The Yeast Arr4p ATPase Binds the Chloride Transporter Gef1p When Copper Is Available in the Cytosol

Redox-regulated chaperones in cell stress responses

Proteostasis and redox homeostasis are tightly interconnected and most protein quality control pathways are under direct redox regulation which allow cells to immediately respond to oxidative stress conditions. The activation of ATP-independent chaperones serves as a first line of defense to counteract oxidative unfolding and aggregation of proteins. Conserved cysteine residues evolved as redox-sensitive switches which upon reversible oxidation induce substantial conformational rearrangements and the formation of chaperone-active complexes. In addition to harnessing unfolding proteins, these chaperone holdases interact with ATP-dependent chaperone systems to facilitate client refolding and restoring proteostasis during stress recovery. This minireview gives an insight into highly orchestrated mechanisms regulating the stress-specific activation and inactivation of redox-regulated chaperones and their role in cell stress responses.

Identification of C. elegans ASNA-1 domains and tissue requirements that differentially influence platinum sensitivity and growth control

ASNA1 plays an essential role in cisplatin chemotherapy response, type 2 diabetes, and heart disease. It is also an important biomarker in the treatment response of many diseases. Biochemically, ASNA1 has two mutually exclusive redox-modulated roles: a tail-anchored protein (TAP) targeting function in the reduced state and a holdase/chaperone function in the oxidized state. Assigning biochemical roles of mammalian ASNA1 to biomedical functions is crucial for successful therapy development. Our previous work showed the relevance of the C. elegans ASNA-1 homolog in modeling cisplatin response and insulin secretion. Here we analyzed two-point mutants in highly conserved residues in C. elegans ASNA-1 and determined their importance in separating the cisplatin response function from its roles in insulin secretion. asna-1(ΔHis164) and asna-1(A63V) point mutants, which both preferentially exist in the oxidized state, displayed cisplatin sensitivity phenotype as well as TAP insertion defect but not an insulin secretion defect. Further, using targeted depletion we analyzed the tissue requirements of asna-1 for C. elegans growth and development. Somatic depletion of ASNA-1 as well as simultaneous depletion of ASNA-1 in neurons and intestines resulted in an L1 arrest. We concluded that, targeting single residues in ASNA-1 affecting Switch I/Switch II domain function, in comparison to complete knockdown counteracted cisplatin resistance without jeopardizing other important biological functions. Taken together, our study shows that effects on health caused by ASNA1 mutations can have different biochemical bases.

From guide to guard-activation mechanism of the stress-sensing chaperone Get3

Oxidative stress conditions can cause ATP depletion, oxidative protein unfolding, and potentially toxic protein aggregation. To alleviate this proteotoxic stress, the highly conserved yeast protein, Get3, switches from its guiding function as an ATP-dependent targeting factor for tail-anchored proteins to its guarding function as an ATP-independent molecular chaperone that prevents irreversible protein aggregation. Here, we demonstrate that activation of Get3's chaperone function follows a tightly orchestrated multi-step process, centered around the redox status of two conserved cysteines, whose reactivity is directly controlled by Get3's nucleotide-binding state. Thiol oxidation causes local unfolding and the transition into chaperone-active oligomers. Vice versa, inactivation requires the reduction of Get3's cysteines followed by ATP-binding, which allows the transfer of bound client proteins to ATP-dependent chaperone systems for their effective refolding. Manipulating this fine-tuned cycle of activation and inactivation in yeast impairs oxidative stress resistance and growth, illustrating the necessity to tightly control Get3's intrinsic chaperone function.

Regulated targeting of the monotopic hairpin membrane protein Erg1 requires the GET pathway

The guided entry of tail-anchored proteins (GET) pathway targets C-terminally anchored transmembrane proteins and protects cells from lipotoxicity. Here, we reveal perturbed ergosterol production in ∆get3 cells and demonstrate the sensitivity of GET pathway mutants to the sterol synthesis inhibiting drug terbinafine. Our data uncover a key enzyme of sterol synthesis, the hairpin membrane protein squalene monooxygenase (Erg1), as a non-canonical GET pathway client, thus rationalizing the lipotoxicity phenotypes of GET pathway mutants. Get3 recognizes the hairpin targeting element of Erg1 via its classical client-binding pocket. Intriguingly, we find that the GET pathway is especially important for the acute upregulation of Erg1 induced by low sterol conditions. We further identify several other proteins anchored to the endoplasmic reticulum (ER) membrane exclusively via a hairpin as putative clients of the GET pathway. Our findings emphasize the necessity of dedicated targeting pathways for high-efficiency targeting of particular clients during dynamic cellular adaptation and highlight hairpin proteins as a potential novel class of GET clients.

Alternative redox forms of ASNA-1 separate insulin signaling from tail-anchored protein targeting and cisplatin resistance in C. elegans

Cisplatin is a frontline cancer therapeutic, but intrinsic or acquired resistance is common. We previously showed that cisplatin sensitivity can be achieved by inactivation of ASNA-1/TRC40 in mammalian cancer cells and in Caenorhabditis elegans. ASNA-1 has two more conserved functions: in promoting tail-anchored protein (TAP) targeting to the endoplasmic reticulum membrane and in promoting insulin secretion. However, the relation between its different functions has remained unknown. Here, we show that ASNA-1 exists in two redox states that promote TAP-targeting and insulin secretion separately. The reduced state is the one required for cisplatin resistance: an ASNA-1 point mutant, in which the protein preferentially was found in the oxidized state, was sensitive to cisplatin and defective for TAP targeting but had no insulin secretion defect. The same was true for mutants in wrb-1, which we identify as the C. elegans homolog of WRB, the ASNA1/TRC40 receptor. Finally, we uncover a previously unknown action of cisplatin induced reactive oxygen species: cisplatin induced ROS drives ASNA-1 into the oxidized form, and selectively prevents an ASNA-1-dependent TAP substrate from reaching the endoplasmic reticulum. Our work suggests that ASNA-1 acts as a redox-sensitive target for cisplatin cytotoxicity and that cisplatin resistance is likely mediated by ASNA-1-dependent TAP substrates. Treatments that promote an oxidizing tumor environment should be explored as possible means to combat cisplatin resistance.

Thiol-based switching mechanisms of stress-sensing chaperones

Thiol-based redox switches evolved as efficient post-translational regulatory mechanisms that enable individual proteins to rapidly respond to sudden environmental changes. While some protein functions need to be switched off to save resources and avoid potentially error-prone processes, protective functions become essential and need to be switched on. In this review, we focus on thiol-based activation mechanisms of stress-sensing chaperones. Upon stress exposure, these chaperones convert into high affinity binding platforms for unfolding proteins and protect cells against the accumulation of potentially toxic protein aggregates. Their chaperone activity is independent of ATP, a feature that becomes especially important under oxidative stress conditions, where cellular ATP levels drop and canonical ATP-dependent chaperones no longer operate. Vice versa, reductive inactivation and substrate release require the restoration of ATP levels, which ensures refolding of client proteins by ATP-dependent foldases. We will give an overview over the different strategies that cells evolved to rapidly increase the pool of ATP-independent chaperones upon oxidative stress and provide mechanistic insights into how stress conditions are used to convert abundant cellular proteins into ATP-independent holding chaperones.

The natural history of Get3-like chaperones

Get3 in yeast or TRC40 in mammals is an ATPase that, in eukaryotes, is a central element of the GET or TRC pathway involved in the targeting of tail-anchored proteins. Get3 has also been shown to possess chaperone holdase activity. A bioinformatic assessment was performed across all domains of life on functionally important regions of Get3 including the TRC40-insert and the hydrophobic groove essential for tail-anchored protein binding. We find that such a hydrophobic groove is much more common in bacterial Get3 homologs than previously appreciated based on a directed comparison of bacterial ArsA and yeast Get3. Furthermore, our analysis shows that the region containing the TRC40-insert varies in length and methionine content to an unexpected extent within eukaryotes and also between different phylogenetic groups. In fact, since the TRC40-insert is present in all domains of life, we suggest that its presence does not automatically predict a tail-anchored protein targeting function. This opens up a new perspective on the function of organellar Get3 homologs in plants which feature the TRC40-insert but have not been demonstrated to function in tail-anchored protein targeting. Our analysis also highlights a large diversity of the ways Get3 homologs dimerize. Thus, based on the structural features of Get3 homologs, these proteins may have an unexplored functional diversity in all domains of life.

Two interacting ATPases protect Mycobacterium tuberculosis from glycerol and nitric oxide toxicity

The Mycobacterium tuberculosis H37Rv genome has been sequenced and annotated over 20 years ago, yet roughly half of the protein-coding genes still lack a predicted function. We characterized two genes of unknown function, rv3679 and rv3680, for which inconsistent findings regarding their importance for virulence in mice have been reported. We confirmed that a rv3679-80 deletion mutant (Δrv3679-80) was virulent in mice and discovered that Δrv3679-80 suffered from a glycerol-dependent recovery defect on agar plates following mouse infection. Glycerol also exacerbated killing of Δrv3679-80 by nitric oxide. Rv3679-Rv3680 have previously been shown to form a complex with ATPase activity and we demonstrate that the ability of M. tuberculosis to cope with elevated levels of glycerol and nitric oxide requires intact ATP-binding motifs in both Rv3679 and Rv3680. Inactivation of glycerol kinase or Rv2370c, a protein of unknown function, suppressed glycerol mediated toxicity in Δrv3679-80 Glycerol catabolism led to increased intracellular methylglyoxal pools and Δrv3679-80 was hypersusceptible to extracellular methylglyoxal suggesting that glycerol toxicity in Δrv3679-80 is caused by methylglyoxal. Rv3679 and Rv3680 interacted with Rv1509, and Rv3679 had numerous additional interactors including proteins of the type II fatty acid synthase (FASII) pathway and mycolic acid modifying enzymes linking Rv3679 to fatty acid or lipid synthesis. This work provides experimentally determined roles for Rv3679 and Rv3680 and stimulates future research on these and other proteins of unknown function.Importance A better understanding of the pathogenesis of tuberculosis requires a better understanding of gene function in M. tuberculosis This work provides the first functional insight into the Rv3679/Rv3680 ATPase complex. We demonstrate that M. tuberculosis requires this complex and specifically its ATPase activity to resist glycerol and nitric oxide toxicity. We provide evidence that the glycerol-derived metabolite methylglyoxal causes toxicity in the absence of Rv3679/Rv3680. We further show that glycerol-dependent toxicity is reversed when glycerol kinase (GlpK) is inactivated. Our work uncovered other genes of unknown function that interact with Rv3679 and/or Rv3680 genetically or physically, underscoring the importance of understanding uncharacterized genes.

A trap mutant reveals the physiological client spectrum of TRC40

The transmembrane recognition complex (TRC) pathway targets tail-anchored (TA) proteins to the membrane of the endoplasmic reticulum (ER). While many TA proteins are known to be able to use this pathway, it is essential for the targeting of only a few. Here, we uncover a large number of TA proteins that engage with TRC40 when other targeting machineries are fully operational. We use a dominant-negative ATPase-impaired mutant of TRC40 in which aspartate 74 was replaced by a glutamate residue to trap TA proteins in the cytoplasm. Manipulation of the hydrophobic TA-binding groove in TRC40 (also known as ASNA1) reduces interaction with most, but not all, substrates suggesting that co-purification may also reflect interactions unrelated to precursor protein targeting. We confirm known TRC40 substrates and identify many additional TA proteins interacting with TRC40. By using the trap approach in combination with quantitative mass spectrometry, we show that Golgi-resident TA proteins such as the golgins golgin-84, CASP and giantin as well as the vesicle-associated membrane-protein-associated proteins VAPA and VAPB interact with TRC40. Thus, our results provide new avenues to assess the essential role of TRC40 in metazoan organisms.

This article has an associated First Person interview with the first author of the paper.

Summary: A strategy to decipher which tail-anchored proteins do (as opposed to can or must) use the TRC pathway in intact cells generates a comprehensive list of human TRC40 clients.

The ATPase activity of Asna1/TRC40 is required for pancreatic progenitor cell survival

Asna1, also known as TRC40, is implicated in the delivery of tail-anchored (TA) proteins into the endoplasmic reticulum (ER), in vesicle-mediated transport, and in chaperoning unfolded proteins during oxidative stress/ATP depletion. Here, we show that Asna1 inactivation in pancreatic progenitor cells leads to redistribution of the Golgi TA SNARE proteins syntaxin 5 and syntaxin 6, Golgi fragmentation, and accumulation of cytosolic p62⁺ puncta. Asna1^−/− multipotent progenitor cells (MPCs) selectively activate integrated stress response signaling and undergo apoptosis, thereby disrupting endocrine and acinar cell differentiation, resulting in pancreatic agenesis. Rescue experiments implicate the Asna1 ATPase activity and a CXXC di-cysteine motif in ensuring Golgi integrity, syntaxin 5 localization and MPC survival. Ex vivo inhibition of retrograde transport reproduces the perturbed Golgi morphology, and syntaxin 5 and syntaxin 6 expression, whereas modulation of p53 activity, using PFT-α and Nutlin-3, prevents or reproduces apoptosis in Asna1-deficient and wild-type MPCs, respectively. These findings support a role for the Asna1 ATPase activity in ensuring the survival of pancreatic MPCs, possibly by counteracting p53-mediated apoptosis.

Summary: Conditional inactivation of Asna1/TRC40 in pancreatic progenitor cells results in pancreatic agenesis resulting from pancreatic progenitor cell apoptosis, thus revealing a crucial role for Asna1/TRC40 in pancreatic progenitor cell survival.

Stress-Activated Chaperones: A First Line of Defense

Proteins are constantly challenged by environmental stress conditions that threaten their structure and function. Especially problematic are oxidative, acid, and severe heat stress which induce very rapid and widespread protein unfolding and generate conditions that make canonical chaperones and/or transcriptional responses inadequate to protect the proteome. We review here recent advances in identifying and characterizing stress-activated chaperones which are inactive under non-stress conditions but become potent chaperones under specific protein-unfolding stress conditions. We discuss the post-translational mechanisms by which these chaperones sense stress, and consider the role that intrinsic disorder plays in their regulation and function. We examine their physiological roles under both non-stress and stress conditions, their integration into the cellular proteostasis network, and their potential as novel therapeutic targets.

Loss of GET pathway orthologs in Arabidopsis thaliana causes root hair growth defects and affects SNARE abundance

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins are key players in cellular trafficking and coordinate vital cellular processes, such as cytokinesis, pathogen defense, and ion transport regulation. With few exceptions, SNAREs are tail-anchored (TA) proteins, bearing a C-terminal hydrophobic domain that is essential for their membrane integration. Recently, the Guided Entry of Tail-anchored proteins (GET) pathway was described in mammalian and yeast cells that serve as a blueprint of TA protein insertion [Schuldiner M, et al. (2008) Cell 134(4):634-645; Stefanovic S, Hegde RS (2007) Cell 128(6):1147-1159]. This pathway consists of six proteins, with the cytosolic ATPase GET3 chaperoning the newly synthesized TA protein posttranslationally from the ribosome to the endoplasmic reticulum (ER) membrane. Structural and biochemical insights confirmed the potential of pathway components to facilitate membrane insertion, but the physiological significance in multicellular organisms remains to be resolved. Our phylogenetic analysis of 37 GET3 orthologs from 18 different species revealed the presence of two different GET3 clades. We identified and analyzed GET pathway components in Arabidopsis thaliana and found reduced root hair elongation in Atget lines, possibly as a result of reduced SNARE biogenesis. Overexpression of AtGET3a in a receptor knockout (KO) results in severe growth defects, suggesting presence of alternative insertion pathways while highlighting an intricate involvement for the GET pathway in cellular homeostasis of plants.

Protein quality control under oxidative stress conditions

Accumulation of reactive oxygen and chlorine species (RO/CS) is generally regarded to be a toxic and highly undesirable event, which serves as contributing factor in aging and many age-related diseases. However, it is also put to excellent use during host defense, when high levels of RO/CS are produced to kill invading microorganisms and regulate bacterial colonization. Biochemical and cell biological studies of how bacteria and other microorganisms deal with RO/CS have now provided important new insights into the physiological consequences of oxidative stress, the major targets that need protection, and the cellular strategies employed by organisms to mitigate the damage. This review examines the redox-regulated mechanisms by which cells maintain a functional proteome during oxidative stress. We will discuss the well-characterized redox-regulated chaperone Hsp33, and we will review recent discoveries demonstrating that oxidative stress-specific activation of chaperone function is a much more widespread phenomenon than previously anticipated. New members of this group include the cytosolic ATPase Get3 in yeast, the Escherichia coli protein RidA, and the mammalian protein α2-macroglobulin. We will conclude our review with recent evidence showing that inorganic polyphosphate (polyP), whose accumulation significantly increases bacterial oxidative stress resistance, works by a protein-like chaperone mechanism. Understanding the relationship between oxidative and proteotoxic stresses will improve our understanding of both host-microbe interactions and how mammalian cells combat the damaging side effects of uncontrolled RO/CS production, a hallmark of inflammation.

The protein targeting factor Get3 functions as ATP-independent chaperone under oxidative stress conditions

Exposure of cells to reactive oxygen species (ROS) causes a rapid and significant drop in intracellular ATP levels. This energy depletion negatively affects ATP-dependent chaperone systems, making ROS-mediated protein unfolding and aggregation a potentially very challenging problem. Here we show that Get3, a protein involved in ATP-dependent targeting of tail-anchored (TA) proteins under nonstress conditions, turns into an effective ATP-independent chaperone when oxidized. Activation of Get3's chaperone function, which is a fully reversible process, involves disulfide bond formation, metal release, and its conversion into distinct, higher oligomeric structures. Mutational studies demonstrate that the chaperone activity of Get3 is functionally distinct from and likely mutually exclusive with its targeting function, and responsible for the oxidative stress-sensitive phenotype that has long been noted for yeast cells lacking functional Get3. These results provide convincing evidence that Get3 functions as a redox-regulated chaperone, effectively protecting eukaryotic cells against oxidative protein damage.

The GEF1 proton-chloride exchanger affects tombusvirus replication via regulation of copper metabolism in yeast

Replication of plus-strand RNA viruses [(+)RNA viruses] is performed by viral replicases, whose function is affected by many cellular factors in infected cells. In this paper, we demonstrate a surprising role for Gef1p proton-chloride exchanger in replication of Tomato bushy stunt virus (TBSV) model (+)RNA virus. A genetic approach revealed that Gef1p, which is the only proton-chloride exchanger in Saccharomyces cerevisiae, is required for TBSV replication in the yeast model host. We also show that the in vitro activity of the purified tombusvirus replicase from gef1Δ yeast was low and that the in vitro assembly of the viral replicase in a cell extract was inhibited by the cytosolic fraction obtained from gef1Δ yeast. Altogether, our data reveal that Gef1p modulates TBSV replication via regulating Cu(2+) metabolism in the cell. This conclusion is supported by several lines of evidence, including the direct inhibitory effect of Cu(2+) ions on the in vitro assembly of the viral replicase, on the activity of the viral RNA-dependent RNA polymerase, and an inhibitory effect of deletion of CCC2 copper pump on TBSV replication in yeast, while altered iron metabolism did not reduce TBSV replication. In addition, applying a chloride channel blocker impeded TBSV replication in Nicotiana benthamiana protoplasts or in whole plants. Overall, blocking Gef1p function seems to inhibit TBSV replication through altering Cu(2+) ion metabolism in the cytosol, which then inhibits the normal functions of the viral replicase.

Get3 is a holdase chaperone and moves to deposition sites for aggregated proteins when membrane targeting is blocked

The endomembrane system of yeast contains different tail-anchored proteins that are post-translationally targeted to membranes via their C-terminal transmembrane domain. This hydrophobic segment could be hazardous in the cytosol if membrane insertion fails, resulting in the need for energy-dependent chaperoning and the degradation of aggregated tail-anchored proteins. A cascade of GET proteins cooperates in a conserved pathway to accept newly synthesized tail-anchored proteins from ribosomes and guide them to a receptor at the endoplasmic reticulum, where membrane integration takes place. It is, however, unclear how the GET system reacts to conditions of energy depletion that might prevent membrane insertion and hence lead to the accumulation of hydrophobic proteins in the cytosol. Here we show that the ATPase Get3, which accommodates the hydrophobic tail anchor of clients, has a dual function: promoting tail-anchored protein insertion when glucose is abundant and serving as an ATP-independent holdase chaperone during energy depletion. Like the generic chaperones Hsp42, Ssa2, Sis1 and Hsp104, we found that Get3 moves reversibly to deposition sites for protein aggregates, hence supporting the sequestration of tail-anchored proteins under conditions that prevent tail-anchored protein insertion. Our findings support a ubiquitous role for the cytosolic GET complex as a triaging platform involved in cellular proteostasis.

A directed RNAi screen based on larval growth arrest reveals new modifiers of C. elegans insulin signaling

Genes regulating Caenorhabditis elegans insulin/IGF signaling (IIS) have largely been identified on the basis of their involvement in dauer development or longevity. A third IIS phenotype is the first larval stage (L1) diapause, which is also influenced by asna-1, a regulator of DAF-28/insulin secretion. We reasoned that new regulators of IIS strength might be identified in screens based on the L1 diapause and the asna-1 phenotype. Eighty- six genes were selected for analysis by virtue of their predicted interaction with ASNA-1 and screened for asna-1-like larval arrest. ykt-6, mrps-2, mrps-10 and mrpl-43 were identified as genes which, when inactivated, caused larval arrest without any associated feeding defects. Several tests indicated that IIS strength was weaker and that insulin secretion was defective in these animals. This study highlights the role of the Golgi network and the mitochondria in insulin secretion and provides a new list of genes that modulate IIS in C. elegans.

The complex process of GETting tail-anchored membrane proteins to the ER

Biosynthesis of membrane proteins requires that hydrophobic transmembrane (TM) regions be shielded from the cytoplasm while being directed to the correct membrane. Tail-anchored (TA) membrane proteins, characterized by a single C-terminal TM, pose an additional level of complexity because they must be post-translationally targeted. In eukaryotes, the GET pathway shuttles TA-proteins to the endoplasmic reticulum. The key proteins required in yeast (Sgt2 and Get1-5) have been under extensive structural and biochemical investigation during recent years. The central protein Get3 utilizes nucleotide linked conformational changes to facilitate substrate loading and targeting. Here we analyze this complex process from a structural perspective, as understood in yeast, and further postulate on similar pathways in other domains of life.

Tail-anchor targeting by a Get3 tetramer: the structure of an archaeal homologue

Efficient delivery of membrane proteins is a critical cellular process. The recently elucidated GET (Guided Entry of TA proteins) pathway is responsible for the targeted delivery of tail-anchored (TA) membrane proteins to the endoplasmic reticulum. The central player is the ATPase Get3, which in its free form exists as a dimer. Biochemical evidence suggests a role for a tetramer of Get3. Here, we present the first crystal structure of an archaeal Get3 homologue that exists as a tetramer and is capable of TA protein binding. The tetramer generates a hydrophobic chamber that we propose binds the TA protein. We use small-angle X-ray scattering to provide the first structural information of a fungal Get3/TA protein complex showing that the overall molecular envelope is consistent with the archaeal tetramer structure. Moreover, we show that this fungal tetramer complex is capable of TA insertion. This allows us to suggest a model where a tetramer of Get3 sequesters a TA protein during targeting to the membrane.

ASNA-1 activity modulates sensitivity to cisplatin

Cancer can be cured by platinum-based chemotherapy, but resistance is a major cause of treatment failure. Here we present the nematode Caenorhabditis elegans as a model to study interactions between the platinum drug cisplatin and signaling pathways in vivo. Null mutation in a single gene, asna-1, makes worms hypersensitive to cisplatin. The metalloregulated ATPase ASNA-1 promotes insulin secretion and membrane insertion of tail-anchored proteins. Using structural data from ASNA-1 homologues, we identify specific ASNA-1 mutants that are sensitive to cisplatin while still able to promote insulin signaling. Mutational analysis reveals that hypersensitivity of ASNA-1 mutants to cisplatin remains in absence of CEP-1/p53 or apoptosis. Human ASNA1 can substitute for the worm gene, indicating a conserved function. Cisplatin sensitivity is not affected by decreased insulin signaling in wild-type nematodes or restored insulin signaling in asna-1 mutants. These findings provide a functional insight into ASNA-1, demonstrate that C. elegans can be used to characterize cisplatin resistance mechanisms, and suggest that rationally designed drugs against ASNA-1 can sensitize cancer cells to cisplatin.

Functional Promiscuity of Homologues of the Bacterial ArsA ATPases

The ArsA ATPase of E. coli plays an essential role in arsenic detoxification. Published evidence implicates ArsA in the energization of As(III) efflux via the formation of an oxyanion-translocating complex with ArsB. In addition, eukaryotic ArsA homologues have several recognized functions unrelated to arsenic resistance. By aligning ArsA homologues, constructing phylogenetic trees, examining ArsA encoding operons, and estimating the probable coevolution of these homologues with putative transporters and auxiliary proteins unrelated to ArsB, we provide evidence for new functions for ArsA homologues. They may play roles in carbon starvation, gas vesicle biogenesis, and arsenic resistance. The results lead to the proposal that ArsA homologues energize four distinct and nonhomologous transporters, ArsB, ArsP, CstA, and Acr3.

Structures of Get3, Get4, and Get5 provide new models for TA membrane protein targeting

The GET pathway, using several proteins (Gets 1-5 and probably Sgt2), posttranslationally conducts tail-anchored (TA) proteins to the endoplasmic reticulum (ER). At the ER, TA proteins are inserted into the lipid bilayer and then sorted and directed to their respective destinations in the secretory pathway. Until last year, there was no structural information on any of the GET components but now there are ten crystal structures of Get3 in a variety of nucleotide-bound states and conformations. The structures of Get4 and a portion of Get5 also emerged in 2010. This minireview provides a detailed comparison of the GET structures and discusses their mechanistic relevance to TA protein insertion. It also addresses the outstanding gaps in detailed molecular information on this system, including the structures of Get5, Sgt2, and the transmembrane complex comprising Get1 and Get2.

Genes involved in novel adaptive aluminum resistance in Rhodotorula glutinis

Rhodotorula glutinis IFO1125 acquired increased aluminum (Al) resistance from 50 muM to more than 5 mM by repetitive culturing with stepwise increases in Al concentration at pH 4.0. In our previous study we performed differential display to find that three genes (RgFET3, RgGET3, and RgCMK) encoding proteins homologous to Saccharomyces cerevisiae FET3p, GET3p, and CMK1p and CMK2p, respectively, were up-regulated in the Al-resistant cells. In this study we cloned these genes and found they were indeed up-regulated in Al-resistant strains. The cloned genes were introduced into S. cerevisiae and corresponding mutants to test their relevance to Al resistance. The introduction of RgFET3 and RgGET3 conferred Al resistance to the host, but that of RgCMK did not. Green fluorescent protein (GFP)-tagged RgFet3p was localized at the cell periphery in the host. GFP-tagged RgGet3p formed more punctate bodies in the host under Al stress than in the absence of Al. Different growth responses to Fe (III), Cu (II), Ca ions, and cyclosporin A in the wild type and resistant cells of R. glutinis suggested the involvement and possible links of the three genes in Al resistance.

Biogenesis of tail-anchored proteins: the beginning for the end?

Tail-anchored proteins are a distinct class of integral membrane proteins located in several eukaryotic organelles, where they perform a diverse range of functions. These proteins have in common the C-terminal location of their transmembrane anchor and the resulting post-translational nature of their membrane insertion, which, unlike the co-translational membrane insertion of most other proteins, is not coupled to ongoing protein synthesis. The study of tail-anchored proteins has provided a paradigm for understanding the components and pathways that mediate post-translational biogenesis of membrane proteins at the endoplasmic reticulum. In this Commentary, we review recent studies that have converged at a consensus regarding the molecular mechanisms that underlie this process--namely, that multiple pathways underlie the biogenesis of tail-anchored proteins at the endoplasmic reticulum.

Structural insight into the membrane insertion of tail-anchored proteins by Get3

Tail anchored (TA) proteins, which are important for numerous cellular processes, are defined by a single transmembrane domain (TMD) near the C-terminus. The membrane insertion of TA proteins is mediated by the highly conserved ATPase Get3. Here we report the crystal structures of Get3 in ADP-bound and nucleotide-free forms at 3.0 A and 2.8 A resolutions, respectively. Get3 consists of a nucleotide binding domain and a helical domain. Both structures exhibit a Zn(2+)-mediated homodimer in a head-to-head orientation, representing an open dimer conformation. Our cross-link experiments indicated the closed dimer-stimulating ATP hydrolysis, which might be coupled with TA-protein release. Further, our coexpression-based binding assays using a model TA protein Sec22p revealed the direct interaction between the helical domain of Get3 and the Sec22p TMD. This interaction is independent of ATP and dimer formation. Finally, we propose a structural mechanism that links ATP hydrolysis with the TA-protein insertion mediated by the conserved DTAPTGH motif.

Structural insights into tail-anchored protein binding and membrane insertion by Get3

Tail-anchored (TA) membrane proteins are involved in a variety of important cellular functions, including membrane fusion, protein translocation, and apoptosis. The ATPase Get3 (Asna1, TRC40) was identified recently as the endoplasmic reticulum targeting factor of TA proteins. Get3 consists of an ATPase and alpha-helical subdomain enriched in methionine and glycine residues. We present structural and biochemical analyses of Get3 alone as well as in complex with a TA protein, ribosome-associated membrane protein 4 (Ramp4). The ATPase domains form an extensive dimer interface that encloses 2 nucleotides in a head-to-head orientation and a zinc ion. Amide proton exchange mass spectrometry shows that the alpha-helical subdomain of Get3 displays considerable flexibility in solution and maps the TA protein-binding site to the alpha-helical subdomain. The non-hydrolyzable ATP analogue AMPPNP-Mg(2+)- and ADP-Mg(2+)-bound crystal structures representing the pre- and posthydrolysis states are both in a closed form. In the absence of a TA protein cargo, ATP hydrolysis does not seem to be possible. Comparison with the ADP.AlF(4)(-)-bound structure representing the transition state (Mateja A, et al. (2009) Nature 461:361-366) indicates how the presence of a TA protein is communicated to the ATP-binding site. In vitro membrane insertion studies show that recombinant Get3 inserts Ramp4 in a nucleotide- and receptor-dependent manner. Although ATP hydrolysis is not required for Ramp4 insertion per se, it seems to be required for efficient insertion. We postulate that ATP hydrolysis is needed to release Get3 from its receptor. Taken together, our results provide mechanistic insights into posttranslational targeting of TA membrane proteins by Get3.

Model for eukaryotic tail-anchored protein binding based on the structure of Get3

The Get3 ATPase directs the delivery of tail-anchored (TA) proteins to the endoplasmic reticulum (ER). TA-proteins are characterized by having a single transmembrane helix (TM) at their extreme C terminus and include many essential proteins, such as SNAREs, apoptosis factors, and protein translocation components. These proteins cannot follow the SRP-dependent co-translational pathway that typifies most integral membrane proteins; instead, post-translationally, these proteins are recognized and bound by Get3 then delivered to the ER in the ATP dependent Get pathway. To elucidate a molecular mechanism for TA protein binding by Get3 we have determined three crystal structures in apo and ADP forms from Saccharomyces cerevisae (ScGet3-apo) and Aspergillus fumigatus (AfGet3-apo and AfGet3-ADP). Using structural information, we generated mutants to confirm important interfaces and essential residues. These results point to a model of how Get3 couples ATP hydrolysis to the binding and release of TA-proteins.

The structural basis of tail-anchored membrane protein recognition by Get3

Targeting of newly synthesized membrane proteins to the endoplasmic reticulum is an essential cellular process. Most membrane proteins are recognized and targeted co-translationally by the signal recognition particle. However, nearly 5% of membrane proteins are 'tail-anchored' by a single carboxy-terminal transmembrane domain that cannot access the co-translational pathway. Instead, tail-anchored proteins are targeted post-translationally by a conserved ATPase termed Get3. The mechanistic basis for tail-anchored protein recognition or targeting by Get3 is not known. Here we present crystal structures of yeast Get3 in 'open' (nucleotide-free) and 'closed' (ADP.AlF(4)(-)-bound) dimer states. In the closed state, the dimer interface of Get3 contains an enormous hydrophobic groove implicated by mutational analyses in tail-anchored protein binding. In the open state, Get3 undergoes a striking rearrangement that disrupts the groove and shields its hydrophobic surfaces. These data provide a molecular mechanism for nucleotide-regulated binding and release of tail-anchored proteins during their membrane targeting by Get3.

The GET complex mediates insertion of tail-anchored proteins into the ER membrane

Tail-anchored (TA) proteins, defined by the presence of a single C-terminal transmembrane domain (TMD), play critical roles throughout the secretory pathway and in mitochondria, yet the machinery responsible for their proper membrane insertion remains poorly characterized. Here we show that Get3, the yeast homolog of the TA-interacting factor Asna1/Trc40, specifically recognizes TMDs of TA proteins destined for the secretory pathway. Get3 recognition represents a key decision step, whose loss can lead to misinsertion of TA proteins into mitochondria. Get3-TA protein complexes are recruited for endoplasmic reticulum (ER) membrane insertion by the Get1/Get2 receptor. In vivo, the absence of Get1/Get2 leads to cytosolic aggregation of Get3-TA complexes and broad defects in TA protein biogenesis. In vitro reconstitution demonstrates that the Get proteins directly mediate insertion of newly synthesized TA proteins into ER membranes. Thus, the GET complex represents a critical mechanism for ensuring efficient and accurate targeting of TA proteins.

Coactivation of G protein signaling by cell-surface receptors and an intracellular exchange factor

G protein-coupled receptors (GPCRs) mediate responses to a broad range of chemical and environmental signals. In yeast, a pheromone-binding GPCR triggers events leading to the fusion of haploid cells. In general, GPCRs function as guanine-nucleotide exchange factors (GEFs); upon agonist binding, the receptor induces a conformational change in the G protein alpha subunit, resulting in exchange of guanine diphosphate (GDP) for guanine triphosphate (GTP) and in signal initiation. Signaling is terminated when GTP is hydrolyzed to GDP [1]. This well-established paradigm has in recent years been revised to include new components that rates of GDP release, GTP binding [2-8], and GTP hydrolysis[9, 10]. Here we report the discovery of a nonreceptor GEF, Arr4. Like receptors, Arr4 binds directly to the G protein,accelerates guanine-nucleotide exchange, and stabilizes the nucleotide-free state of the a subunit. Moreover, Arr4 promotes G protein-dependent cellular responses, including mitogen-activated protein kinase (MAPK) phosphorylation,new-gene transcription, and mating. In contrast to knownGPCRs, however, Arr4 is not a transmembrane receptor,but rather a soluble intracellular protein. Our data suggest that intracellular proteins function in cooperation with mating pheromones to amplify G protein signaling, thereby leading to full pathway activation.

Distinct targeting pathways for the membrane insertion of tail-anchored (TA) proteins

Tail-anchored (TA) proteins are characterised by a C-terminal transmembrane region that mediates post-translational insertion into the membrane of the endoplasmic reticulum (ER). We have investigated the requirements for membrane insertion of three TA proteins, RAMP4, Sec61beta and cytocrome b5. We show here that newly synthesised RAMP4 and Sec61beta can accumulate in a cytosolic, soluble complex with the ATPase Asna1 before insertion into ER-derived membranes. Membrane insertion of these TA proteins is stimulated by ATP, sensitive to redox conditions and blocked by alkylation of SH groups by N-ethylmaleimide (NEM). By contrast, membrane insertion of cytochrome b5 is not found to be mediated by Asna1, not stimulated by ATP and not affected by NEM or an oxidative environment. The Asna1-mediated pathway of membrane insertion of RAMP4 and Sec61beta may relate to functions of these proteins in the ER stress response.

CLC chloride channels and transporters: from genes to protein structure, pathology and physiology

CLC genes are expressed in species from bacteria to human and encode Cl(-)-channels or Cl(-)/H(+)-exchangers. CLC proteins assemble to dimers, with each monomer containing an ion translocation pathway. Some mammalian isoforms need essential beta -subunits (barttin and Ostm1). Crystal structures of bacterial CLC Cl(-)/H(+)-exchangers, combined with transport analysis of mammalian and bacterial CLCs, yielded surprising insights into their structure and function. The large cytosolic carboxy-termini of eukaryotic CLCs contain CBS domains, which may modulate transport activity. Some of these have been crystallized. Mammals express nine CLC isoforms that differ in tissue distribution and subcellular localization. Some of these are plasma membrane Cl(-) channels, which play important roles in transepithelial transport and in dampening muscle excitability. Other CLC proteins localize mainly to the endosomal-lysosomal system where they may facilitate luminal acidification or regulate luminal chloride concentration. All vesicular CLCs may be Cl(-)/H(+)-exchangers, as shown for the endosomal ClC-4 and -5 proteins. Human diseases and knockout mouse models have yielded important insights into their physiology and pathology. Phenotypes and diseases include myotonia, renal salt wasting, kidney stones, deafness, blindness, male infertility, leukodystrophy, osteopetrosis, lysosomal storage disease and defective endocytosis, demonstrating the broad physiological role of CLC-mediated anion transport.

A CLC chloride channel plays an essential role in copper homeostasis in Aspergillus nidulans at increased extracellular copper concentrations

A putative CLC voltage-gated anion channel gene from Aspergillus nidulans (AnCLCA) is characterised. The expression of the AnCLCA cDNA restored the iron-limited growth of the Saccharomyces cerevisiae CLC null mutant strain (gef1) suggesting that AnCLCA functions as a chloride channel. An AnCLCA conditional mutant was created and exhibited a strong and specific growth inhibition in the presence of extracellular copper concentrations >18 microM. This sensitivity was shown to be the result of a hyper-accumulation of copper by the conditional mutant, which generates superoxide to toxic levels inhibiting the growth. Further analysis revealed that copper dependent enzymes were disrupted in the AnCLCA conditional null mutant, specifically, a reduced activity of the copper-zinc superoxide dismutase (CuZn-SOD) and enhanced activity of the cytochrome oxidase (COX). These results suggest that AnCLCA plays a key role in copper homeostasis in A. nidulans and that a malfunction of this chloride channel results in disrupted intracellular copper trafficking.

Identification of a Targeting Factor for Posttranslational Membrane Protein Insertion into the ER

Hundreds of proteins are anchored in intracellular membranes by a single transmembrane domain (TMD) close to the C terminus. Although these tail-anchored (TA) proteins serve numerous essential roles in cells, components of their targeting and insertion pathways have long remained elusive. Here we reveal a cytosolic TMD recognition complex (TRC) that targets TA proteins for insertion into the ER membrane. The highly conserved, 40 kDa ATPase subunit of TRC (which we termed TRC40) was identified as Asna-1. TRC40/Asna-1 interacts posttranslationally with TA proteins in a TMD-dependent manner for delivery to a proteinaceous receptor at the ER membrane. Subsequent release from TRC40/Asna-1 and insertion into the membrane depends on ATP hydrolysis. Consequently, an ATPase-deficient mutant of TRC40/Asna-1 dominantly inhibited TA protein insertion selectively without influencing other translocation pathways. Thus, TRC40/Asna-1 represents an integral component of a posttranslational pathway of membrane protein insertion whose targeting is mediated by TRC.

The conserved ATPase Get3/Arr4 modulates the activity of membrane-associated proteins in Saccharomyces cerevisiae

The regulation of cellular membrane dynamics is crucial for maintaining proper cell growth and division. The Cdc48-Npl4-Ufd1 complex is required for several regulated membrane-associated processes as part of the ubiquitin-proteasome system, including ER-associated degradation and the control of lipid composition in yeast. In this study we report the results of a genetic screen in Saccharomyces cerevisiae for extragenic suppressors of a temperature-sensitive npl4 allele and the subsequent analysis of one suppressor, GET3/ARR4. The GET3 gene encodes an ATPase with homology to the regulatory component of the bacterial arsenic pump. Mutants of GET3 rescue several phenotypes of the npl4 mutant and transcription of GET3 is coregulated with the proteasome, illustrating a functional relationship between GET3 and NPL4 in the ubiquitin-proteasome system. We have further found that Get3 biochemically interacts with the trans-membrane domain proteins Get1/Mdm39 and Get2/Rmd7 and that Deltaget3 is able to suppress phenotypes of get1 and get2 mutants, including sporulation defects. In combination, our characterization of GET3 genetic and biochemical interactions with NPL4, GET1, and GET2 implicates Get3 in multiple membrane-dependent pathways.

Targeted disruption of the mouseAsna1gene results in embryonic lethality

The bacterial ArsA ATPase is the catalytic component of an oxyanion pump that is responsible for resistance to arsenicals and antimonials. Homologues of the bacterial ArsA ATPase are widespread in nature. We had earlier identified the mouse homologue (Asna1) that exhibits 27% identity to the bacterial ArsA ATPase. To identify the physiological role of the protein, heterozygous Asna1 knockout mice (Asna1+/-) were generated by homologous recombination. The Asna1+/- mice displayed similar phenotype as the wild-type mice. However, early embryonic lethality was observed in homozygous Asna1 knockout embryos, between E3.5 (E=embryonic day) and E8.5 stage. These findings indicate that Asna1 plays a crucial role during early embryonic development.