Tail-anchored protein biosynthesis at the endoplasmic reticulum: the same but different

The Function, Structure, and Origins of the ER Membrane Protein Complex

The endoplasmic reticulum (ER) is the site of membrane protein insertion, folding, and assembly in eukaryotes. Over the past few years, a combination of genetic and biochemical studies have implicated an abundant factor termed the ER membrane protein complex (EMC) in several aspects of membrane protein biogenesis. This large nine-protein complex is built around a deeply conserved core formed by the EMC3-EMC6 subcomplex. EMC3 belongs to the universally conserved Oxa1 superfamily of membrane protein transporters, whereas EMC6 is an ancient, widely conserved obligate partner. EMC has an established role in the insertion of transmembrane domains (TMDs) and less understood roles during the later steps of membrane protein folding and assembly. Several recent structures suggest hypotheses about the mechanism(s) of TMD insertion by EMC, with various biochemical and proteomics studies beginning to reveal the range of EMC's membrane protein substrates. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Structures and Functions of VZV Glycoproteins

The virions of all enveloped viruses, including those of the Herpesviridae, must bind to the cell surface then undergo a process of membrane fusion between the cell plasma membrane and the virus particle envelope. As for all herpesviruses, glycoproteins in the virion envelope are the modus operandi of these events.

Go your own way: membrane-targeting sequences

Membrane-targeting sequences, connected targeting mechanisms, and co-factors orchestrate primary targeting of proteins to membranes.

Neutralization of Junín virus by single domain antibodies targeted against the nucleoprotein

The syndrome viral haemorrhagic fever (VHF) designates a broad range of diseases that are caused by different viruses including members of the family Arenaviridae. Prophylaxis for Argentine Haemorrhagic Fever (AHF), caused by the arenavirus Junín (JUNV), has been achieved by the use of a live attenuated vaccine, named Candid#1. The standard treatment of AHF is transfusion of convalescent human plasma. Our aim was to develop an alternative and safer treatment for AHF based on the use of virus-neutralizing single domain antibodies (VHHs). We describe the first reported VHHs directed against an arenavirus. These VHHs could neutralize Candid#1 by altering virion binding/fusion. Surprisingly, the neutralizing VHHs appeared to be specific for the viral nucleoprotein (N) that is not known to be involved in arenavirus entry. Candid#1 VHH-escape viruses had acquired a predicted N-glycosylation site in the surface glycoprotein GP1 that is present in highly pathogenic JUNV strains. Accordingly, the Candid#1-neutralizing VHHs could not neutralize pathogenic JUNV strains, but they could still bind to cells infected with a pathogenic strain or the escape mutant viruses. These results show that the attenuated strains of JUNV can be potently neutralized by nucleoprotein-specific VHHs.

Structural and functional characterization of ybr137wp implicates its involvement in the targeting of tail-anchored proteins to membranes

Nearly 5% of membrane proteins are guided to nuclear, endoplasmic reticulum (ER), mitochondrial, Golgi, or peroxisome membranes by their C-terminal transmembrane domain and are classified as tail-anchored (TA) membrane proteins. In Saccharomyces cerevisiae, the guided entry of TA protein (GET) pathway has been shown to function in the delivery of TA proteins to the ER. The sorting complex for this pathway is comprised of Sgt2, Get4, and Get5 and facilitates the loading of nascent tail-anchored proteins onto the Get3 ATPase. Multiple pulldown assays also indicated that Ybr137wp associates with this complex in vivo. Here, we report a 2.8-Å-resolution crystal structure for Ybr137wp from Saccharomyces cerevisiae. The protein is a decamer in the crystal and also in solution, as observed by size exclusion chromatography and analytical ultracentrifugation. In addition, isothermal titration calorimetry indicated that the C-terminal acidic motif of Ybr137wp interacts with the tetratricopeptide repeat (TPR) domain of Sgt2. Moreover, an in vivo study demonstrated that Ybr137wp is induced in yeast exiting the log phase and ameliorates the defect of TA protein delivery and cell viability derived by the impaired GET system under starvation conditions. Therefore, this study suggests a possible role for Ybr137wp related to targeting of tail-anchored proteins.

Comparison between the behavior of different hydrophobic peptides allowing membrane anchoring of proteins

Membrane binding of proteins such as short chain dehydrogenase reductases or tail-anchored proteins relies on their N- and/or C-terminal hydrophobic transmembrane segment. In this review, we propose guidelines to characterize such hydrophobic peptide segments using spectroscopic and biophysical measurements. The secondary structure content of the C-terminal peptides of retinol dehydrogenase 8, RGS9-1 anchor protein, lecithin retinol acyl transferase, and of the N-terminal peptide of retinol dehydrogenase 11 has been deduced by prediction tools from their primary sequence as well as by using infrared or circular dichroism analyses. Depending on the solvent and the solubilization method, significant structural differences were observed, often involving α-helices. The helical structure of these peptides was found to be consistent with their presumed membrane binding. Langmuir monolayers have been used as membrane models to study lipid-peptide interactions. The values of maximum insertion pressure obtained for all peptides using a monolayer of 1,2-dioleoyl-sn-glycero-3-phospho-ethanolamine (DOPE) are larger than the estimated lateral pressure of membranes, thus suggesting that they bind membranes. Polarization modulation infrared reflection absorption spectroscopy has been used to determine the structure and orientation of these peptides in the absence and in the presence of a DOPE monolayer. This lipid induced an increase or a decrease in the organization of the peptide secondary structure. Further measurements are necessary using other lipids to better understand the membrane interactions of these peptides.

WRB and CAML are necessary and sufficient to mediate tail-anchored protein targeting to the ER membrane

Tail-Anchored (TA) proteins are inserted into the endoplasmic reticulum (ER) membrane of yeast cells via the posttranslational Guided Entry of Tail-Anchored protein (GET) pathway. The key component of this targeting machinery is the ATPase Get3 that docks to the ER membrane by interacting with a receptor complex formed by the proteins Get1 and Get2. A conserved pathway is present in higher eukaryotes and is mediated by TRC40, homolog of Get3, and the recently identified membrane receptors WRB and CAML. Here, we used yeast lacking the GET1 and GET2 genes and substituted them with WRB and CAML. This rescued the growth phenotypes of the GET receptor mutant. We demonstrate that WRB and CAML efficiently recruit Get3 to the ER membrane and promote the targeting of the TA proteins in vivo. Our results show that the membrane spanning segments of CAML are essential to create a functional receptor with WRB and to ensure TA protein membrane insertion. Finally, we determined the binding parameters of TRC40 to the WRB/CAML receptor. We conclude that together, WRB and CAML are not only necessary but also sufficient to create a functional membrane receptor complex for TRC40. The yeast complementation assay can be used to further dissect the structure-function relationship of the WRB/CAML heteromultimer in the absence of endogenous receptor proteins.

The association of BAG6 with SGTA and tail-anchored proteins

Background

The BAG6 protein is a subunit of a heterotrimeric complex that binds a range of membrane and secretory protein precursors localized to the cytosol, enforcing quality control and influencing their subsequent fate.

Methodology and Principal Findings

BAG6 has an N-terminal ubiquitin-like domain, and a C-terminal Bcl-2-associated athanogene domain, separated by a large central proline-rich region. We have used in vitro binding approaches to identify regions of BAG6 important for its interactions with: i) the small-glutamine rich tetratricopeptide repeat-containing protein alpha (SGTA) and ii) two model tail-anchored membrane proteins as a paradigm for its hydrophobic substrates. We show that the BAG6-UBL is essential for binding to SGTA, and find that the UBL of a second subunit of the BAG6-complex, ubiquitin-like protein 4A (UBL4A), competes for SGTA binding. Our data show that this binding is selective, and suggest that SGTA can bind either BAG6, or UBL4A, but not both at the same time. We adapted our in vitro binding assay to study the association of BAG6 with an immobilized tail-anchored protein, Sec61β, and find both the UBL and BAG domains are dispensable for binding this substrate. This conclusion was further supported using a heterologous subcellular localization assay in yeast, where the BAG6-dependent nuclear relocalization of a second tail-anchored protein, GFP-Sed5, also required neither the UBL, nor the BAG domain of BAG6.

Significance

On the basis of these findings, we propose a working model where the large central region of the BAG6 protein provides a binding site for a diverse group of substrates, many of which expose a hydrophobic stretch of polypeptide. This arrangement would enable the BAG6 complex to bring together its substrates with potential effectors including those recruited via its N-terminal UBL. Such effectors may include SGTA, and the resulting assemblies influence the subsequent fate of the hydrophobic BAG6 substrates.

HIV-1 Vpr triggers mitochondrial destruction by impairing Mfn2-mediated ER-mitochondria interaction

Human immunodeficiency virus 1 (HIV-1) viral protein R (Vpr) has been shown to induce host cell death by increasing the permeability of mitochondrial outer membrane (MOM). The mechanism underlying the damage to the mitochondria by Vpr, however, is not clearly illustrated. In this study, Vpr that is introduced, via transient transfection or lentivirus infection, into the human embryonic kidney cell line HEK293, human CD4⁺ T lymphoblast cell line SupT1, or human primary CD4⁺ T cells serves as the model system to study the molecular mechanism of Vpr-mediated HIV-1 pathogenesis. The results show that Vpr injures MOM and causes a loss in membrane potential (MMP) by posttranscriptionally reducing the expression of mitofusin 2 (Mfn2) via VprBP-DDB1-CUL4A ubiquitin ligase complex, gradually weakening MOM, and increasing mitochondrial deformation. Vpr also markedly decreases cytoplasmic levels of dynamin-related protein 1 (DRP1) and increases bulging in mitochondria-associated membranes (MAM), the specific regions of endoplasmic reticulum (ER) which form physical contacts with the mitochondria. Overexpression of Mfn2 and DRP1 significantly decreased the loss of MMP and apoptotic cell death caused by Vpr. Furthermore, by employing time-lapse confocal fluorescence microscopy, we identify the transport of Vpr protein from the ER, via MAM to the mitochondria. Taken together, our results suggest that Vpr-mediated cellular damage may occur on an alternative protein transport pathway from the ER, via MAM to the mitochondria, which are modulated by Mfn2 and DRP1.

Interaction of modified tail-anchored proteins with liposomes: effect of extensions of hydrophilic segment at the COOH-terminus of holo-cytochromes b₅

A group of membrane proteins having a single COOH-terminal hydrophobic domain capable of post-translational insertion into lipid bilayer is known as tail-anchored (TA) proteins. To clarify the insertion mechanism of the TA-domain of human cytochrome b(5) (Hcytb5) into ER membranes, we produced and purified various membrane-bound forms of Hcytb5 with their heme b-bound, in which various truncated forms of NH(2)-terminal bovine opsin sequence were appended at the COOH-terminus of the native form. We analyzed the integration of the TA-domains of these forms onto protein-free liposomes. The integration occurred efficiently even in the presence of a small amount of sodium cholate and, once incorporated, such proteoliposomes were very stable. The mode of the integration was further analyzed by treatment of the proteoliposomes with trypsin either on the extravesicular side or on the luminal side. LC-MS analyses of the trypsin digests obtained from the proteoliposomes indicated that most of the C-terminal hydrophilic segment of the native Hcytb5 were exposed towards the lumen of the vesicles and, further, a significant part of the population of the extended C-terminal hydrophilic segments of the modified Hcytb5 were exposed in the lumen as well, suggesting efficient translocation ability of the TA-domain without any assistance from other protein factors. Present results opened a route for the use of the C-terminal TA-domain as a convenient tool for the transport of proteins as well as short peptides into artificial liposomes.

Reductive evolution of proteomes and protein structures

The lengths of orthologous protein families in Eukarya are almost double the lengths found in Bacteria and Archaea. Here we examine protein structures in 745 genomes and show that protein length differences between superkingdoms arise as much shorter prokaryotic nondomain linker sequences. Eukaryotic, bacterial, and archaeal linkers are 250, 86, and 73 aa residues in length, respectively, whereas folded domain sequences are 281, 280, and 256 residues, respectively. Cryptic domains match linkers (P < 0.0001) with probabilities ranging between 0.022 and 0.042; accordingly, they do not affect length estimates significantly. Linker sequences support intermolecular binding within proteomes and they are probably enriched in intrinsically disordered regions as well. Reductively evolved linker sequence lengths in growth rate maximized cells should be proportional to proteome diversity. By using total in-frame coding capacity of a genome [i.e., coding sequence (CDS)] as a reliable measure of proteome diversity, we find linker lengths of prokaryotes clearly evolve in proportion to CDS values, whereas those of eukaryotes are more randomly larger than expected. Domain lengths scarcely change over the entire range of CDS values. Thus, the protein linkers of prokaryotes evolve reductively whereas those of eukaryotes do not.

Structural basis for tail-anchored membrane protein biogenesis by the Get3-receptor complex

Tail-anchored (TA) proteins are involved in cellular processes including trafficking, degradation, and apoptosis. They contain a C-terminal membrane anchor and are posttranslationally delivered to the endoplasmic reticulum (ER) membrane by the Get3 adenosine triphosphatase interacting with the hetero-oligomeric Get1/2 receptor. We have determined crystal structures of Get3 in complex with the cytosolic domains of Get1 and Get2 in different functional states at 3.0, 3.2, and 4.6 angstrom resolution. The structural data, together with biochemical experiments, show that Get1 and Get2 use adjacent, partially overlapping binding sites and that both can bind simultaneously to Get3. Docking to the Get1/2 complex allows for conformational changes in Get3 that are required for TA protein insertion. These data suggest a molecular mechanism for nucleotide-regulated delivery of TA proteins.

Bacterial transmembrane proteins that lack N-terminal signal sequences

Tail-anchored membrane proteins (TAMPs), a class of proteins characterized by their lack of N-terminal signal sequence and Sec-independent membrane targeting, play critical roles in apoptosis, vesicle trafficking and other vital processes in eukaryotic organisms. Until recently, this class of membrane proteins has been unknown in bacteria. Here we present the results of bioinformatic analysis revealing proteins that are superficially similar to eukaryotic TAMPs in the bacterium Streptomyces coelicolor. We demonstrate that at least four of these proteins are bona fide membrane-spanning proteins capable of targeting to the membrane in the absence of their N-terminus and the C-terminal membrane-spanning domain is sufficient for membrane targeting. Several of these proteins, including a serine/threonine kinase and the SecE component of the Sec translocon, are widely conserved in bacteria.

Tail-anchored membrane proteins: exploring the complex diversity of tail-anchored-protein targeting in plant cells

Tail-anchored (TA) proteins are special class of integral membrane proteins that in recent years have received a considerable amount of attention due to their diverse cellular functions and unique targeting and insertion mechanisms. Defined by the presence of a single, hydrophobic membrane-spanning domain at or near their C terminus, TA proteins must be inserted into membranes post-translationally and are orientated such that their larger N-terminal domain (most often the functional domain) faces the cytosol, while their shorter C-terminal domain faces the interior of the organelle. The C-terminal domain of TA proteins also usually contains the information responsible for their selective targeting to the proper subcellular membrane, a process that, based primarily on studies with yeasts and mammals, appears to be highly complex due to the presence of multiple pathways. Within this context, we discuss here the biogenesis of plant TA proteins and the potential for hundreds of new TA proteins identified via bioinformatics screens to contribute to the already remarkable number of roles that this class of membrane proteins participates in throughout plant growth and development.

Characterization of varicella-zoster virus-encoded ORF0 gene--comparison of parental and vaccine strains

The varicella-zoster virus (VZV) Oka vaccine strain (vOka) differs from the parental strain (pOka) at several amino acid positions, but the mutations responsible for the attenuation of vOka have not been clearly defined. The ORF0 of vOka carries some of the mutations. Although we found that the ORF0 of both strains was incorporated into virus particles, the C-terminal region of vOka ORF0 was presented on the virion surface and was N-glycosylated, suggesting that the mutation in vOka ORF0 changes it into a novel envelope glycoprotein. In a mutant virus in which pOka ORF0 was replaced by vOka ORF0, the molecular weight of ORF0 was altered, but the plaque size was not. In addition, a pOka recombinant virus lacking the hydrophobic domain of ORF0 grew equally well as the wild-type virus, indicating that the mutation in ORF0 is not by itself sufficient for the attenuation of the vOka virus.

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.

Subcellular distribution of tail-anchored proteins in Arabidopsis

Tail-anchored (TA) proteins function in key cellular processes in eukaryotic cells, such as vesicle trafficking, protein translocation and regulation of transcription. They anchor to internal cell membranes by a C-terminal transmembrane domain, which also serves as a targeting sequence. Targeting occurs post-translationally, via pathways that are specific to the precursor, which makes TA proteins a model system for investigating post-translational protein targeting. Bioinformatics approaches have previously been used to identify potential TA proteins in yeast and humans, yet little is known about TA proteins in plants. The identification of plant TA proteins is important for extending the post-translational model system to plastids, in addition to general proteome characterization, and the identification of functional homologues characterized in other organisms. We identified 454 loci that potentially encode TA proteins in Arabidopsis, and combined published data with new localization experiments to assign localizations to 130 proteins, including 29 associated with plastids. By analysing the tail anchor sequences of characterized proteins, we have developed a tool for predicting localization and estimate that 138 TA proteins are localized to plastids.

The role of cytosolic proteins in the insertion of tail-anchored proteins into phospholipid bilayers

Tail-anchored (TA) proteins are membrane proteins that contain an N-terminal domain exposed to the cytosol and a single transmembrane segment near the C-terminus followed by few or no polar residues. TA proteins with a mildly hydrophobic transmembrane domain, such as cytochrome b5 (b5), are able to insert post-translationally into pure lipid vesicles without assistance from membrane proteins. Here, we investigated whether any cytosolic proteins are needed to maintain b5 in a competent state for transmembrane integration. Using b5 constructs translated in vitro or produced in bacteria, we demonstrate that cytosolic proteins are neither necessary nor facilitatory for the unassisted translocation of b5. Furthermore, we demonstrate that no cytosolic protein is involved in the translocation of a C-terminal domain of 85 residues appended to the transmembrane domain of b5. Nevertheless, b5 does bind cytosolic proteins, and in their presence but not in their absence, its insertion into liposomes is inhibited by the thiol oxidant diamide and the alkylating agent N-ethylmaleimide. The effect of diamide is also observed in living cells. Thus, the specific in vivo targeting of b5 might be achieved by interaction with redox-sensitive targeting factors that hinder its nonspecific insertion into any permissive bilayer.

Delivering proteins for export from the cytosol

Correct protein function depends on delivery to the appropriate cellular or subcellular compartment. Following the initiation of protein synthesis in the cytosol, many bacterial and eukaryotic proteins must be integrated into or transported across a membrane to reach their site of function. Whereas in the post-translational delivery pathway ATP-dependent factors bind to completed polypeptides and chaperone them until membrane translocation is initiated, a GTP-dependent co-translational pathway operates to couple ongoing protein synthesis to membrane transport. These distinct pathways provide different solutions for the maintenance of proteins in a state that is competent for membrane translocation and their delivery for export from the cytosol.

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.

Lipid droplets as dynamic organelles connecting storage and efflux of lipids

Neutral lipids are stored in the cytosol in so-called lipid droplets. These are dynamic organelles with neutral lipids as the core surrounded by a monolayer of amphipathic lipids (phospholipids and cholesterol) and specific proteins (PAT proteins and proteins involved in the turnover of lipids and in the formation and trafficking of the droplets). Lipid droplets are formed at microsomal membranes as primordial droplets with a diameter of 0.1-0.4 microm and increase in size by fusion. In this article, we review the assembly and fusion of lipid droplets, and the processes involved in the secretion of triglycerides. Triglycerides are secreted from cells by two principally different processes. In the mammary gland, lipid droplets interact with specific regions of the plasma membrane and bud off with an envelope consisting of the membrane, to form milk globules. In the liver and intestine, very low-density lipoproteins (VLDL) and chylomicrons are secreted by using the secretory pathway of the cell. Finally, we briefly review the importance of lipid droplets in the development of insulin resistance and atherosclerosis.

A precursor-specific role for Hsp40/Hsc70 during tail-anchored protein integration at the endoplasmic reticulum

Tail-anchored (TA) protein synthesis at the endoplasmic reticulum (ER) represents a distinct and novel process that provides a paradigm for understanding post-translational membrane insertion in eukaryotes. The major route for delivering TA proteins to the ER requires both ATP and one or more cytosolic factors that facilitate efficient membrane insertion. Until recently, the identity of these cytosolic components was elusive, but two candidates have now been suggested to promote ATP-dependent TA protein integration. The first is the cytosolic chaperone complex of Hsp40/Hsc70, and the second is a novel ATPase denoted Asna-1 or TRC40. In this study we focus on the role of the Hsp40/Hsc70 complex in promoting TA protein biogenesis at the ER. We show that the membrane integration of most TA proteins is stimulated by Hsp40/Hsc70 when using purified components and a reconstituted system. In contrast, when both Hsp40/Hsc70 and Asna-1/TRC40 are provided as a complete system, small molecule inhibition of Hsp40/Hsc70 indicates that only a subset of TA proteins are obligatory clients for this chaperone-mediated delivery route. We show that the hydrophobicity of the TA region dictates whether a precursor is delivered to the ER via the Hsp40/Hsc70 or Asna-1/TRC40-dependent route, and we conclude that these distinct cytosolic ATPases are responsible for two different ATP-dependent pathways of TA protein biogenesis.

Integration of tail-anchored proteins into the mitochondrial outer membrane does not require any known import components

Tail-anchored proteins form a distinct class of membrane proteins that are found in all intracellular membranes exposed to the cytosol. These proteins have a single membrane insertion sequence at their C-terminus and display a large N-terminal portion to the cytosol. Despite their importance for various cellular processes, the mechanisms by which these proteins are recognized at and inserted into their corresponding target membrane remained largely unclear. Here we address this issue and investigate the biogenesis of tail-anchored proteins residing in the mitochondrial outer membrane. To that goal we developed a highly specific assay to monitor the membrane insertion of the model tail-anchored protein Fis1. Using this assay, we show that in contrast to all other import pathways in yeast mitochondria, none of the import components at the outer membrane is involved in the insertion process of Fis1. Both the steady-state levels of Fis1 and its in vitro insertion into isolated mitochondria were unaffected when mitochondria mutated in known import factors were analyzed. Fis1 was inserted into lipid vesicles, and importantly, elevated ergosterol contents in these vesicles inhibited this insertion. Collectively, these results suggest that Fis1 is inserted into mitochondria in a novel pathway where the unique lipid composition of the mitochondrial outer membrane contributes to the selectivity of the process. Thus, this work demonstrates a novel role for lipids in the biogenesis of mitochondrial protein.

SNARE proteins mediate fusion between cytosolic lipid droplets and are implicated in insulin sensitivity

The accumulation of cytosolic lipid droplets in muscle and liver cells has been linked to the development of insulin resistance and type 2 diabetes. Such droplets are formed as small structures that increase in size through fusion, a process that is dependent on intact microtubules and the motor protein dynein. Approximately 15% of all droplets are involved in fusion processes at a given time. Here, we show that lipid droplets are associated with proteins involved in fusion processes in the cell: NSF (N-ethylmaleimide-sensitive-factor), alpha-SNAP (soluble NSF attachment protein) and the SNAREs (SNAP receptors), SNAP23 (synaptosomal-associated protein of 23 kDa), syntaxin-5 and VAMP4 (vesicle-associated membrane protein 4). Knockdown of the genes for SNAP23, syntaxin-5 or VAMP4, or microinjection of a dominant-negative mutant of alpha-SNAP, decreases the rate of fusion and the size of the lipid droplets. Thus, the SNARE system seems to have an important role in lipid droplet fusion. We also show that oleic acid treatment decreases the insulin sensitivity of heart muscle cells, and this sensitivity is completely restored by transfection with SNAP23. Thus, SNAP23 might be a link between insulin sensitivity and the inflow of fatty acids to the cell.

Subcellular localization of hepatitis E virus (HEV) replicase

Hepatitis E virus (HEV) is a hepatotropic virus with a single sense-strand RNA genome of approximately 7.2 kb in length. Details of the intracellular site of HEV replication can pave further understanding of HEV biology. In-frame fusion construct of functionally active replicase-enhanced green fluorescent protein (EGFP) gene was made in eukaryotic expression vector. The functionality of replicase-EGFP fusion protein was established by its ability to synthesize negative-strand viral RNA in vivo, by strand-specific anchored RT-PCR and molecular beacon binding. Subcellular co-localization was carried out using organelle specific fluorophores and by immuno-electron microscopy. Fluorescence Resonance Energy Transfer (FRET) demonstrated the interaction of this protein with the 3' end of HEV genome. The results show localization of replicase on the endoplasmic reticulum membranes. The protein regions responsible for membrane localization was predicted and identified by use of deletion mutants. Endoplasmic reticulum was identified as the site of replicase localization and possible site of replication.

The origins of modern proteomes

Distributions of phylogenetically related protein domains (fold superfamilies), or FSFs, among the three Superkingdoms (trichotomy) are assessed. Very nearly 900 of the 1200 FSFs of the trichotomy are shared by two or three Superkingdoms. Parsimony analysis of FSF distributions suggests that the FSF complement of the last common ancestor to the trichotomy was more like that of modern eukaryotes than that of archaea and bacteria. Studies of length distributions among members of orthologous families of proteins present in all three Superkingdoms reveal that such lengths are significantly longer among eukaryotes than among bacteria and archaea. The data also reveal that proteins lengths of eukaryotes are more broadly distributed than they are within archaeal and bacterial members of the same orthologous families. Accordingly, selective pressure for a minimal size is significantly greater for orthologous protein lengths in archaea and bacteria than in eukaryotes. Alignments of orthologous proteins of archaea, bacteria and eukaryotes are characterized by greater sequence variation at their N-terminal and C-terminal domains, than in their central cores. Length variations tend to be localized in the terminal sequences; the conserved sequences of orthologous families are localized in a central core. These data are consistent with the interpretation that the genomes of the last common ancestor (LUCA) encoded a cohort of FSFs not very different from that of modern eukaryotes. Divergence of bacterial and archaeal genomes from that common ancestor may have been accompanied by more intensive reductive evolution of proteomes than that expressed in eukaryotes. Dollo's Law suggests that the evolution of novel FSFs is a very slow process, while laboratory experiments suggests that novel protein genesis from preexisting FSFs can be relatively rapid. Reassortment of FSFs to create novel proteins may have been mediated by genetic recombination before the advent of more efficient splicing mechanisms.

Membrane protein chaperones: a new twist in the tail?

The integration of tail-anchored membrane proteins at the endoplasmic reticulum occurs via a specialised ATP-dependent pathway, but the cytosolic factors involved have proven elusive. A novel ATPase that mediates this process has now been identified.

How tails guide tail-anchored proteins to their destinations

A large group of diverse, functionally important, and differently localized transmembrane proteins shares a particular membrane topology, consisting of a cytosolic N-terminal region, followed by a transmembrane domain close to the C-terminus. Because of their structure, these C-tail-anchored (TA) proteins must insert into all their target membranes by post-translational pathways. Recent work, based on the development of stringent and sensitive biochemical assays, has demonstrated that novel unexplored mechanisms underlie these post-translational targeting and membrane insertion pathways. Unravelling these pathways will shed light on the biosynthesis and regulation of an important group of membrane proteins and is likely to lead to new concepts in the field of membrane biogenesis.

Targeting and retention of type 1 ryanodine receptors to the endoplasmic reticulum

Most ryanodine receptors and their relatives, inositol 1,4,5-trisphosphate receptors, are expressed in the sarcoplasmic or endoplasmic reticulum (ER), where they mediate Ca(2+) release. We expressed fragments of ryanodine receptor type 1 (RyR1) in COS cells alone or fused to intercellular adhesion molecule-1 (ICAM-1), each tagged with yellow fluorescent protein, and used confocal imaging and glycoprotein analysis to identify the determinants of ER targeting and retention. Single transmembrane domains (TMD) of RyR1 taken from the first (TMD1-TMD2) or last (TMD5-TMD6) pair were expressed in the ER membrane. TMD3-TMD4 was expressed in the outer mitochondrial membrane. The TMD outer pairs (TMD1-TMD2 and TMD5-TMD6) retained ICAM-1, a plasma membrane-targeted protein, within the ER membrane. TMD1 alone provided a strong ER retention signal and TMD6 a weaker signal, but the other single TMD were unable to retain ICAM-1 in the ER. We conclude that TMD1 provides the first and sufficient signal for ER targeting of RyR1. The TMD outer pairs include redundant ER retention signals, with TMD1 providing the strongest signal.

Post-translational integration of tail-anchored proteins is facilitated by defined molecular chaperones

Tail-anchored (TA) proteins provide an ideal model for studying post-translational integration at the endoplasmic reticulum (ER) of eukaryotes. There are multiple pathways for delivering TA proteins from the cytosol to the ER membrane yet, whereas an ATP-dependent route predominates, none of the cytosolic components involved had been identified. In this study we have directly addressed this issue and identify novel interactions between a model TA protein and the two cytosolic chaperones Hsp40 and Hsc70. To investigate their function, we have reconstituted the membrane integration of TA proteins using purified components. Remarkably, we find that a combination of Hsc70 and Hsp40 can completely substitute for the ATP-dependent factors present in cytosol. On the basis of this in vitro analysis, we conclude that this chaperone pair can efficiently facilitate the ATP-dependent integration of TA proteins.

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 C-terminus of cytochrome b5 confers endoplasmic reticulum specificity by preventing spontaneous insertion into membranes

The molecular mechanisms that determine the correct subcellular localization of proteins targeted to membranes by tail-anchor sequences are poorly defined. Previously, we showed that two isoforms of the tung oil tree [Vernicia (Aleurites) fordii] tail-anchored Cb5 (cytochrome b5) target specifically to ER (endoplasmic reticulum) membranes both in vivo and in vitro [Hwang, Pelitire, Henderson, Andrews, Dyer and Mullen (2004) Plant Cell 16, 3002-3019]. In the present study, we examine the targeting of various tung Cb5 fusion proteins and truncation mutants to purified intracellular membranes in vitro in order to assess the importance of the charged CTS (C-terminal sequence) in targeting to specific membranes. Removal of the CTS from tung Cb5 proteins resulted in efficient binding to both ER and mitochondria. Results from organelle competition, liposome-binding and membrane proteolysis experiments demonstrated that removal of the CTS results in spontaneous insertion of tung Cb5 proteins into lipid bilayers. Our results indicate that the CTSs from plant Cb5 proteins provide ER specificity by preventing spontaneous insertion into incorrect subcellular membranes.

Unassisted translocation of large polypeptide domains across phospholipid bilayers

Although transmembrane proteins generally require membrane-embedded machinery for integration, a few can insert spontaneously into liposomes. Previously, we established that the tail-anchored (TA) protein cytochrome b(5) (b5) can posttranslationally translocate 28 residues downstream to its transmembrane domain (TMD) across protein-free bilayers (Brambillasca, S., M. Yabal, P. Soffientini, S. Stefanovic, M. Makarow, R.S. Hegde, and N. Borgese. 2005. EMBO J. 24:2533–2542). In the present study, we investigated the limits of this unassisted translocation and report that surprisingly long (85 residues) domains of different sequence and charge placed downstream of b5's TMD can posttranslationally translocate into mammalian microsomes and liposomes at nanomolar nucleotide concentrations. Furthermore, integration of these constructs occurred in vivo in translocon-defective yeast strains. Unassisted translocation was not unique to b5 but was also observed for another TA protein (protein tyrosine phosphatase 1B) whose TMD, like the one of b5, is only moderately hydrophobic. In contrast, more hydrophobic TMDs, like synaptobrevin's, were incapable of supporting unassisted integration, possibly because of their tendency to aggregate in aqueous solution. Our data resolve long-standing discrepancies on TA protein insertion and are relevant to membrane evolution, biogenesis, and physiology.

Targeting of the tail-anchored peroxisomal membrane proteins PEX26 and PEX15 occurs through C-terminal PEX19-binding sites

Tail-anchored proteins contain a single transmembrane domain (TMD) followed by a short C-terminal domain extending into the organellar lumen. Tail-anchored proteins are thought to target to the correct subcellular compartment by virtue of general physicochemical properties of their C-termini; however, the machineries that enable correct sorting remain largely elusive. Here we analyzed targeting of the human peroxisomal tail-anchored protein PEX26. Its C-terminal-targeting signal contains two binding sites for PEX19, the import receptor for several peroxisomal membrane proteins. One PEX19-binding site overlapped with the TMD, the other was contained within the luminal domain. Although the PEX19-binding site containing the TMD targeted to peroxisomes to some extent, the luminal site proved essential for correct targeting of the full-length protein, as it prevented PEX26 from mislocalization to mitochondria. Its function as a targeting motif was proved by its ability to insert a heterologous TMD-containing fragment into the peroxisomal membrane. Finally we show that PEX19 is essential for PEX26 import. Analysis of the yeast tail-anchored protein Pex15p revealed that it also harbors a luminal PEX19-binding site that acts as a peroxisomal-targeting motif. We conclude that C-terminal PEX19-binding sites mark tail-anchored proteins for delivery to peroxisomes.

The diverse roles of J-proteins, the obligate Hsp70 co-chaperone

Hsp70s and J-proteins, which constitute one of the most ubiquitous types of molecular chaperone machineries, function in a wide variety of cellular processes. J-proteins play a central role by stimulating an Hsp70's ATPase activity, thereby stabilizing its interaction with client proteins. However, while all J-proteins serve this core purpose, individual proteins are both structurally and functionally diverse. Some, but not all, J-proteins interact with client polypeptides themselves, facilitating their binding to an Hsp70. Some J-proteins have many client proteins, others only one. Certain J-proteins, while not others, are tethered to particular locations within a cellular compartment, thus "recruiting" Hsp70s to the vicinity of their clients. Here we review recent work on the diverse family of J-proteins, outlining emerging themes concerning their function.

Transmembrane topogenesis of a tail-anchored protein is modulated by membrane lipid composition

A large class of proteins with cytosolic functional domains is anchored to selected intracellular membranes by a single hydrophobic segment close to the C-terminus. Although such tail-anchored (TA) proteins are numerous, diverse, and functionally important, the mechanism of their transmembrane insertion and the basis of their membrane selectivity remain unclear. To address this problem, we have developed a highly specific, sensitive, and quantitative in vitro assay for the proper membrane-spanning topology of a model TA protein, cytochrome b5 (b5). Selective depletion from membranes of components involved in cotranslational protein translocation had no effect on either the efficiency or topology of b5 insertion. Indeed, the kinetics of transmembrane insertion into protein-free phospholipid vesicles was the same as for native ER microsomes. Remarkably, loading of either liposomes or microsomes with cholesterol to levels found in other membranes of the secretory pathway sharply and reversibly inhibited b5 transmembrane insertion. These results identify the minimal requirements for transmembrane topogenesis of a TA protein and suggest that selectivity among various intracellular compartments can be imparted by differences in their lipid composition.