type GTPases form GTP-dependent homo- and heterodimers and deviate from the canonical switch paradigm in that no GAPs have been identified. Here we show that the YlxH protein activates the SRP-GTPase FlhF. The crystal structure of the Bacillus subtilis FlhF-effector complex revealed that the effector does not contribute a catalytic residue but positions the catalytic machinery already present in SRP-GTPases. We provide a general concept that might also apply to the RNA-driven activation of the universally conserved, co-translational protein-targeting machinery comprising the SRP-GTPases Ffh and FtsY. Our study exemplifies the evolutionary transition from RNA- to protein-driven activation in SRP-GTPases and suggests that the current view on SRP-mediated protein targeting is incomplete.

Biophysical studies of the membrane-embedded and cytoplasmic forms of the glucose-specific Enzyme II of the E. coli phosphotransferase system (PTS)

The glucose Enzyme II transporter complex of the Escherichia coli phosphotransferase system (PTS) exists in at least two physically distinct forms: a membrane-integrated dimeric form, and a cytoplasmic monomeric form, but little is known about the physical states of these enzyme forms. Six approaches were used to evaluate protein-protein and protein-lipid interactions in this system. Fluorescence energy transfer (FRET) using MBP-II(Glc)-YFP and MBP-II(Glc)-CFP revealed that the homodimeric Enzyme II complex in cell membranes is stable (FRET(-)) but can be dissociated and reassociated to the heterodimer only in the presence of Triton X100 (FRET(+)). The monomeric species could form a heterodimeric species (FRET(+)) by incubation and purification without detergent exposure. Formaldehyde cross linking studies, conducted both in vivo and in vitro, revealed that the dimeric MBP-II(Glc) activity decreased dramatically with increasing formaldehyde concentrations due to both aggregation and activity loss, but that the monomeric MBP-II(Glc) retained activity more effectively in response to the same formaldehyde treatments, and little or no aggregation was observed. Electron microscopy of MBP-II(Glc) indicated that the dimeric form is larger than the monomeric form. Dynamic light scattering confirmed this conclusion and provided quantitation. NMR analyses provided strong evidence that the dimeric form is present primarily in a lipid bilayer while the monomeric form is present as micelles. Finally, lipid analyses of the different fractions revealed that the three lipid species (PE, PG and CL) are present in all fractions, but the monomeric micellar structure contains a higher percentage of anionic lipids (PG & CL) while the dimeric bilayer form has a higher percentage of zwitterion lipids (PE). Additionally, evidence for a minor dimeric micellar species, possibly an intermediate between the monomeric micellar and the dimeric bilayer forms, is presented. These results provide convincing evidence for interconvertible physical forms of Enzyme-II(Glc).

Signal sequence-independent SRP-SR complex formation at the membrane suggests an alternative targeting pathway within the SRP cycle

Our study reveals an alternative route in the SRP-dependent protein targeting pathway that includes a preassembled, membrane-bound SRP-SR complex. This alternative route is fully sufficient to maintain cell viability in the absence of a soluble SRP.

Protein targeting by the signal recognition particle (SRP) and the bacterial SRP receptor FtsY requires a series of closely coordinated steps that monitor the presence of a substrate, the membrane, and a vacant translocon. Although the influence of substrate binding on FtsY-SRP complex formation is well documented, the contribution of the membrane is largely unknown. In the current study, we found that negatively charged phospholipids stimulate FtsY-SRP complex formation. Phospholipids act on a conserved positively charged amphipathic helix in FtsY and induce a conformational change that strongly enhances the FtsY-lipid interaction. This membrane-bound, signal sequence–independent FtsY-SRP complex is able to recruit RNCs to the membrane and to transfer them to the Sec translocon. Significantly, the same results were also observed with an artificial FtsY-SRP fusion protein, which was tethered to the membrane via a transmembrane domain. This indicates that substrate recognition by a soluble SRP is not essential for cotranslational targeting in Escherichia coli. Our findings reveal a remarkable flexibility of SRP-dependent protein targeting, as they indicate that substrate recognition can occur either in the cytosol via ribosome-bound SRP or at the membrane via a preassembled FtsY-SRP complex.

Lipids trigger a conformational switch that regulates signal recognition particle (SRP)-mediated protein targeting

Co-translational protein targeting to the membrane is mediated by the signal recognition particle and its receptor (FtsY). Their homologous GTPase domains interact at the membrane and form a heterodimer in which both GTPases are activated. The prerequisite for protein targeting is the interaction of FtsY with phospholipids. However, the mechanism of FtsY regulation by phospholipids remained unclear. Here we show that the N terminus of FtsY (A domain) is natively unfolded in solution and define the complete membrane-targeting sequence. We show that the membrane-targeting sequence is highly dynamic in solution, independent of nucleotides and directly responds to the density of anionic phospholipids by a random coil-helix transition. This conformational switch is essential for tethering FtsY to membranes and activates the GTPase for its subsequent interaction with the signal recognition particle. Our results underline the dynamics of lipid-protein interactions and their importance in the regulation of protein targeting and translocation across biological membranes.

The potassium channel KcsA: a model protein in studying membrane protein oligomerization and stability of oligomeric assembly?

Many membrane proteins are functional as stable oligomers. An understanding of the conditions that elicit and enhance oligomerization is important in many therapeutics. In this regard, protein-protein and protein-lipid interactions play crucial roles in the assembly and stability of oligomeric complexes. Recent years have seen a rapid increase in the mechanistic information on the importance of cytoplasmic termini in determining subunit assembly and stability of oligomeric complexes. In addition, the role of specific protein-lipid interaction between anionic phospholipids and "hot spots" on the protein surface has also become evident in stabilizing oligomeric assemblies. This review focuses on several contemporary developments of membrane proteins that stabilize oligomers by taking the potassium channel KcsA as an exemplary ion channel.

The prediction of novel multiple lipid-binding regions in protein translocation motor proteins: a possible general feature

Protein translocation is an important cellular process. SecA is an essential protein component in the Sec system, as it contains the molecular motor that facilitates protein translocation. In this study, a bioinformatics approach was applied in the search for possible lipid-binding helix regions in protein translocation motor proteins. Novel lipid-binding regions in Escherichia coli SecA were identified. Remarkably, multiple lipid-binding sites were also identified in other motor proteins such as BiP, which is involved in ER protein translocation. The prokaryotic signal recognition particle receptor FtsY, though not a motor protein, is in many ways related to SecA, and was therefore included in this study. The results demonstrate a possible general feature for motor proteins involved in protein translocation.

Genetic evidence for functional interaction of the Escherichia coli signal recognition particle receptor with acidic lipids in vivo

The mechanism underlying the interaction of the Escherichia coli signal recognition particle receptor FtsY with the cytoplasmic membrane has been studied in detail. Recently, we proposed that FtsY requires functional interaction with inner membrane lipids at a late stage of the signal recognition particle pathway. In addition, an essential lipid-binding α-helix was identified in FtsY of various origins. Theoretical considerations and in vitro studies have suggested that it interacts with acidic lipids, but this notion is not yet fully supported by in vivo experimental evidence. Here, we present an unbiased genetic clue, obtained by serendipity, supporting the involvement of acidic lipids. Utilizing a dominant negative mutant of FtsY (termed NG), which is defective in its functional interaction with lipids, we screened for E. coli genes that suppress the negative dominant phenotype. In addition to several unrelated phenotype-suppressor genes, we identified pgsA, which encodes the enzyme phosphatidylglycerophosphate synthase (PgsA). PgsA is an integral membrane protein that catalyzes the committed step to acidic phospholipid synthesis, and we show that its overexpression increases the contents of cardiolipin and phosphatidylglycerol. Remarkably, expression of PgsA also stabilizes NG and restores its biological function. Collectively, our results strongly support the notion that FtsY functionally interacts with acidic lipids.

Lipid activation of the signal recognition particle receptor provides spatial coordination of protein targeting

Phospholipid binding leads to accelerated assembly of the bacterial SRP receptor FtsY and SRP, allowing cargo proteins to be delivered to target membranes more efficiently.

The signal recognition particle (SRP) and SRP receptor comprise the major cellular machinery that mediates the cotranslational targeting of proteins to cellular membranes. It remains unclear how the delivery of cargos to the target membrane is spatially coordinated. We show here that phospholipid binding drives important conformational rearrangements that activate the bacterial SRP receptor FtsY and the SRP–FtsY complex. This leads to accelerated SRP–FtsY complex assembly, and allows the SRP–FtsY complex to more efficiently unload cargo proteins. Likewise, formation of an active SRP–FtsY GTPase complex exposes FtsY’s lipid-binding helix and enables stable membrane association of the targeting complex. Thus, membrane binding, complex assembly with SRP, and cargo unloading are inextricably linked to each other via conformational changes in FtsY. These allosteric communications allow the membrane delivery of cargo proteins to be efficiently coupled to their subsequent unloading and translocation, thus providing spatial coordination during protein targeting.

The Sec translocase

The vast majority of proteins trafficking across or into the bacterial cytoplasmic membrane occur via the translocon. The translocon consists of the SecYEG complex that forms an evolutionarily conserved heterotrimeric protein-conducting membrane channel that functions in conjunction with a variety of ancillary proteins. For posttranslational protein translocation, the translocon interacts with the cytosolic motor protein SecA that drives the ATP-dependent stepwise translocation of unfolded polypeptides across the membrane. For the cotranslational integration of membrane proteins, the translocon interacts with ribosome-nascent chain complexes and membrane insertion is coupled to polypeptide chain elongation at the ribosome. These processes are assisted by the YidC and SecDF(yajC) complex that transiently interacts with the translocon. This review summarizes our current understanding of the structure-function relationship of the translocon and its interactions with ancillary components during protein translocation and membrane protein insertion. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Component interactions, regulation and mechanisms of chloroplast signal recognition particle-dependent protein transport

The chloroplast proteome comprises nuclear- and plastome-encoded proteins. In order to function correctly these proteins must be transported, either cotranslationally or posttranslationally, to their final destination in the chloroplast. Here the chloroplast signal recognition particle (cpSRP) which is present in two different stromal pools plays an essential role. On the one hand, the conserved 54kDa subunit (cpSRP54) is associated with 70S ribosomes to function in the cotranslational transport of the plastid-encoded thylakoid membrane protein D1. On the other hand, the cpSRP consists of cpSRP54 and a unique 43kDa subunit (cpSRP43) and facilitates the transport of nuclear-encoded light-harvesting chlorophyll-binding proteins (LHCPs), the most abundant membrane proteins of the thylakoids. In addition to cpSRP, the cpSRP receptor cpFtsY and the thylakoid membrane protein Alb3 are required for posttranslational LHCP integration in a GTP-dependent manner. In contrast to the universally conserved cytosolic SRP, the chloroplast SRP of higher plants lacks an SRP-RNA component. Interestingly, cpSRP-RNA genes have been identified in the plastome of lower plants, indicating that their cpSRP structure resembles the cytosolic SRP.

Early targeting events during membrane protein biogenesis in Escherichia coli

All living cells have co-translational pathways for targeting membrane proteins. Co-translation pathways for secretory proteins also exist but mostly in eukaryotes. Unlike secretory proteins, the biosynthetic pathway of most membrane proteins is conserved through evolution and these proteins are usually synthesized by membrane-bound ribosomes. Translation on the membrane requires that both the ribosomes and the mRNAs be properly localized. Theoretically, this can be achieved by several means. (i) The current view is that the targeting of cytosolic mRNA-ribosome-nascent chain complexes (RNCs) to the membrane is initiated by information in the emerging hydrophobic nascent polypeptides. (ii) The alternative model suggests that ribosomes may be targeted to the membrane also constitutively, whereas the appropriate mRNAs may be carried on small ribosomal subunits or targeted by other cellular factors to the membrane-bound ribosomes. Importantly, the available experimental data do not rule out the possibility that cells may also utilize both pathways in parallel. In any case, it is well documented that a major player in the targeting pathway is the signal recognition particle (SRP) system composed of the SRP and its receptor (SR). Although the functional core of the SRP system is evolutionarily conserved, its composition and biological practice come with different flavors in various organisms. This review is dedicated mainly to the Escherichia (E.) coli SRP, where the biochemical and structural properties of components of the SRP system have been relatively characterized, yielding essential information about various aspects of the pathway. In addition, several cellular interactions of the SRP and its receptor have been described in E. coli, providing insights into their spatial function. Collectively, these in vitro studies have led to the current view of the targeting pathway [see (i) above]. Interestingly, however, in vivo studies of the role of the SRP and its receptor, with emphasis on the temporal progress of the pathway, elicited an alternative hypothesis [see (ii) above]. This article is part of a Special Issue entitled Protein translocation across or insertion into membranes.

Biogenesis of bacterial inner-membrane proteins

All cells must traffic proteins into and across their membranes. In bacteria, several pathways have evolved to enable protein transfer across the inner membrane, the periplasm, and the outer membrane. The major route of protein translocation in and across the cytoplasmic membrane is the general secretion pathway (Sec-pathway). The biogenesis of membrane proteins not only requires protein translocation but also coordinated targeting to the membrane beforehand and folding and assembly into their protein complexes afterwards to function properly in the cell. All these processes are responsible for the biogenesis of membrane proteins that mediate essential functions of the cell such as selective transport, energy conversion, cell division, extracellular signal sensing, and motility. This review will highlight the most recent developments on the structure and function of bacterial membrane proteins, focusing on the journey that integral membrane proteins take to find their final destination in the inner membrane.

Archaea signal recognition particle shows the way

Archaea SRP is composed of an SRP RNA molecule and two bound proteins named SRP19 and SRP54. Regulated by the binding and hydrolysis of guanosine triphosphates, the RNA-bound SRP54 protein transiently associates not only with the hydrophobic signal sequence as it emerges from the ribosomal exit tunnel, but also interacts with the membrane-associated SRP receptor (FtsY). Comparative analyses of the archaea genomes and their SRP component sequences, combined with structural and biochemical data, support a prominent role of the SRP RNA in the assembly and function of the archaea SRP. The 5e motif, which in eukaryotes binds a 72 kilodalton protein, is preserved in most archaea SRP RNAs despite the lack of an archaea SRP72 homolog. The primary function of the 5e region may be to serve as a hinge, strategically positioned between the small and large SRP domain, allowing the elongated SRP to bind simultaneously to distant ribosomal sites. SRP19, required in eukaryotes for initiating SRP assembly, appears to play a subordinate role in the archaea SRP or may be defunct. The N-terminal A region and a novel C-terminal R region of the archaea SRP receptor (FtsY) are strikingly diverse or absent even among the members of a taxonomic subgroup.