Structure and biosynthesis of the signal-sequence receptor

A Putative TRAPα Protein of Microsporidia Nosema bombycis Exhibits Non-Canonical Alternative Polyadenylation in Transcripts

Microsporidia are obligate intracellular eukaryotic parasites that have significantly reduced genomes and that have lost most of their introns. In the current study, we characterized a gene in microsporidia Nosema bombycis, annotated as TRAPα (HNbTRAPα). The homologous of TRAPα are a functional component of ER translocon and facilitates the initiation of protein translocation in a substrate-specific manner, which is conserved in animals but absent from most fungi. The coding sequence of HNbTRAPα consists of 2226 nucleotides, longer than the majority of homologs in microsporidia. A 3′ RACE analysis indicated that there were two mRNA isoforms resulting from non-canonical alternative polyadenylation (APA), and the polyadenylate tail was synthesized after the C951 or C1167 nucleotide, respectively. Indirect immunofluorescence analysis showed two different localization characteristics of HNbTRAPα, which are mainly located around the nuclear throughout the proliferation stage and co-localized with the nuclear in mature spores. This study demonstrated that the post-transcriptional regulation mechanism exists in Microsporidia and expands the mRNA isoform repertoire.

Requirement for translocon-associated protein (TRAP) α in insulin biogenesis

The mechanistic basis for the biogenesis of peptide hormones and growth factors is poorly understood. Here, we show that the conserved endoplasmic reticulum membrane translocon-associated protein α (TRAPα), also known as signal sequence receptor 1, plays a critical role in the biosynthesis of insulin. Genetic analysis in the nematode Caenorhabditis elegans and biochemical studies in pancreatic β cells reveal that TRAPα deletion impairs preproinsulin translocation while unexpectedly disrupting distal steps in insulin biogenesis including proinsulin processing and secretion. The association of common intronic single-nucleotide variants in the human TRAPα gene with susceptibility to type 2 diabetes and pancreatic β cell dysfunction suggests that impairment of preproinsulin translocation and proinsulin trafficking may contribute to the pathogenesis of type 2 diabetes.

DC2 and KCP2 mediate the interaction between the oligosaccharyltransferase and the ER translocon

The STT3A isoform of the oligosaccharyltransferase is adjacent to the protein translocation channel to catalyze co-translational N-glycosylation of proteins in the endoplasmic reticulum. Shrimal et al. show that the DC2 and KCP2 subunits of the STT3A isoform of the oligosaccharyltransferase are responsible for mediating the interaction between the STT3A complex and the protein translocation channel to allow co-translational N-glycosylation of proteins.

In metazoan organisms, the STT3A isoform of the oligosaccharyltransferase is localized adjacent to the protein translocation channel to catalyze co-translational N-linked glycosylation of proteins in the endoplasmic reticulum. The mechanism responsible for the interaction between the STT3A complex and the translocation channel has not been addressed. Using genetically modified human cells that are deficient in DC2 or KCP2 proteins, we show that loss of DC2 causes a defect in co-translational N-glycosylation of proteins that mimics an STT3A^−/− phenotype. Biochemical analysis showed that DC2 and KCP2 are responsible for mediating the interaction between the protein translocation channel and the STT3A complex. Importantly, DC2- and KCP2-deficient STT3A complexes are stable and enzymatically active. Deletion mutagenesis revealed that a conserved motif in the C-terminal tail of DC2 is critical for assembly into the STT3A complex, whereas the lumenal loop and the N-terminal cytoplasmic segment are necessary for the functional interaction between the STT3A and Sec61 complexes.

Dissecting the molecular organization of the translocon-associated protein complex

In eukaryotic cells, one-third of all proteins must be transported across or inserted into the endoplasmic reticulum (ER) membrane by the ER protein translocon. The translocon-associated protein (TRAP) complex is an integral component of the translocon, assisting the Sec61 protein-conducting channel by regulating signal sequence and transmembrane helix insertion in a substrate-dependent manner. Here we use cryo-electron tomography (CET) to study the structure of the native translocon in evolutionarily divergent organisms and disease-linked TRAP mutant fibroblasts from human patients. The structural differences detected by subtomogram analysis form a basis for dissecting the molecular organization of the TRAP complex. We assign positions to the four TRAP subunits within the complex, providing insights into their individual functions. The revealed molecular architecture of a central translocon component advances our understanding of membrane protein biogenesis and sheds light on the role of TRAP in human congenital disorders of glycosylation.

Proteomic responses reveal the differential effects induced by cadmium in mussels Mytilus galloprovincialis at early life stages

Cadmium (Cd) has become an important metal contaminant and posed severe risk on the organisms in the coastal environments of the Bohai Sea. Marine mussel Mytilus galloprovincialis is widely distributed along the Bohai coast and consumed as seafood by local residents. Evidences indicate that the early stages of marine organisms are more sensitive to metal contaminants. In this study, we applied two-dimensional electrophoresis-based proteomics to characterize the biological effects of Cd (50 μg L(-1)) in the early life stages (D-shape larval and juvenile) of mussels. The different proteomic responses demonstrated the differential responsive mechanisms to Cd exposure in these two early life stages of mussels. In details, results indicated that Cd mainly induced immune and oxidative stresses in both D-shape larval and juvenile mussels via different pathways. In addition, the significant up-regulation of triosephosphate isomerase and metallothionein confirmed the enhanced energy demand and mobilized detoxification mechanism in D-shape larval mussels exposed to Cd. In juvenile mussels, Cd exposure also induced clear apoptosis. Overall, this work suggests that Cd is a potential immune toxicant to mussel M. galloprovincialis at early life stages.

Sequence-specific retention and regulated integration of a nascent membrane protein by the endoplasmic reticulum Sec61 translocon

A defining feature of eukaryotic polytopic protein biogenesis involves integration, folding, and packing of hydrophobic transmembrane (TM) segments into the apolar environment of the lipid bilayer. In the endoplasmic reticulum, this process is facilitated by the Sec61 translocon. Here, we use a photocross-linking approach to examine integration intermediates derived from the ATP-binding cassette transporter cystic fibrosis transmembrane conductance regulator (CFTR) and show that the timing of translocon-mediated integration can be regulated at specific stages of synthesis. During CFTR biogenesis, the eighth TM segment exits the ribosome and enters the translocon in proximity to Sec61alpha. This interaction is initially weak, and TM8 spontaneously dissociates from the translocon when the nascent chain is released from the ribosome. Polypeptide extension by only a few residues, however, results in stable TM8-Sec61alpha photocross-links that persist after peptidyl-tRNA bond cleavage. Retention of these untethered polypeptides within the translocon requires ribosome binding and is mediated by an acidic residue, Asp924, near the center of the putative TM8 helix. Remarkably, at this stage of synthesis, nascent chain release from the translocon is also strongly inhibited by ATP depletion. These findings contrast with passive partitioning models and indicate that Sec61alpha can retain TMs and actively inhibit membrane integration in a sequence-specific and ATP-dependent manner.

The concept of translocational regulation

Biological processes are regulated to provide cells with exquisite adaptability to changing environmental conditions and cellular demands. The mechanisms regulating secretory and membrane protein translocation into the endoplasmic reticulum (ER) are unknown. A conceptual framework for translocational regulation is proposed based on our current mechanistic understanding of ER protein translocation and general principles of regulatory control.

Biogenesis of CFTR and other polytopic membrane proteins: new roles for the ribosome-translocon complex

Polytopic protein biogenesis represents a critical, yet poorly understood area of modern biology with important implications for human disease. Inherited mutations in a growing array of membrane proteins frequently lead to improper folding and/or trafficking. The cystic fibrosis transmembrane conductance regulator (CFTR) is a primary example in which point mutations disrupt CFTR folding and lead to rapid degradation in the endoplasmic reticulum (ER). It has been difficult, however, to discern the mechanistic principles of such disorders, in part, because membrane protein folding takes place coincident with translation and within a highly specialized environment formed by the ribosome, Sec61 translocon, and the ER membrane. This ribosome-translocon complex (RTC) coordinates the synthesis, folding, orientation and integration of transmembrane segments across and into the ER membrane. At the same time, RTC function is controlled by specific sequence determinants within the nascent polypeptide. Recent studies of CFTR and other native membrane proteins have begun to define novel variations in translocation pathways and to elucidate the specific steps that establish complex topology. This article will attempt to reconcile advances in our understanding of protein biogenesis with emerging models of RTC function. In particular, it will emphasize how information within the nascent polypeptide is interpreted by and in turn controls RTC dynamics to generate the broad structural and functional diversity observed for naturally occurring membrane proteins.

Substrate-specific function of the translocon-associated protein complex during translocation across the ER membrane

Although the transport of model proteins across the mammalian ER can be reconstituted with purified Sec61p complex, TRAM, and signal recognition particle receptor, some substrates, such as the prion protein (PrP), are inefficiently or improperly translocated using only these components. Here, we purify a factor needed for proper translocation of PrP and identify it as the translocon-associated protein (TRAP) complex. Surprisingly, TRAP also stimulates vectorial transport of many, but not all, other substrates in a manner influenced by their signal sequences. Comparative analyses of several natural signal sequences suggest that a dependence on TRAP for translocation is not due to any single physical parameter, such as hydrophobicity of the signal sequence. Instead, a functional property of the signal, efficiency of its post-targeting role in initiating substrate translocation, correlates inversely with TRAP dependence. Thus, maximal translocation independent of TRAP can only be achieved with a signal sequence, such as the one from prolactin, whose strong interaction with the translocon mediates translocon gating shortly after targeting. These results identify the TRAP complex as a functional component of the translocon and demonstrate that it acts in a substrate-specific manner to facilitate the initiation of protein translocation.

Mammalian Sec61 is associated with Sec62 and Sec63

In yeast, efficient protein transport across the endoplasmic reticulum (ER) membrane may occur co-translationally or post-translationally. The latter process is mediated by a membrane protein complex that consists of the Sec61p complex and the Sec62p-Sec63p subcomplex. In contrast, in mammalian cells protein translocation is almost exclusively co-translational. This transport depends on the Sec61 complex, which is homologous to the yeast Sec61p complex and has been identified in mammals as a ribosome-bound pore-forming membrane protein complex. We report here the existence of ribosome-free mammalian Sec61 complexes that associate with two ubiquitous proteins of the ER membrane. According to primary sequence analysis both proteins display homology to the yeast proteins Sec62p and Sec63p and are therefore named Sec62 and Sec63, respectively. The probable function of the mammalian Sec61-Sec62-Sec63 complex is discussed with respect to its abundance in ER membranes, which, in contrast to yeast ER membranes, apparently lack efficient post-translational translocation activity.

In vitro formation of the endoplasmic reticulum occurs independently of microtubules by a controlled fusion reaction

We have established an in vitro system for the formation of the endoplasmic reticulum (ER). Starting from small membrane vesicles prepared from Xenopus laevis eggs, an elaborate network of membrane tubules is formed in the presence of cytosol. In the absence of cytosol, the vesicles only fuse to form large spheres. Network formation requires a ubiquitous cytosolic protein and nucleoside triphosphates, is sensitive to N-ethylmaleimide and high cytosolic Ca(2+) concentrations, and proceeds via an intermediate stage in which vesicles appear to be clustered. Microtubules are not required for membrane tubule and network formation. Formation of the ER network shares significant similarities with formation of the nuclear envelope. Our results suggest that the ER network forms in a process in which cytosolic factors modify and regulate a basic reaction of membrane vesicle fusion.

The translocon: a dynamic gateway at the ER membrane

Cotranslational protein translocation across and integration into the membrane of the endoplasmic reticulum (ER) occur at sites termed translocons. Translocons are composed of several ER membrane proteins that associate to form an aqueous pore through which secretory proteins and lumenal domains of membrane proteins pass from the cytoplasm to the ER lumen. These sites are not passive holes in the bilayer, but instead are quite dynamic both structurally and functionally. Translocons cycle between ribosome-bound and ribosome-free states, and convert between translocation and integration modes of operation. These changes in functional state are accompanied by structural rearrangements that alter translocon conformation, composition, and interactions with ligands such as the ribosome and BiP. Recent studies have revealed that the translocon is a complex and sophisticated molecular machine that regulates the movement of polypeptides through the bilayer, apparently in both directions as well as laterally into the bilayer, all while maintaining the membrane permeability barrier.

Translocon-associated protein alpha transcripts are induced by granulocyte-macrophage colony-stimulating factor and exhibit complex alternative polyadenylation

The cloning of full length cDNA for the translocon-associated protein alpha subunit, previously called signal sequence receptor alpha, is reported as a result of differential display experiments in search of genes induced by granulocyte-macrophage colony-stimulating factor. Its messenger RNA was more abundant in growing cells than in either factor-deprived cells or quiescent cells and comprised four species, each having microheterogeneity, as a result of complex alternative polyadenylation apparently dependent on arrays of non-canonical polyadenylation signals. Radiation hybrid mapping of the gene showed that the gene is on the short arm of chromosome 6.

Targeting of the synaptic vesicle protein synaptobrevin in the axon of cultured hippocampal neurons: evidence for two distinct sorting steps

Synaptic vesicles are concentrated in the distal axon, far from the site of protein synthesis. Integral membrane proteins destined for this organelle must therefore make complex targeting decisions. Short amino acid sequences have been shown to act as targeting signals directing proteins to a variety of intracellular locations. To identify synaptic vesicle targeting sequences and to follow the path that proteins travel en route to the synaptic vesicle, we have used a defective herpes virus amplicon expression system to study the targeting of a synaptobrevin-transferrin receptor (SB-TfR) chimera in cultured hippocampal neurons. Addition of the cytoplasmic domain of synaptobrevin onto human transferrin receptor was sufficient to retarget the transferrin receptor from the dendrites to presynaptic sites in the axon. At the synapse, the SB-TfR chimera did not localize to synaptic vesicles, but was instead found in an organelle with biochemical and functional characteristics of an endosome. The chimera recycled in parallel with synaptic vesicle proteins demonstrating that the nerve terminal efficiently sorts transmembrane proteins into different pathways. The synaptobrevin sequence that controls targeting to the presynaptic endosome was not localized to a single, 10- amino acid region of the molecule, indicating that this targeting signal may be encoded by a more distributed structural conformation. However, the chimera could be shifted to synaptic vesicles by deletion of amino acids 61-70 in synaptobrevin, suggesting that separate signals encode the localization of synaptobrevin to the synapse and to the synaptic vesicle.

Incomplete endoplasmic reticulum (ER) retention in immature thymocytes as revealed by surface expression of "ER-resident" molecular chaperones

The folding and assembly of nascent proteins in the endoplasmic reticulum (ER) is assisted by molecular chaperones that are themselves retained within the ER. We now report that a number of different ER proteins, including molecular chaperones, are selectively expressed on the surface of immature thymocytes, but their surface expression is extinguished upon further differentiation. Escape from the ER is only possible for newly synthesized ER proteins before they become permanently retained. Thus, the cellular process of ER retention is incomplete in immature thymocytes and provides an explanation for surface expression of partial receptor complexes that transduce differentiative signals during thymic development.

Translocon-associated protein TRAP delta and a novel TRAP-like protein are coordinately expressed with pro-opiomelanocortin in Xenopus intermediate pituitary

In the intermediate pituitary gland of Xenopus laevis, the expression levels of the prohormone pro-opiomelanocortin (POMC) can be readily manipulated. When the animal is placed on a black background, the gene for POMC is actively transcribed, whereas on a white background the gene is virtually inactive. In this study, we characterized two genes whose transcript levels in the intermediate pituitary are regulated in coordination with that for POMC. One of these codes for a protein homologous to translocon-associated protein TRAP delta, a subunit of a transmembrane protein complex located at the site where nascent secretory proteins enter the endoplasmic reticulum (ER). Both Xenopus and mice were found to express an alternatively spliced transcript that gives rise to a previously unknown version of the TRAP delta protein. The product of the second gene is a novel and highly conserved protein with structural similarity to glycoprotein gp25L, a constituent of another translocon-associated protein complex. A database search revealed the existence of a novel family of gp25L-related proteins whose members occur throughout the animal kingdom. Together, our data imply that (i) the group of ER proteins surrounding translocating polypeptide chains may be far more complex than previously expected, and (ii) a number of the accessory components of the translocon participate in early steps of prohormone biosynthesis.

Subcellular localization of human voltage-dependent anion channel isoforms

The voltage-dependent anion channel of the outer mitochondrial membrane, VDAC (also known as mitochondrial porin), is a small abundant protein which forms a voltage-gated pore when incorporated into planar lipid bilayers. This protein forms the primary pathway for movement of major metabolites through the outer membrane. Recently, it has been demonstrated that two human VDAC genes, HVDAC1 and HVDAC2, produce three proteins that differ most significantly at their amino termini. These results suggest that the distinct amino termini lead to the targeting of individual VDAC isoforms to different cellular compartments. Consistent with this hypothesis, recent reports suggest that HIV-DAC1 is found in the plasma membrane of mammalian cells. To define the subcellular location of HVDAC isoforms, HVDAC genes were modified so that the encoded proteins contain COOH-terminal epitopes recognized by either of two monoclonal antibodies. Introduction of these epitope tags had no effect on the function of modified VDAC proteins. Epitope-tagged proteins were then individually expressed in COS7 cells or rat astrocytes and the intracellular location of each isoform subsequently identified by subcellular fractionation, light level immunofluorescence, and immunoelectron microscopy. Our results demonstrate that each HVDAC protein is exclusively located in fractions or subcellular regions containing mitochondrial marker proteins. In addition, immunofluorescence and immunoelectron microscopy show that an individual mitochondrion can contain both HVDAC1 and HVDAC2. Our results call into question previous reports demonstrating VDAC molecules in the plasma membrane and suggest that functional differences between individual VDAC isoforms may result in distinct regulatory processes within a single mitochondrion.

Protein phosphatase 1 regulates the cytoplasmic dynein-driven formation of endoplasmic reticulum networks in vitro

Interphase Xenopus egg extracts form extensive tubular membrane networks in vitro. These networks are identified here as endoplasmic reticulum by the presence of ER resident proteins, as shown by immunofluorescence, and by the presence of single ribosomes and polysomes, as shown by electron microscopy. The effect of phosphorylation on ER movement in interphase was tested using the phosphatase inhibitor, okadaic acid. Okadaic acid treatment resulted in an increase of up to 27-fold in the number of ER tubules moving and in the extent of ER networks formed compared to control extracts. This activation was blocked by the broad-specificity kinase inhibitor 6-dimethylaminopurine. Okadaic acid had no effect, however, on the direction of ER tubule movement, which occurred towards the minus end of microtubules, and was sensitive to low concentrations of vanadate. Inhibition of phosphatases also had no effect on the speed or duration of ER tubule extensions, and did not stimulate the activity of soluble cytoplasmic dynein. The sensitivity of ER movement to okadaic acid closely matched that of protein phosphatase 1. Although the amount of ER motility was greatly increased by inhibiting protein phosphatase 1 (PP1), the amount of cytoplasmic dynein associated with the membrane was not altered. The data support a model in which phosphorylation regulates ER movement by controlling the activity of cytoplasmic dynein bound to the ER membrane.

The N-terminal region of the alpha-subunit of the TRAP complex has a conserved cluster of negative charges

The alpha-subunit of the TRAP complex (TRAP alpha) is a single-spanning membrane protein of the endoplasmic reticulum (ER) which is found in proximity of nascent polypeptide chains translocating across the membrane. Here, we demonstrate the widespread occurrence of TRAP alpha in eukaryotes as indicated by its existence in man, fish and plants. Despite the fact that the sequence homology is much lower than for other proteins in the translocation site, the overall topology, the location of the glycosylation sites and, most interestingly, the distribution of charges are conserved. These data indicate that the TRAP complex has a ubiquitous function.

An LI and ML motif in the cytoplasmic tail of the MHC-associated invariant chain mediate rapid internalization

Invariant chain (Ii) is a transmembrane protein that associates with the MHC class II molecules in the endoplasmic reticulum. Two regions of the 30 residue cytoplasmic tail of Ii contain sorting information able to direct Ii to the endocytic pathway. The full-length cytoplasmic tail of Ii and the two tail regions were fused to neuraminidase (NA) forming chimeric proteins (INA). Ii is known to form trimers and when INA was transfected into COS cells it assembled as a tetramer like NA. The INA molecules were targeted to the endosomal pathway and cotransfection with Ii showed that both molecules appeared in the same vesicles. By labelling the INA fusion proteins with iodinated antibody it was found that molecules with either endocytosis signal were expressed at the plasma membrane and internalized rapidly. Point mutations revealed that an LI motif within the first region of the cytoplasmic tail and an ML motif in the second region were essential for efficient internalization. The region containing the LI motif is required for Ii to induce large endosomes but a functional LI internalization motif was not fundamental for this property. The cytoplasmic tail of Ii is essential for efficient targeting of the class II molecules to endosomes and the dual LI and ML motif may thus be responsible for directing these molecules to the endosomal pathway, possibly via the plasma membrane.

The invariant chain inhibits presentation of endogenous antigens by a human fibroblast cell line

The human fibroblast cell line, M1, expressing the products of transfected DRA and DRB1*0101 genes (M1-DR1) was unable to present intact influenza antigens to a series of DR1-restricted human T cell lines and clones, but was fully able to present synthetic peptides for T cell recognition. In contrast, M1-DR1 cells infected with live influenza virus were recognized by two polyclonal hemagglutinin- or whole virus-specific T cell lines and one of four T cell clones. This difference could not be accounted for simply by the ability of infectious virus to overcome a defect in antigen uptake by the M1-DR1 cells, in that direct studies of endocytosis showed that the M1 cells were more efficient than human B cells in the internalization of exogenous protein. These data suggested that the M1 cells were unable to present exogenous antigens but were capable of loading major histocompatibility complex (MHC) class II molecules with peptides derived from endogenous antigens. To investigate this further, the M1-DR1 cells were super-transfected with a cDNA encoding the p33 and p35 forms of the human invariant chain (Ii). Expression of the Ii chain was detected by intracytoplasmic staining of transfectants, and by metabolic labeling. Equimolar amounts of the p33 and p35 forms were detected, and the high level of p35 Ii was reflected by extensive retention of Ii protein in the endoplasmic reticulum. Addition of the Ii chain led to no recovery of presentation of intact antigens with DR1, but inhibited the presentation of live virus. These data indicate that MHC class II molecules in the M1-DR1 cells can be loaded with peptides derived from endogenous proteins, possibly in the biosynthetic pathway, and that the Ii chain has a role in limiting this route of class II antigen presentation.

Molecular cloning and subcellular localization of three GTP-binding proteins of the rab subfamily

Small GTPases of the rab subfamily are involved in regulation of intracellular membrane transport events. We recently used a PCR approach to isolate short cDNA fragments of a number of novel rab sequences. These PCR fragments have not been used with cDNA library screening and PCR-based techniques to clone the cDNAs encoding three of these proteins, rab12, rab22, and rab24. By northern blot analysis, the messages were found to be present in a wide variety of mouse tissues. However, quantitative differences in the mRNA levels between the tissues were detected. We determined the subcellular localization of the GTPases by expressing the c-myc epitope-tagged proteins with the Semliki Forest virus and the vaccinia T7 vector systems. Transiently expressed rab12 was localized to the Golgi complex. This localization was confirmed using a polyclonal anti-peptide antibody detecting the endogenous protein in BHK cells. rab22 expressed from the cDNA was localized to endosomal compartments and to the plasma membrane. After longer periods of expression, the protein was found on abnormally large perinuclear endosomal structures, suggesting that it is a potent regulator of events in the endocytic pathway. Finally, rab24 was found in the endoplasmic reticulum/cis-Golgi region and on late endosomal structures. The localization of rab24 may indicate its involvement in autophagy-related processes.

The signal sequence moves through a ribosomal tunnel into a noncytoplasmic aqueous environment at the ER membrane early in translocation

The signal sequence is in an aqueous milieu at an early stage in the translocation of a nascent secretory protein across the endoplasmic reticulum membrane. This was determined using fluorescent probes incorporated into the signal sequence of fully assembled ribosome-nascent chain-membrane complexes: the fluorescence lifetimes revealed that the probes were in an aqueous environment rather than buried in the nonpolar core of the membrane. Since these membrane-bound probes were not susceptible to collisional quenching by iodide ions, the space containing the signal sequence is sealed off from the cytoplasm by a tight ribosome-membrane junction. The nascent chain inside the ribosome is also not exposed to the cytoplasm and apparently passes through an aqueous tunnel in the ribosome.

A tetrameric complex of membrane proteins in the endoplasmic reticulum

The translocation site (translocon), at which nascent polypeptides pass through the endoplasmic reticulum membrane, contains a component previously called 'signal sequence receptor' that is now renamed as 'translocon-associated protein' (TRAP). Two glycosylated subunits of the TRAP complex have been identified before (alpha and beta subunits). We now show that TRAP complex is actually comprised of four membrane proteins (alpha, beta, gamma, delta), present in a stoichiometric relation, which are genuine neighbours in intact microsomes. The amino acid sequences of the additional, non-glycosylated subunits were deduced from cloning of the corresponding cDNAs. The delta subunit spans the membrane only once and has its major portion, containing a disulfide bridge, at the lumenal side. The gamma subunit is predicted to span the membrane four times.

Assembly of vaccinia virus: role of the intermediate compartment between the endoplasmic reticulum and the Golgi stacks

Vaccinia virus, the prototype of the Poxviridae, is a large DNA virus which replicates in the cytoplasm of the host cell. The assembly pathway of vaccinia virus displays several unique features, such as the production of two structurally distinct, infectious forms. One of these, termed intracellular naked virus (INV), remains cells associated while the other, termed extracellular enveloped virus (EEV), is released from the cell. In addition, it has long been believed that INVs acquire their lipid envelopes by a unique example of de novo membrane biogenesis. To examine the structure and assembly of vaccinia virus we have used immunoelectron microscopy using antibodies to proteins of different subcellular compartments as well as a phospholipid analysis of purified INV and EEV. Our data are not consistent with the de novo model of viral membrane synthesis but rather argue that the vaccinia virus DNA becomes enwrapped by a membrane cisterna derived from the intermediate compartment between the ER and the Golgi stacks, thus acquiring two membranes in one step. Phospholipid analysis of purified INV supports its derivation from an early biosynthetic compartment. This unique assembly process is repeated once more when the INV becomes enwrapped by an additional membrane cisterna, in agreement with earlier reports. The available data suggest that after fusion between the outer envelope and the plasma membrane, mature EEV is released from the cell.

A mammalian homolog of SEC61p and SECYp is associated with ribosomes and nascent polypeptides during translocation

SEC61p is essential for protein translocation across the endoplasmic reticulum membrane of S. cerevisiae. We have found a mammalian homolog that shows more than 50% sequence identity with the yeast protein. Moreover, several regions of SEC61p have significant similarities with corresponding ones of SecYp of bacteria, indicating a strong evolutionary conservation of the mechanism of protein translocation. Mammalian Sec61p, like the yeast protein, is located in the immediate vicinity of nascent polypeptides during their membrane passage. It is tightly associated with membrane-bound ribosomes, suggesting that the nascent chain passes directly from the ribosome into a protein-conducting channel. These results define Sec61p as a ubiquitous key component of the protein translocation apparatus.

A protein of the endoplasmic reticulum involved early in polypeptide translocation

To identify components of the mammalian endoplasmic reticulum involved in the translocation of secretory proteins, crosslinking and reconstitution methods were combined. A multispanning abundant membrane glycoprotein was found which is in proximity to nascent chains early in translocation. In reconstituted proteoliposomes, this protein is stimulatory or required for the translocation of secretory proteins.

The signal sequence receptor, unlike the signal recognition particle receptor, is not essential for protein translocation

Detergent extracts of canine pancreas rough microsomal membranes were depleted of either the signal recognition particle receptor (SR), which mediates the signal recognition particle (SRP)-dependent targeting of the ribosome/nascent chain complex to the membrane, or the signal sequence receptor (SSR), which has been proposed to function as a membrane bound receptor for the newly targeted nascent chain and/or as a component of a multi-protein translocation complex responsible for transfer of the nascent chain across the membrane. Depletion of the two components was performed by chromatography of detergent extracts on immunoaffinity supports. Detergent extracts lacking either SR or SSR were reconstituted and assayed for activity with respect to SR dependent elongation arrest release, nascent chain targeting, ribosome binding, secretory precursor translocation, and membrane protein integration. Depletion of SR resulted in the loss of elongation arrest release activity, nascent chain targeting, secretory protein translocation, and membrane protein integration, although ribosome binding was unaffected. Full activity was restored by addition of immunoaffinity purified SR before reconstitution of the detergent extract. Surprisingly, depletion of SSR was without effect on any of the assayed activities, indicating that SSR is either not required for translocation or is one of a family of functionally redundant components.

An ATP-binding membrane protein is required for protein translocation across the endoplasmic reticulum membrane

The role of nucleotides in providing energy for polypeptide transfer across the endoplasmic reticulum (ER) membrane is still unknown. To address this question, we treated ER-derived mammalian microsomal vesicles with a photoactivatable analogue of ATP, 8-N3ATP. This treatment resulted in a progressive inhibition of translocation activity. Approximately 20 microsomal membrane proteins were labeled by [alpha 32P]8-N3ATP. Two of these were identified as proteins with putative roles in translocation, alpha signal sequence receptor (SSR), the 35-kDa subunit of the signal sequence receptor complex, and ER-p180, a putative ribosome receptor. We found that there was a positive correlation between inactivation of translocation activity and photolabeling of alpha SSR. In contrast, our data demonstrate that the ATP-binding domain of ER-p180 is dispensable for translocation activity and does not contribute to the observed 8-N3ATP sensitivity of the microsomal vesicles.

The protein translocation apparatus of the rough endoplasmic reticulum, its associated proteins, and the mechanism of translocation

The demonstration that the yeast Sec61, Sec62, and Sec63 proteins are assembled into a multisubunit complex in the yeast endoplasmic reticulum was of particular significance in a year when protein, and nucleic-sequence analyses revealed interesting homologies between pathways of protein transport in mammals and yeast, and possibly in Escherichia coli.

Photoaffinity labeling of dog pancreas microsomes with 8-azido-ATP inhibits association of nascent preprolactin with the signal sequence receptor complex

Transport of bovine preprolactin into dog pancreas microsomes involves a microsomal protein which is sensitive to photoaffinity labeling with azido-ATP and which is distinct from the ATP-binding protein, immunoglobulin heavy chain binding protein. Here we addressed the question of what stage of preprolactin transport is affected. Thus a nascent presecretory protein which is related to preprolactin, termed ppl-86mer, was employed. Here we show that the nascent preprolactin did not become associated with the alpha-subunit of the signal sequence receptor complex after photoaffinity labeling of microsomes with azido-ATP. Therefore, we conclude that the microsomal protein which is sensitive to photoaffinity labeling with azido-ATP acts prior to the signal sequence receptor complex.

Protein export in prokaryotes and eukaryotes. Theme with variations

Protein export in prokaryotes as well as in eukaryotes can be defined as protein transport across the plasma membrane. In both types of organisms there are various apparently ATP-dependent transport mechanisms which can be distinguished from one another and which show similarities when the prokaryotic mechanism is compared with the respective eukaryotic mechanism. First, one can distinguish between transport mechanisms which involve so-called signal or leader peptides and those which do not. The latter mechanisms seem to employ ATP-dependent transport systems which belong to the family of oligopeptide permeases and multiple drug resistance proteins. Second, in signal or leader peptide-dependent transport one can distinguish between transport mechanisms which involve ribonucleoparticles and those which employ molecular chaperones. Both mechanisms appear to converge at the level of ATP-dependent translocases.

ER translocation intermediates are adjacent to a nonglycosylated 34-kD integral membrane protein

We have used the homobifunctional cross-linking reagent disuccinimidyl suberate (DSS) to identify proteins that are adjacent to nascent polypeptides undergoing translocations across mammalian rough ER. Translocation intermediates were assembled by supplementing cell free translations of truncated mRNAs with the signal recognition particle (SRP) and microsomal membrane vesicles. Two prominent cross-linked products of 45 and 64 kD were detected. The 64-kD product was obtained when the cell free translation contained SRP, while formation of the 45-kD product required both SRP and translocation competent microsomal membrane vesicles. In agreement with previous investigators, we suggest that the 64-kD product arises by cross-linking of the nascent polypeptide to the 54-kD subunit of SRP. The 45-kD product resists alkaline extraction from the membrane, so we conclude that the 11-kD nascent polypeptide has been crosslinked to an integral membrane protein of approximately 34 kD (imp34). The cross-linked product does not bind to ConA Sepharose, nor is it sensitive to endoglycosidase H digestion; hence imp34 is not identical to the alpha or beta subunits of the signal sequence receptor (SSR). We propose that imp34 functions in concert with SSR to form a translocation site through which nascent polypeptides pass in traversing the membrane bilayer of the rough endoplasmic reticulum.

Requirement of GTP hydrolysis for dissociation of the signal recognition particle from its receptor

The signal recognition particle (SRP) directs signal sequence specific targeting of ribosomes to the rough endoplasmic reticulum. Displacement of the SRP from the signal sequence of a nascent polypeptide is a guanosine triphosphate (GTP)-dependent reaction mediated by the membrane-bound SRP receptor. A nonhydrolyzable GTP analog can replace GTP in the signal sequence displacement reaction, but the SRP then fails to dissociate from the membrane. Complexes of the SRP with its receptor containing the nonhydrolyzable analog are incompetent for subsequent rounds of protein translocation. Thus, vectorial targeting of ribosomes to the endoplasmic reticulum is controlled by a GTP hydrolysis cycle that regulates the affinity between the SRP, signal sequences, and the SRP receptor.

Biochemical fractionation and assembly of the membrane components that mediate nascent chain targeting and translocation

Fractionation of a microsomal detergent extract with ammonium sulfate allows separation of the signal recognition particle receptor (SR alpha), which is required for targeting of the nascent chain, from other microsomal proteins, such as signal peptidase, whose activity is displayed during subsequent translocation. The reconstituted SR alpha-enriched fraction is functional in assays of precursor targeting and elongation arrest release but lacks translocation activity. This defect can be complemented by addition, prior to reconstitution, of a separate protein subfraction. In addition, protein components necessary for translocation can be reversibly depleted from the complementary fraction, under conditions where precursor targeting is retained, by sulfhydryl-directed chromatography. Thus, precursor binding and translocation can be biochemically uncoupled, indicating that they are sequential reactions mediated by distinct components.

The identification of proteins in the proximity of signal-anchor sequences during their targeting to and insertion into the membrane of the ER

Using a photocross-linking approach we have investigated the cytosolic and membrane components involved in the targeting and insertion of signal-anchor proteins into the membrane of the ER. The nascent chains of both type I and type II signal-anchor proteins can be cross-linked to the 54-kD subunit of the signal recognition particle. Upon addition of rough microsomes the type I and type II signal-anchor proteins interact with a number of components. Both types of protein interact with an integral membrane protein, the signal sequence receptor, previously identified by its proximity to preprolactin during its translocation (Wiedmann, M., T.V. Kurzchalia, E. Hartmann, and T.A. Rapoport. 1987. Nature [Lond.] 328:830-833). Three proteins, previously unidentified, were found to be cross-linked to the nascent chains of the signal-anchor proteins. Among them was a 37-kD protein that was found to be the main component interacting with the type I SA protein used. These proteins were not seen in the absence of membranes suggesting they are components of the ER. The ability of the nascent chains to be cross-linked to these identified proteins was shown to be abolished by prior treatment with agents known to disrupt translocation intermediates or ribosomes. We propose that the newly identified proteins function either in the membrane insertion of only a subset of proteins or only at a specific stage of insertion.

A nascent membrane protein is located adjacent to ER membrane proteins throughout its integration and translation

The immediate environment of nascent membrane proteins undergoing integration into the ER membrane was investigated by photocrosslinking. Nascent polypeptides of different lengths, each containing a single IgM transmembrane sequence that functions either as a stop-transfer or a signal-anchor sequence, were synthesized by in vitro translation of truncated mRNAs in the presence of N epsilon-(5-azido-2-nitrobenzoyl)-Lys-tRNA, signal recognition particle, and microsomal membranes. This yielded nascent chains with photoreactive probes at one end of the transmembrane sequence where two lysine residues are located. When irradiated, these nascent chains reacted covalently with several ER proteins. One prominent crosslinking target was a glycoprotein similar in size to a protein termed mp39, shown previously to be situated adjacent to a secretory protein during its translocation across the ER membrane (Krieg, U. C., A. E. Johnson, and P. Walter. 1989. J. Cell Biol. 109:2033-2043; Wiedmann, M., D. Goerlich, E. Hartmann, T. V. Kurzchalia, and T. A. Rapoport. 1989. FEBS (Fed. Eur. Biochem. Soc.) Lett. 257:263-268) and likely to be identical to a protein previously designated the signal sequence receptor (Wiedmann, M., T. V. Kurzchalia, E. Hartmann, and T. A. Rapoport. 1987. Nature (Lond.). 328:830-833). Changing the orientation of the transmembrane domain in the bilayer, or making the transmembrane domain the first topogenic sequence in the nascent chain instead of the second, did not significantly alter the identities of the ER proteins that were the primary crosslinking targets. Furthermore, the nascent chains crosslinked to the mp39-like glycoprotein and other microsomal proteins even after the cytoplasmic tail of the nascent chain had been lengthened by nearly 100 amino acids beyond the stop-transfer sequence. Yet when the nascent chain was allowed to terminate normally, the major photocrosslinks were no longer observed, including in particular that to the mp39-like glycoprotein. These results show that the transmembrane segment of a nascent membrane protein is located adjacent to the mp39-like glycoprotein and other ER proteins during the integration process, and that at least a portion of the nascent chain remains in close proximity to these ER proteins until translation has been completed.

The signal sequence receptor has a second subunit and is part of a translocation complex in the endoplasmic reticulum as probed by bifunctional reagents

Bifunctional cross-linking reagents were used to probe the protein environment in the ER membrane of the signal sequence receptor (SSR), a 24-kD integral membrane glycoprotein (Wiedmann, M., T. V. Kurzchalia, E. Hartmann, and T. A. Rapoport. 1987. Nature [Lond.]. 328:830-833). The proximity of several polypeptides was demonstrated. A 22-kD glycoprotein was identified tightly bound to the 34-kD SSR even after membrane solubilization. The 34-kD polypeptide, now termed alpha SSR, and the 22-kD polypeptide, the beta SSR, represent a heterodimer. We report on the sequence of the beta SSR, its membrane topology, and on the mechanism of its integration into the membrane. Cross-linking also produced dimers of the alpha-subunit of the SSR indicating that oligomers of the SSR exist in the ER membrane. Various bifunctional cross-linking reagents were used to study the relation to ER membrane proteins of nascent chains of preprolactin and beta-lactamase at different stages of their translocation through the membrane. The predominant cross-linked products obtained in high yields contained the alpha SSR, indicating in conjunction with previous results that it is a major membrane protein in the neighborhood of translocating nascent chains of secretory proteins. The results support the existence of a translocon, a translocation complex involving the SSR, which constitutes the specific site of protein translocation across the ER membrane.

Protein transport across the ER membrane

Protein transport across the endoplasmic reticulum (ER) membrane may be divided into two phases: an initiation or targeting cycle, which has been fairly well characterized, and the actual transfer of the polypeptide chain through the membrane, the mechanism of which is still unknown. In this review, the initiation cycle is discussed with emphasis on the mechanism of signal sequence recognition by the 54 kDa polypeptide of the signal recognition particle (SRP) and on the efficiency of targeting of nascent chains. Recent results are reviewed suggesting the transfer of the polypeptide chain by means of a translocation complex, a constituent of which appears to be the signal sequence receptor protein (SSR).