Discrete cross-linking products identified during membrane protein biosynthesis

The ER Protein Translocation Channel Subunit Sbh1 Controls Virulence of Cryptococcus neoformans

The fungal pathogen Cryptococcus neoformans is distinguished by a cell-wall-anchored polysaccharide capsule that is critical for virulence. Biogenesis of both cell wall and capsule relies on the secretory pathway. Protein secretion begins with polypeptide translocation across the endoplasmic reticulum (ER) membrane through a highly conserved channel formed by three proteins: Sec61, Sbh1, and Sss1. Sbh1, the most divergent, contains multiple phosphorylation sites, which may allow it to regulate entry into the secretory pathway in a species- and protein-specific manner. Absence of SBH1 causes a cell-wall defect in both Saccharomyces cerevisiae and C. neoformans, although other phenotypes differ. Notably, proteomic analysis showed that when cryptococci are grown in conditions that mimic aspects of the mammalian host environment (tissue culture medium, 37°C, 5% CO₂), a set of secretory and transmembrane proteins is upregulated in wild-type, but not in Δsbh1 mutant cells. The Sbh1-dependent proteins show specific features of their ER targeting sequences that likely cause them to transit less efficiently into the secretory pathway. Many also act in cell-wall biogenesis, while several are known virulence factors. Consistent with these observations, the C. neoformans Δsbh1 mutant is avirulent in a mouse infection model. We conclude that, in the context of conditions encountered during infection, Sbh1 controls the entry of virulence factors into the secretory pathway of C. neoformans, and thereby regulates fungal pathogenicity. IMPORTANCE Cryptococcus neoformans is a yeast that causes almost 200,000 deaths worldwide each year, mainly of immunocompromised individuals. The surface structures of this pathogen, a protective cell wall surrounded by a polysaccharide capsule, are made and maintained by proteins that are synthesized inside the cell and travel outwards through the secretory pathway. A protein called Sbh1 is part of the machinery that determines which polypeptides enter this export pathway. We found that when Sbh1 is absent, both C. neoformans and the model yeast S. cerevisiae show cell-wall defects. Lack of Sbh1 also changes the pattern of secretion of both transmembrane and soluble proteins, in a manner that depends on characteristics of their sequences. Notably, multiple proteins that are normally upregulated in conditions similar to those encountered during infection, including several needed for cryptococcal virulence, are no longer increased. Sbh1 thereby regulates the ability of this important pathogen to cause disease.

Sec61 channel subunit Sbh1/Sec61β promotes ER translocation of proteins with suboptimal targeting sequences and is fine-tuned by phosphorylation

The highly conserved endoplasmic reticulum (ER) protein translocation channel contains one nonessential subunit, Sec61β/Sbh1, whose function is poorly understood so far. Its intrinsically unstructured cytosolic domain makes transient contact with ER-targeting sequences in the cytosolic channel vestibule and contains multiple phosphorylation sites suggesting a potential for regulating ER protein import. In a microscopic screen, we show that 12% of a GFP-tagged secretory protein library depends on Sbh1 for translocation into the ER. Sbh1-dependent proteins had targeting sequences with less pronounced hydrophobicity and often no charge bias or an inverse charge bias which reduces their insertion efficiency into the Sec61 channel. We determined that mutating two N-terminal, proline-flanked phosphorylation sites in the Sbh1 cytosolic domain to alanine phenocopied the temperature-sensitivity of a yeast strain lacking SBH1 and its ortholog SBH2. The phosphorylation site mutations reduced translocation into the ER of a subset of Sbh1-dependent proteins, including enzymes whose concentration in the ER lumen is critical for ER proteostasis. In addition, we found that ER import of these proteins depended on the activity of the phospho-S/T-specific proline isomerase Ess1 (PIN1 in mammals). We conclude that Sbh1 promotes ER translocation of substrates with suboptimal targeting sequences and that its activity can be regulated by a conformational change induced by N-terminal phosphorylation.

Cancer associated mutations in Sec61γ alter the permeability of the ER translocase

Translocation of secretory and integral membrane proteins across or into the ER membrane occurs via the Sec61 complex, a heterotrimeric protein complex possessing two essential sub-units, Sec61p/Sec61α and Sss1p/Sec61γ and the non-essential Sbh1p/Sec61β subunit. In addition to forming a protein conducting channel, the Sec61 complex maintains the ER permeability barrier, preventing flow of molecules and ions. Loss of Sec61 integrity is detrimental and implicated in the progression of disease. The Sss1p/Sec61γ C-terminus is juxtaposed to the key gating module of Sec61p/Sec61α and is important for gating the translocon. Inspection of the cancer genome database identifies six mutations in highly conserved amino acids of Sec61γ/Sss1p. We identify that five out of the six mutations identified affect gating of the ER translocon, albeit with varying strength. Together, we find that mutations in Sec61γ that arise in malignant cells result in altered translocon gating dynamics, this offers the potential for the translocon to represent a target in co-therapy for cancer treatment.

Mechanistic insights into the inhibition of Sec61-dependent co- and post-translational translocation by mycolactone

The virulence factor mycolactone is responsible for the immunosuppression and tissue necrosis that characterise Buruli ulcer, a disease caused by infection with Mycobacterium ulcerans In this study, we confirm that Sec61, the protein-conducting channel that coordinates entry of secretory proteins into the endoplasmic reticulum, is a primary target of mycolactone, and characterise the nature of its inhibitory effect. We conclude that mycolactone constrains the ribosome-nascent-chain-Sec61 complex, consistent with its broad-ranging perturbation of the co-translational translocation of classical secretory proteins. In contrast, the effect of mycolactone on the post-translational ribosome-independent translocation of short secretory proteins through the Sec61 complex is dependent on both signal sequence hydrophobicity and the translocation competence of the mature domain. Changes to protease sensitivity strongly suggest that mycolactone acts by inducing a conformational change in the pore-forming Sec61α subunit. These findings establish that mycolactone inhibits Sec61-mediated protein translocation and highlight differences between the co- and post-translational routes that the Sec61 complex mediates. We propose that mycolactone also provides a useful tool for further delineating the molecular mechanisms of Sec61-dependent protein translocation.

Reorientation of the first signal-anchor sequence during potassium channel biogenesis at the Sec61 complex

The majority of the polytopic proteins that are synthesized at the ER (endoplasmic reticulum) are integrated co-translationally via the Sec61 translocon, which provides lateral access for their hydrophobic TMs (transmembrane regions) to the phospholipid bilayer. A prolonged association between TMs of the potassium channel subunit, TASK-1 [TWIK (tandem-pore weak inwardly rectifying potassium channel)-related acid-sensitive potassium channel 1], and the Sec61 complex suggests that the ER translocon co-ordinates the folding/assembly of the TMs present in the nascent chain. The N-terminus of both TASK-1 and Kcv (potassium channel protein of chlorella virus), another potassium channel subunit of viral origin, has access to the N-glycosylation machinery located in the ER lumen, indicating that the Sec61 complex can accommodate multiple arrangements/orientations of TMs within the nascent chain, both in vitro and in vivo. Hence the ER translocon can provide the ribosome-bound nascent chain with a dynamic environment in which it can explore a range of different conformations en route to its correct transmembrane topology and final native structure.

The Sec61 translocon provides an unexpectedly flexible and dynamic environment within which transmembrane regions of nascent polypeptides can be completely reoriented during the biosynthesis of multiple-spanning membrane proteins.

N-acetylation and phosphorylation of Sec complex subunits in the ER membrane

Background

Covalent modifications of proteins provide a mechanism to control protein function. Here, we have investigated modifications of the heptameric Sec complex which is responsible for post-translational protein import into the endoplasmic reticulum (ER). It consists of the Sec61 complex (Sec61p, Sbh1p, Sss1p) which on its own mediates cotranslational protein import into the ER and the Sec63 complex (Sec63p, Sec62p, Sec71p, Sec72p). Little is known about the biogenesis and regulation of individual Sec complex subunits.

Results

We show that Sbh1p when it is part of the Sec61 complex is phosphorylated on T5 which is flanked by proline residues. The phosphorylation site is conserved in mammalian Sec61ß, but only partially in birds, and not in other vertebrates or unicellular eukaryotes, suggesting convergent evolution. Mutation of T5 to A did not affect the ability of mutant Sbh1p to complement the growth defect in a Δsbh1Δsbh2 strain, and did not result in a hypophosphorylated protein which shows that alternate sites can be used by the T5 kinase. A survey of yeast phosphoproteome data shows that Sbh1p can be phosphorylated on multiple sites which are organized in two patches, one at the N-terminus of its cytosolic domain, the other proximal to the transmembrane domain. Surprisingly, although N-acetylation has been shown to interfere with ER targeting, we found that both Sbh1p and Sec62p are cotranslationally N-acetylated by NatA, and N-acetyl-proteome data indicate that Sec61p is modified by the same enzyme. Mutation of the N-acetylation site, however, did not affect Sec62p function in posttranslational protein import into the ER. Disabling NatA resulted in growth retardation, but not in co- or posttranslational translocation defects or instability of Sec62p or Sbh1p.

Conclusions

We conclude that N-acetylation of transmembrane and tail-anchored proteins does not interfere with their ER-targeting, and that Sbh1p phosphorylation on T5, which is not present in Sbh2p, plays a non-essential role specific to the Sec61 complex.

Endoplasmic reticulum-associated degradation of a degron-containing polytopic membrane protein

The presence of two basic amino acids strategically located within a single spanning transmembrane region has previously been shown to act as a signal for the endoplasmic reticulum associated degradation (ERAD) of several polypeptides. In contrast, the functionality of this degron motif within the context of a polytopic membrane protein has not been established. Using opsin as a model system, we have investigated the consequences of inserting the degron motif in the first of its seven transmembrane (TM) spans. Whilst these basic residue reduce the binding of the targeting factor, signal recognition particle, to the first TM span, this has no effect on membrane integration in vitro or in vivo. This most likely reflects the presence of multiple TM spans that can act as targeting signals within in the nascent opsin chain. We find that the degron motif leads to the efficient retention of mutant opsin chains at the endoplasmic reticulum. The mutant opsin polypeptides are degraded via a proteasomal pathway that involves the actions of the E3 ubiquitin ligase HRD1. In contrast, wild-type opsin remains stable for a prolonged period even when artificially accumulated at the endoplasmic reticulum. We conclude that a single dibasic degron motif is sufficient to initiate both the ER retention and subsequent degradation of ospin via an ERAD pathway.

Dissecting the physiological role of selective transmembrane-segment retention at the ER translocon

The membrane integration of polytopic proteins is coordinated at the endoplasmic reticulum (ER) by the conserved Sec61 translocon, which facilitates the lateral release of transmembrane (TM) segments into the lipid phase during polypeptide translocation. Here we use a site-specific crosslinking strategy to study the membrane integration of a new model protein and show that the TM segments of the P2X2 receptor are retained at the Sec61 complex for the entire duration of the biosynthetic process. This extremely prolonged association implicates the Sec61 complex in the regulation of the membrane integration process, and we use both in vitro and in vivo analyses to study this effect further. TM-segment retention depends on the association of the ribosome with the Sec61 complex, and complete lateral exit of the P2X2 TM segments was only induced by the artificial termination of translation. In the event of the premature release of P2X2 TM1 from the ER translocon, the truncated polypeptide fragment was to found aggregate in the ER membrane, suggesting a distinct physiological requirement for the delayed release of TM segments from the ER translocon site.

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.

Specific transmembrane segments are selectively delayed at the ER translocon during opsin biogenesis

A site-specific cross-linking approach was used to study the integration of TM (transmembrane) segments 4-7 of the polytopic membrane protein, opsin, at the ER (endoplasmic reticulum). We found that although TM4 exits the ER translocon rapidly, TM segments 5, 6 and 7 are all retained at the translocon until opsin biosynthesis is terminated. Furthermore, although artificial extension of the nascent chain is not sufficient to release the C-terminal region of opsin from the translocon, substitution of the native TM segment 7 with a more hydrophobic TM segment results in its rapid lateral exit into the lipid bilayer. We conclude that the intrinsic properties of a TM segment determine the timing of its membrane integration rather than its relative location within the polypeptide chain. A pronounced and prolonged association of opsin TM5 with the translocon-associated component PAT-10 was also observed, suggesting that PAT-10 may facilitate the assembly of distinct opsin subdomains during membrane integration. The results of the present study strongly support a model in which the ER translocon co-ordinates the integration of selected TM segments in response to the specific requirements of the precursor being synthesized.

The oligomeric state of Derlin-1 is modulated by endoplasmic reticulum stress

The endoplasmic reticulum (ER) is a major site of protein synthesis in eukaryotes. Newly synthesized proteins are monitored by a process of quality control, which removes misfolded or unassembled polypeptides from the ER for degradation by the proteasome. This requires the retrotranslocation of the misfolded proteins from the ER lumen into the cytosol via a pathway that, for some substrates, involves members of the recently discovered Derlin family. The Derlin-1 isoform is present as a dimer in the ER, and we now show that its dimerization is modulated by ER stress. Three distinct types of chemically-induced ER stress substantially reduce the levels of Derlin-1 dimer as assayed by both cross-linking and co-immunoprecipitation. The potential function of the different Derlin-1 populations with respect to ER quality control is investigated by analysing their capacity to associate with a misfolded membrane protein fragment. We show for the first time that Derlin-1 can associate with an aberrant membrane protein fragment in the absence of the viral component US11, and conclude that it is the monomeric form of Derlin-1 that interacts with this potential ER-associated degradation substrate. On the basis of these data we propose a model where the pool of active Derlin-1 in the ER membrane can be modulated in response to ER stress.

Inhibition of Vascular Endothelial Growth Factor Cotranslational Translocation by the Cyclopeptolide CAM741

The cyclopeptolide CAM741 inhibits cotranslational translocation of vascular cell adhesion molecule 1 (VCAM1), which is dependent on its signal peptide. We now describe the identification of the signal peptide of vascular endothelial growth factor (VEGF) as the second target of CAM741. The mechanism by which the compound inhibits translocation of VEGF is very similar or identical to that of VCAM1, although the signal peptides share no obvious sequence similarities. By mutagenesis of the VEGF signal peptide, two important regions, located in the N-terminal and hydrophobic segments, were identified as critical for compound sensitivity. CAM741 alters positioning of the VEGF signal peptide at the translocon, and increasing hydrophobicity in the h-region reduces compound sensitivity and causes a different, possibly more efficient, interaction with the translocon. Although CAM741 is effective against translocation of both VEGF and VCAM1, the derivative NFI028 is able to inhibit only VCAM1, suggesting that chemical derivatization can alter not only potency, but also the specificity of the compounds.

Active and passive displacement of transmembrane domains both occur during opsin biogenesis at the Sec61 translocon

We used a site-specific crosslinking approach to study the membrane integration of the polytopic protein opsin at the endoplasmic reticulum. We show that transmembrane domain 1 occupies two distinct Sec61-based environments during its integration. However, transmembrane domains 2 and 3 exit the Sec61 translocon more rapidly in a process that suggests a displacement model for their integration where the biosynthesis of one transmembrane domain would facilitate the exit of another. In order to investigate this hypothesis further, we studied the integration of the first and third transmembrane domains of opsin in the absence of any additional C-terminal transmembrane domains. In the case of transmembrane domain 1, we found that its lateral exit from the translocon is clearly dependent upon the synthesis of subsequent transmembrane domains. By contrast, the lateral exit of the third transmembrane domain occurred independently of any such requirement. Thus, even within a single polypeptide chain, distinct transmembrane domains display different requirements for their integration through the endoplasmic reticulum translocon, and the displacement of one transmembrane domain by another is not a global requirement for membrane integration.

The translocation inhibitor CAM741 interferes with vascular cell adhesion molecule 1 signal peptide insertion at the translocon

The cyclopeptolide CAM741 selectively inhibits cotranslational translocation of vascular cell adhesion molecule 1 (VCAM1), a process that is dependent on its signal peptide. In this study we identified the C-terminal (C-) region upstream of the cleavage site of the VCAM1 signal peptide as most critical for inhibition of translocation by CAM741, but full sensitivity to the compound also requires residues of the hydrophobic (h-) region and the first amino acid of the VCAM1 mature domain. The murine VCAM1 signal peptide, which is less susceptible to translocation inhibition by CAM741, can be converted into a fully sensitive signal peptide by two amino acid substitutions identified as critical for compound sensitivity of the human VCAM1 signal peptide. Using cysteine substitutions of non-critical residues in the human VCAM1 signal peptide and chemical cross-linking of targeted short nascent chains we show that, in the presence of CAM741, the N- and C-terminal segments of the VCAM1 signal peptide could be cross-linked to the cytoplasmic tail of Sec61beta, indicating altered positioning of the VCAM1 signal peptide relative to this translocon component. Moreover, translocation of a tag fused N-terminal to the VCAM1 signal peptide is selectively inhibited by CAM741. Our data indicate that the compound inhibits translocation of VCAM1 by interfering with correct insertion of its signal peptide into the translocon.

Molecular mechanisms of aquaporin biogenesis by the endoplasmic reticulum Sec61 translocon

The past decade has witnessed remarkable advances in our understanding of aquaporin (AQP) structure and function. Much, however, remains to be learned regarding how these unique and vitally important molecules are generated in living cells. A major obstacle in this respect is that AQP biogenesis takes place in a highly specialized and relatively inaccessible environment formed by the ribosome, the Sec61 translocon and the ER membrane. This review will contrast the folding pathways of two AQP family members, AQP1 and AQP4, and attempt to explain how six TM helices can be oriented across and integrated into the ER membrane in the context of current (and somewhat conflicting) translocon models. These studies indicate that AQP biogenesis is intimately linked to translocon function and that the ribosome and translocon form a highly dynamic molecular machine that both interprets and is controlled by specific information encoded within the nascent AQP polypeptide. AQP biogenesis thus has wide ranging implications for mechanisms of translocon function and general membrane protein folding pathways.

Preparation of ribosomes loaded with truncated nascent proteins to study ribosome binding to mammalian mitochondria

Supporting a co-translational model of protein import into mitochondria, we have previously shown that ribosome-nascent chain complexes (RNCs) specifically bind to mitochondria. When producing RNCs using the rabbit reticulocyte lysate in vitro translation system, it was necessary to maximize ribosome loading with truncated nascent proteins because it had a direct impact on RNC binding. We describe here the optimal conditions for preparing RNCs. We show that translation temperature and reaction time are two critical factors, with 30 degrees Celsius and 15min being optimal, respectively. We also show that transcription reactions can be used directly in the translation reaction to create RNCs.

Structure of the E. coli protein-conducting channel bound to a translating ribosome

Secreted and membrane proteins are translocated across or into cell membranes through a protein-conducting channel (PCC). Here we present a cryo-electron microscopy reconstruction of the Escherichia coli PCC, SecYEG, complexed with the ribosome and a nascent chain containing a signal anchor. This reconstruction shows a messenger RNA, three transfer RNAs, the nascent chain, and detailed features of both a translocating PCC and a second, non-translocating PCC bound to mRNA hairpins. The translocating PCC forms connections with ribosomal RNA hairpins on two sides and ribosomal proteins at the back, leaving a frontal opening. Normal mode-based flexible fitting of the archaeal SecYEbeta structure into the PCC electron microscopy densities favours a front-to-front arrangement of two SecYEG complexes in the PCC, and supports channel formation by the opening of two linked SecY halves during polypeptide translocation. On the basis of our observation in the translocating PCC of two segregated pores with different degrees of access to bulk lipid, we propose a model for co-translational protein translocation.

Early encounters of a nascent membrane protein: specificity and timing of contacts inside and outside the ribosome

An unbiased photo–cross-linking approach was used to probe the “molecular path” of a growing nascent Escherichia coli inner membrane protein (IMP) from the peptidyl transferase center to the surface of the ribosome. The nascent chain was initially in proximity to the ribosomal proteins L4 and L22 and subsequently contacted L23, which is indicative of progression through the ribosome via the main ribosomal tunnel. The signal recognition particle (SRP) started to interact with the nascent IMP and to target the ribosome–nascent chain complex to the Sec–YidC complex in the inner membrane when maximally half of the transmembrane domain (TM) was exposed from the ribosomal exit. The combined data suggest a flexible tunnel that may accommodate partially folded nascent proteins and parts of the SRP and SecY. Intraribosomal contacts of the nascent chain were not influenced by the presence of a functional TM in the ribosome.

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.

Double-spanning plant viral movement protein integration into the endoplasmic reticulum membrane is signal recognition particle-dependent, translocon-mediated, and concerted

The current model for cell-to-cell movement of plant viruses holds that transport requires virus-encoded movement proteins that intimately associate with endoplasmic reticulum membranes. We have examined the early stages of the integration into endoplasmic reticulum membranes of a double-spanning viral movement protein using photocross-linking. We have discovered that this process is cotranslational and proceeds in a signal recognition particle-dependent manner. In addition, nascent chain photocross-linking to Sec61alpha and translocating chain-associated membrane protein reveal that viral membrane protein insertion takes place via the translocon, as with most eukaryotic membrane proteins, but that the two transmembrane segments of the viral protein leave the translocon and enter the lipid bilayer together.

A misassembled transmembrane domain of a polytopic protein associates with signal peptide peptidase

The endoplasmic reticulum (ER) exerts a quality control over newly synthesized proteins and a variety of components have been implicated in the specific recognition of aberrant or misfolded polypeptides. We have exploited a site-specific cross-linking approach to search for novel ER components that may specifically recognize the misassembled transmembrane domains present in truncated polytopic proteins. We find that a single probe located in the transmembrane domain of a truncated opsin fragment is cross-linked to several ER proteins. These components are distinct from subunits of the Sec61 complex and represent a 'post-translocon' environment. In this study, we identify one of these post-translocon cross-linking partners as the signal peptide peptidase (SPP). We find that the interaction of truncated opsin chains with SPP is mediated by its second transmembrane domain, and propose that this interaction may contribute to the recognition of misassembled transmembrane domains during membrane protein quality control at the ER.

Conservative sorting in a primitive plastid. The cyanelle of Cyanophora paradoxa

Higher plant chloroplasts possess at least four different pathways for protein translocation across and protein integration into the thylakoid membranes. It is of interest with respect to plastid evolution, which pathways have been retained as a relic from the cyanobacterial ancestor ('conservative sorting'), which ones have been kept but modified, and which ones were developed at the organelle stage, i.e. are eukaryotic achievements as (largely) the Toc and Tic translocons for envelope import of cytosolic precursor proteins. In the absence of data on cyanobacterial protein translocation, the cyanelles of the glaucocystophyte alga Cyanophora paradoxa for which in vitro systems for protein import and intraorganellar sorting were elaborated can serve as a model: the cyanelles are surrounded by a peptidoglycan wall, their thylakoids are covered with phycobilisomes and the composition of their oxygen-evolving complex is another feature shared with cyanobacteria. We demonstrate the operation of the Sec and Tat pathways in cyanelles and show for the first time in vitro protein import across cyanobacteria-like thylakoid membranes and protease protection of the mature protein.

Polytopic membrane protein folding and assembly in vitro and in vivo

The insertion and folding of proteins in biological membranes during protein synthesis in vivo is fundamental to membrane biogenesis. At present, however, certain molecular aspects of this process can only be understood by complementary studies in vitro. We bring together in vitro and in vivo results, highlighting how the studies inform each other and increase our knowledge of the folding and assembly of polytopic membrane proteins. A notable recent advance is the high-resolution crystal structure of the protein machinery responsible for membrane protein insertion into the endoplasmic reticulum. This provides an opportunity to combine in vitro and in vivo studies at a more sophisticated level and address mechanistic aspects of polytopic protein insertion and folding. Quality control is another important aspect of membrane biogenesis, and we give an overview of the current understanding of this process, focusing on cystic fibrosis as a well-studied paradigm. Mutations in the associated membrane protein, the cystic fibrosis transmembrane conductance regulator (CFTR), can cause the quality control mechanisms to prevent the mutant protein reaching its normal site of action, the cell surface. In vitro studies of CFTR shed light on the possible origins of other clinically relevant folding mutants and highlight the potential synergy between in vitro and in vivo approaches.

Ribophorin I associates with a subset of membrane proteins after their integration at the sec61 translocon

The biosynthesis of membrane proteins at the endoplasmic reticulum (ER) involves the integration of the polypeptide at the Sec61 translocon together with a number of maturation events, such as N-glycosylation and signal sequence cleavage, that can occur both during and after synthesis. To better understand the events occurring after the release of the nascent chain from the ER translocon, we investigated the ER components adjacent to the transmembrane-spanning domain of a well characterized fragment of the amyloid precursor protein. Using individual cysteine residues as site-specific cross-linking targets, we found that several ER components can be cross-linked to the fully integrated polypeptide. We identified strong adducts with both the ribophorin I subunit of the oligosaccharyltransferase complex and the 25-kDa subunit of the signal peptidase complex. Focusing on the association with ribophorin I, we found that adduct formation occurred exclusively after the exit of the nascent chain from the Sec61 translocon and was unaffected by the N-glycosylation status of the associated precursor. Only a subset of newly made membrane proteins associated with ribophorin I in vitro, and we could recapitulate a specific association between the amyloid precursor protein fragment and ribophorin I in vivo. Taken together, our data suggest a model where ribophorin I may function to retain potential substrates in close proximity to the catalytic subunit of the oligosaccharyltransferase and thereby stochastically improve the efficiency of the N-glycosylation reaction in vivo. Alternatively ribophorin I may be multifunctional and facilitate additional processes, for example, ER quality control.

Tail-anchored and signal-anchored proteins utilize overlapping pathways during membrane insertion

Tail-anchored proteins are a distinct class of membrane proteins that are characterized by a C-terminal membrane insertion sequence and a capacity for post-translational integration. Although it is now clear that tail-anchored proteins are inserted into the membrane at the endoplasmic reticulum (ER), the molecular basis for their integration is poorly understood. We have used a cross-linking approach to identify ER components that may be involved in the membrane insertion of tail-anchored proteins. We find that several newly synthesized tail-anchored proteins are transiently associated with a defined subset of cellular components. Among these, we identify several ER proteins, including subunits of the Sec61 translocon, Sec62p, Sec63p, and the 25-kDa subunit of the signal peptidase complex. When we analyze the cotranslational membrane insertion of a comparable signal-anchored protein we find the nascent polypeptide associated with a similar set of ER components. We conclude that the pathways for the integration of tail-anchored and signal-anchored membrane proteins at the ER exhibit a substantial degree of overlap, and we propose that this reflects similarities between co- and post-translational membrane insertion.

Sbh1p, a subunit of the Sec61 translocon, interacts with the chaperone calnexin in the yeast Yarrowia lipolytica

The core component of the translocation apparatus, Sec61p or alpha, was previously cloned in Yarrowia lipolytica. Using anti-Sec61p antibodies, we showed that most of the translocation sites are devoted to co-translational translocation in this yeast, which is similar to the situation in mammalian cells but in contrast to the situation in Saccharomyces cerevisiae, where post-translational translocation is predominant. In order to characterize further the minimal translocation apparatus in Y. lipolytica, the beta Sec61 complex subunit, Sbh1p, was cloned by functional complementation of a Deltasbh1, Deltasbh2 S. cerevisiae mutant. The secretion of the reporter protein is not impaired in the Y. lipolytica sbh1 inactivated strain. We screened the Y. lipolytica two-hybrid library to look for partners of this translocon component. The ER-membrane chaperone protein, calnexin, was identified as an interacting protein. By a co-immunoprecipitation approach, we confirmed this association in Yarrowia and then showed that the S. cerevisiae Sbh2p protein was a functional homologue of YlSbh1p. The interaction of Sbh1p with calnexin was shown to occur between the lumenal domain of both proteins. These results suggest that the beta subunit of the Sec61 translocon may relay folding of nascent proteins to their translocation.

Different transmembrane domains associate with distinct endoplasmic reticulum components during membrane integration of a polytopic protein

We have been studying the insertion of the seven transmembrane domain (TM) protein opsin to gain insights into how the multiple TMs of polytopic proteins are integrated at the endoplasmic reticulum (ER). We find that the ER components associated with the first and second TMs of the nascent opsin polypeptide chain are clearly distinct. The first TM (TM1) is adjacent to the alpha and beta subunits of the Sec61 complex, and a novel component, a protein associated with the ER translocon of 10 kDa (PAT-10). The most striking characteristic of PAT-10 is that it remains adjacent to TM1 throughout the biogenesis and membrane integration of the full-length opsin polypeptide. TM2 is also found to be adjacent to Sec61alpha and Sec61beta during its membrane integration. However, TM2 does not form any adducts with PAT-10; rather, a transient association with the TRAM protein is observed. We show that the association of PAT-10 with opsin TM1 does not require the N-glycosylation of the nascent chain and occurs irrespective of the amino acid sequence and transmembrane topology of TM1. We conclude that the precise makeup of the ER membrane insertion site can be distinct for the different transmembrane domains of a polytopic protein. We find that the environment of a particular TM can be influenced by both the "stage" of nascent chain biosynthesis reached, and the TM's relative location within the polypeptide.

Bimodal targeting of microsomal CYP2E1 to mitochondria through activation of an N-terminal chimeric signal by cAMP-mediated phosphorylation

Cytochrome P450 2E1 (CYP2E1) plays an important role in alcohol-induced toxicity and oxidative stress. Recently, we showed that this predominantly microsomal protein is also localized in rat hepatic mitochondria. In this report, we show that the N-terminal 30 amino acids of CYP2E1 contain a chimeric signal for bimodal targeting of the apoprotein to endoplasmic reticulum (ER) and mitochondria. We demonstrate that the cryptic mitochondrial targeting signal at sequence 21-31 of the protein is activated by cAMP-dependent phosphorylation at Ser-129. S129A mutation resulted in lower affinity for binding to cytoplasmic Hsp70, mitochondrial translocases (TOM40 and TIM44) and reduced mitochondrial import. S129A mutation, however, did not affect the extent of binding to the signal recognition particle and association with ER membrane translocator protein Sec61. Addition of saturating levels of signal recognition particle caused only a partial inhibition of CYP2E1 translation under in vitro conditions, and saturating levels of ER resulted only in partial membrane integration. cAMP enhanced the mitochondrial CYP2E1 (referred to as P450MT5) level but did not affect its level in the ER. Our results provide new insights on the mechanism of cAMP-mediated activation of a cryptic mitochondrial targeting signal and regulation of P450MT5 targeting to mitochondria.

Chloroplast quest: a journey from the cytosol into the chloroplast and beyond

Chloroplasts are characteristic organelles of plants and algae and the site of oxygenic photosynthesis. They are surrounded by a double membrane and possess an internal membrane system, the thylakoids, on which the photosynthetic machinery is located. They originated more than 1.2 billion years ago from an endosymbiotic event between an already photosynthetic ancestor of present day cyanobacteria and a mitochondriate host cell. During the transformation of the internalized cyanobacterium into a cell organelle most of the genetic information of the endosymbiot got lost or was transferred into the nucleus of the host. Chloroplast proteins encoded by nuclear genes are synthesized on cytoplasmic ribosomes and have to be relocated into the organelle. This is achieved by a proteinaceous import machinery in the outer and inner envelope of the chloroplasts. Proteins destined for the thylakoid membrane and the thylakoid lumen are further translocated by several different pathways into or across this membrane. The subject of this review is the quest of nuclear encoded chloroplast proteins into the organelle and to their final suborganellar location.

The amino terminus of opsin translocates "posttranslationally" as efficiently as cotranslationally

Opsin, a member of the G-protein-coupled receptor family, is a polytopic membrane protein that does not encode a cleaved amino-terminal signal sequence. The amino terminus of opsin precedes the first known targeting information, suggesting that it translocates across the endoplasmic reticulum (ER) membrane after synthesis, uncoupled from translation. However, translocation across the mammalian ER is believed to be coupled to protein synthesis. In this study we show that opsin, within a range of nascent peptide lengths, targets and translocates equally efficiently co- and posttranslationally. Longer nascent opsin peptides have a lower efficiency of cotranslational translocation but an even lower efficiency of posttranslational translocation. We also show that SRP is required for both co- and posttranslational targeting.

Assembly strategies and GTPase regulation of the eukaryotic and Escherichia coli translocons

The translocation of most proteins across the endoplasmic reticulum or bacterial inner membrane occurs through an aqueous pore that spans the membrane. Substrates that are translocated co-translationally across the membrane are directed to the translocation pore via an interaction between the cytosolic signal recognition particle and its membrane-bound receptor. Together the translocation pore and the receptor are referred to as a translocon. By studying the biogenesis of the translocon a number of alternate targeting and membrane-integration pathways have been discovered that operate independently of the signal recognition particle (SRP) pathway. The novel assembly strategies of the translocon and the ways in which these components interact to ensure the fidelity and unidirectionality of the targeting and translocation process are reviewed here.Key words: protein translocation, translocon, SRP receptor, GTPases.

Biogenesis of inner membrane proteins in Escherichia coli

For a long time, it was generally assumed that the biogenesis of inner membrane proteins in Escherichia coli occurs spontaneously, and that only the translocation of large periplasmic domains requires the aid of a protein machinery, the Sec translocon. However, evidence obtained in recent years indicates that most, if not all, inner membrane proteins require the assistance of protein factors to reach their native conformation in the membrane. Here, we review and discuss recent advances in our understanding of the biogenesis of inner membrane proteins in E. coli.

Evolutionarily related insertion pathways of bacterial, mitochondrial, and thylakoid membrane proteins

The inner membranes of eubacteria and mitochondria, as well as the chloroplast thylakoid membrane, contain essential proteins that function in oxidative phosphorylation and electron transport processes or in photosynthesis. Because most of the organellar proteins are nuclear encoded, they are synthesized in the cytoplasm and subsequently imported into the organelle before they are inserted into the membrane. This review focuses on the pathways of protein insertion into the inner membrane of eubacteria and mitochondria and into the chloroplast thylakoid membrane. In many respects, insertion of proteins into the inner membrane of bacteria is a process similar to that used by proteins of the thylakoid membrane. In both of these systems a signal recognition particle (SRP) and a SecYE-translocase are involved, as in translocation into the endoplasmic reticulum. The pathway of proteins into the mitochondrial membranes appears to be different in that it involves no SecYE-like components. A conservative pathway, recently identified in mitochondria, involves the Oxa1 protein for the insertion of proteins from the matrix. The presence of Oxa1 homologues in eubacteria and chloroplasts suggests that this pathway is evolutionarily conserved.

In vitro binding of ribosomes to the beta subunit of the Sec61p protein translocation complex

The Sec61p complex forms the core element of the protein translocation complex (translocon) in the rough endoplasmic reticulum (rough ER) membrane. Translating or nontranslating ribosomes bind with high affinity to ER membranes that have been stripped of ribosomes or to liposomes containing purified Sec61p. Here we present evidence that the beta subunit of the complex (Sec61beta) makes contact with nontranslating ribosomes. A fusion protein containing the Sec61beta cytoplasmic domain (Sec61beta(c)) prevents the binding of ribosomes to stripped ER-derived membranes and also binds to ribosomes directly with an affinity close to the affinity of ribosomes for stripped ER-derived membranes. The ribosome binding activity of Sec61beta(c), like that of native ER membranes, is sensitive to high salt concentrations and is not based on an unspecific charge-dependent interaction of the relatively basic Sec61beta(c) domain with ribosomal RNA. Like stripped ER membranes, the Sec61beta(c) sequence binds to large ribosomal subunits in preference over small subunits. Previous studies have shown that Sec61beta is inessential for ribosome binding and protein translocation, but translocation is impaired by the absence of Sec61beta, and it has been proposed that Sec61beta assists in the insertion of nascent proteins into the translocation pore. Our results suggest a physical interaction of the ribosome itself with Sec61beta; this may normally occur alongside interactions between the ribosome and other elements of Sec61p, or it may represent one stage in a temporal sequence of binding.

Co-translational interactions of apoprotein B with the ribosome and translocon during lipoprotein assembly or targeting to the proteasome

Hepatic lipoprotein assembly and secretion can be regulated by proteasomal degradation of newly synthesized apoB, especially if lipid synthesis or lipid transfer is low. Our previous studies in HepG2 cells showed that, under these conditions, newly synthesized apoB remains stably associated with the endoplasmic reticulum (ER) membrane (Mitchell, D. M., Zhou, M., Pariyarath, R., Wang, H., Aitchison, J. D., Ginsberg, H. N., and Fisher, E. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 14733-14738). We now show that independent of lipid synthesis, apoB chains that appear full-length are, in fact, incompletely translated polypeptides still engaged by the ribosome and associated with the ER translocon. In the presence of active lipid synthesis and transfer, translation and lipoprotein assembly are completed, and the complexes exit the ER. Upon omitting fatty acids from, or adding a microsomal triglyceride transfer protein inhibitor to, culture media to reduce lipid synthesis or transfer, respectively, apoB was degraded while it remained associated with the ER and complexed with cytosolic hsp70 and proteasomes. Thus, unlike other ER substrates of the proteasome, such as major histocompatibility complex class I molecules, apoB does not fully retrotranslocate to the cytosol before entering the ubiquitin-proteasome pathway. Although, upon immunofluorescence, apoB in proteasome-inhibited cells accumulated in punctate structures similar in appearance to aggresomes (cytosolic structures containing molecules irreversibly lost from the secretory pathway), these apoB molecules could be secreted when lipid synthesis was stimulated. The results suggest a model in which 1) apoB translation does not complete until lipoprotein assembly terminates, and 2) assembly with lipids or entry into the ubiquitin-proteasome pathway occurs while apoB polypeptides remain associated with the translocon and attached to the ribosome.

Identification of sequence determinants that direct different intracellular folding pathways for aquaporin-1 and aquaporin-4

Homologous aquaporin water channels utilize different folding pathways to acquire their transmembrane (TM) topology in the endoplasmic reticulum (ER). AQP4 acquires each of its six TM segments via cotranslational translocation events, whereas AQP1 is initially synthesized with four TM segments and subsequently converted into a six membrane-spanning topology. To identify sequence determinants responsible for these pathways, peptide segments from AQP1 and AQP4 were systematically exchanged. Chimeric proteins were then truncated, fused to a C-terminal translocation reporter, and topology was analyzed by protease accessibility. In each chimeric context, TM1 initiated ER targeting and translocation. However, AQP4-TM2 cotranslationally terminated translocation, while AQP1-TM2 failed to terminate translocation and passed into the ER lumen. This difference in stop transfer activity was due to two residues that altered both the length and hydrophobicity of TM2 (Asn(49) and Lys(51) in AQP1 versus Met(48) and Leu(50) in AQP4). A second peptide region was identified within the TM3-4 peptide loop that enabled AQP4-TM3 but not AQP1-TM3 to reinitiate translocation and cotranslationally span the membrane. Based on these findings, it was possible to convert AQP1 into a cotranslational biogenesis mode similar to that of AQP4 by substituting just two peptide regions at the N terminus of TM2 and the C terminus of TM3. Interestingly, each of these substitutions disrupted water channel activity. These data thus establish the structural basis for different AQP folding pathways and provide evidence that variations in cotranslational folding enable polytopic proteins to acquire and/or maintain primary sequence determinants necessary for function.

Integration of deletion mutants of bovine rhodopsin into the membrane of the endoplasmic reticulum

Newly synthesized eukaryotic membrane proteins must be integrated into the membrane of the endoplasmic reticulum with the correct topology to enable the subsequent acquisition of the correctly folded, functional conformation. Here, an analysis is presented of N-terminal glycosylation and steady-state membrane orientation of a series of truncation mutants of the seven-helix protein rhodopsin expressed in COS-1 cells. Mutants containing one, three, or five N-terminal transmembrane segments of rhodopsin, as well as mutants containing only the first transmembrane segment, but with hydrophilic extensions at the C-terminus were studied. The findings demonstrate that the C-terminal transmembrane segments play a crucial role in determining the final orientation of rhodopsin, and that the commitment to the correct orientation occurs only after the synthesis of at least three transmembrane segments. The experiments also suggest that the molecular machinery involved in the integration of a newly synthesized seven-helix membrane protein into the endoplasmic reticulum membrane is sensitive to the overall hydrophobicity of the sequence that follows the first transmembrane segment.

Role of lipids in the translocation of proteins across membranes

The architecture of cells, with various membrane-bound compartments and with the protein synthesizing machinery confined to one location, dictates that many proteins have to be transported through one or more membranes during their biogenesis. A lot of progress has been made on the identification of protein translocation machineries and their sorting signals in various organelles and organisms. Biochemical characterization has revealed the functions of several individual protein components. Interestingly, lipid components were also found to be essential for the correct functioning of these translocases. This led to the idea that there is a very intimate relationship between the lipid and protein components that enables them to fulfil their intriguing task of transporting large biopolymers through a lipid bilayer without leaking their contents. In this review we focus on the Sec translocases in the endoplasmic reticulum and the bacterial inner membrane. We also highlight the interactions of lipids and proteins during the process of translocation and integrate this into a model that enables us to understand the role of membrane lipid composition in translocase function.

YidC, the Escherichia coli homologue of mitochondrial Oxa1p, is a component of the Sec translocase

In Escherichia coli, both secretory and inner membrane proteins initially are targeted to the core SecYEG inner membrane translocase. Previous work has also identified the peripherally associated SecA protein as well as the SecD, SecF and YajC inner membrane proteins as components of the translocase. Here, we use a cross-linking approach to show that hydrophilic portions of a co-translationally targeted inner membrane protein (FtsQ) are close to SecA and SecY, suggesting that insertion takes place at the SecA/Y interface. The hydrophobic FtsQ signal anchor sequence contacts both lipids and a novel 60 kDa translocase-associated component that we identify as YidC. YidC is homologous to Saccharomyces cerevisiae Oxa1p, which has been shown to function in a novel export pathway at the mitochondrial inner membrane. We propose that YidC is involved in the insertion of hydrophobic sequences into the lipid bilayer after initial recognition by the SecAYEG translocase.

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.

The Sec61 complex is located in both the ER and the ER-Golgi intermediate compartment

The heteromeric Sec61 complex is composed of (alpha), beta and (gamma) subunits and forms the core of the mammalian ER translocon. Oligomers of the Sec61 complex form a transmembrane channel where proteins are translocated across and integrated into the ER membrane. We have studied the subcellular localisation of the Sec61 complex using both wild-type COS1 cells and cells transfected with GFP-tagged Sec61(alpha). By double labelling immunofluorescence microscopy the GFP-tagged Sec61(alpha) was found in both the ER and the ER-Golgi intermediate compartment (ERGIC) but not in the trans-Golgi network. Immunofluorescence studies of endogenous Sec61beta and Sec61(gamma) showed that these proteins are also located in both the ER and the ERGIC. Using the alternative strategy of subcellular fractionation, we have shown that wild-type Sec61(alpha), beta and (gamma), and GFP-tagged Sec61(alpha), are all present in both the ER and the ERGIC/Golgi fractions of the gradient. The presence of the Sec61 subunits in a post-ER compartment suggests that these proteins can escape the ER and be recycled back, despite the fact that none of them contain any known membrane protein retrieval signals such as cytosolic di-lysine or di-arginine motifs. We also found that another translocon component, the glycoprotein TRAM, was present in post-ER compartments as demonstrated by subcellular fractionation. Our data indicate that the core components of the mammalian ER translocon are not permanently resident in the ER, but rather that they are maintained in the ER by a specific retrieval mechanism.

Protein translocation into and across the bacterial plasma membrane and the plant thylakoid membrane

Over the past decade, some familiar themes have emerged on how proteins are inserted into or translocated across the plant chloroplast thylakoid membrane and bacterial inner membranes. In the SecA and signal recognition particle (SRP) pathways, nucleotides and soluble factors are used to translocate proteins across the membrane bilayer in the unfolded state. However, the delta pH-dependent pathway in thylakoids uses a radically different mechanism: transport of proteins across the membrane is driven by the transmembrane pH gradient, and neither stromal factors nor nucleotide triphosphates are needed. In addition, this pathway, which requires the membrane-bound protein Hcf106, appears to translocate proteins in a tightly folded form. Recently, a similar pathway has been shown to operate in eubacteria, and several of its components have been identified.

Old and new pathways of protein export in chloroplasts and bacteria

Targeting of chloroplast proteins to the thylakoid membrane is analogous to bacterial secretion, and much of what we know has been learned from secretory mechanisms in Escherichia coli. However, chloroplasts also use a delta pH-dependent pathway to target thylakoid proteins, at least some of which are folded before transport. Previously, this pathway seemed to have no cognate in bacteria, but recent results have shown that the HCF106 gene in maize encodes a component of this pathway and has bacterial homologues. This delta pH-dependent pathway might be an ancient conserved mechanism for protein translocation that evolved before the endosymbiotic origin of plastids and mitochondria.

The mechanism underlying cystic fibrosis transmembrane conductance regulator transport from the endoplasmic reticulum to the proteasome includes Sec61beta and a cytosolic, deglycosylated intermediary

Endoplasmic reticulum (ER) degradation pathways can selectively route proteins away from folding and maturation. Both soluble and integral membrane proteins can be targeted from the ER to proteasomal degradation in this fashion. The cystic fibrosis transmembrane conductance regulator (CFTR) is an integral, multidomain membrane protein localized to the apical surface of epithelial cells that functions to facilitate Cl- transport. CFTR was among the first membrane proteins for which a role of the proteasome in ER-related degradation was described. However, the signals that route CFTR to ubiquitination and subsequent degradation are not known. Moreover, limited information is available concerning the subcellular localization of polyubiquitinated CFTR or mechanisms underlying retrograde dislocation of CFTR from the ER membrane to the proteasome either before or after ubiquitination. In the present study, we show that proteasome inhibition with clasto-lactacystin beta-lactone (4 microM, 1 h) stabilizes the presence of a deglycosylated CFTR intermediate for up to 5 h without increasing the core glycosylated (band B) form of CFTR. Deglycosylated CFTR is present under the same conditions that result in accumulation of polyubiquitinated CFTR. Moreover, the deglycosylated form of both wild type and DeltaF508 CFTR can be found in the cytosolic fraction. Both the level and stability of cytosolic, deglycosylated CFTR are increased by proteasome blockade. During retrograde translocation from the ER to the cytosol, CFTR associates with the Sec61 trimeric complex. Sec61 is the key component of the mammalian co-translational protein translocation system and has been proposed to function as a two way channel that transports proteins both into the ER and back to the cytosol for degradation. We show that the level of the Sec61.CFTR complexes are highest when CFTR degradation proceeds at the greatest rate (approximately 90 min after pulse labeling). Quantities of Sec61.CFTR complexes are also increased by inhibition of the proteasome. Based on these results, we propose a model in which complex membrane proteins such as CFTR are transported through the Sec61 trimeric complex back to the cytosol, escorted by the beta subunit of Sec61, and degraded by the proteasome or by other proteolytic systems.

Coupled translocation events generate topological heterogeneity at the endoplasmic reticulum membrane

Topogenic determinants that direct protein topology at the endoplasmic reticulum membrane usually function with high fidelity to establish a uniform topological orientation for any given polypeptide. Here we show, however, that through the coupling of sequential translocation events, native topogenic determinants are capable of generating two alternate transmembrane structures at the endoplasmic reticulum membrane. Using defined chimeric and epitope-tagged full-length proteins, we found that topogenic activities of two C-trans (type II) signal anchor sequences, encoded within the seventh and eighth transmembrane (TM) segments of human P-glycoprotein were directly coupled by an inefficient stop transfer (ST) sequence (TM7b) contained within the C-terminus half of TM7. Remarkably, these activities enabled TM7 to achieve both a single- and a double-spanning TM topology with nearly equal efficiency. In addition, ST and C-trans signal anchor activities encoded by TM8 were tightly linked to the weak ST activity, and hence topological fate, of TM7b. This interaction enabled TM8 to span the membrane in either a type I or a type II orientation. Pleiotropic structural features contributing to this unusual topogenic behavior included 1) a short, flexible peptide loop connecting TM7a and TM7b, 2) hydrophobic residues within TM7b, and 3) hydrophilic residues between TM7b and TM8.

PROTEIN TARGETING TO THE THYLAKOID MEMBRANE

▪ Abstract  The assembly of the photosynthetic apparatus at the thylakoid begins with the targeting of proteins from their site of synthesis in the cytoplasm or stroma to the thylakoid membrane. Plastid-encoded proteins are targeted directly to the thylakoid during or after synthesis on plastid ribosomes. Nuclear-encoded proteins undergo a two-step targeting process requiring posttranslational import into the organelle from the cytoplasm and subsequent targeting to the thylakoid membrane. Recent investigations have revealed a single general import machinery at the envelope that mediates the direct transport of preproteins from the cytoplasm to the stroma. In contrast, at least four distinct pathways exist for the targeting of proteins to the thylakoid membrane. At least two of these systems are homologous to translocation systems that operate in bacteria and at the endoplasmic reticulum, indicating that elements of the targeting mechanisms have been conserved from the original prokaryotic endosymbiont.

The Escherichia coli SRP and SecB targeting pathways converge at the translocon

Two distinct protein targeting pathways can direct proteins to the Escherichia coli inner membrane. The Sec pathway involves the cytosolic chaperone SecB that binds to the mature region of pre-proteins. SecB targets the pre-protein to SecA that mediates pre-protein translocation through the SecYEG translocon. The SRP pathway is probably used primarily for the targeting and assembly of inner membrane proteins. It involves the signal recognition particle (SRP) that interacts with the hydrophobic targeting signal of nascent proteins. By using a protein cross-linking approach, we demonstrate here that the SRP pathway delivers nascent inner membrane proteins at the membrane. The SRP receptor FtsY, GTP and inner membranes are required for release of the nascent proteins from the SRP. Upon release of the SRP at the membrane, the targeted nascent proteins insert into a translocon that contains at least SecA, SecY and SecG. Hence, as appears to be the case for several other translocation systems, multiple targeting mechanisms deliver a variety of precursor proteins to a common membrane translocation complex of the E.coli inner membrane.

Membrane integration of Sec61alpha: a core component of the endoplasmic reticulum translocation complex

The Sec61 complex is a central component of the endoplasmic reticulum (ER) translocation site. The complex consists of three subunits: Sec61alpha, Sec61beta and Sec61gamma, at least two of which (alpha and beta) are adjacent to nascent proteins during membrane insertion. Another component of the translocation machinery is the translocating chain-associating membrane (TRAM) protein, which is also adjacent to many nascent proteins during membrane insertion. Sec61alpha functions as the major component of a transmembrane channel formed by oligomers of the Sec61 complex. This channel is the site of secretory protein translocation and membrane protein integration at the ER membrane. Sec61alpha is a polytopic integral membrane protein, and we have studied its biosynthesis and membrane integration in vitro. Using a cross-linking approach to analyse the environment of a series of discrete Sec61alpha membrane-integration intermediates, we find: (i) newly synthesized Sec61alpha is adjacent to known components of the ER membrane-insertion site, namely Sec61alpha, Sec61beta and TRAM, and thus the integration of Sec61alpha appears to require a pre-existing Sec61 complex; (ii) a site-specific cross-linking analysis indicates that the first transmembrane domain of Sec61alpha remains adjacent to protein components of the ER-insertion site (specifically TRAM and Sec61beta) during the insertion of at least three subsequent transmembrane domains; and (iii) the membrane integration of Sec61alpha requires ER targeting by the signal-recognition particle.

The role of the ribosome-translocon complex in translation and assembly of polytopic membrane proteins

Newly synthesized polytopic membrane proteins and secretory proteins often share the same target membrane as their primary destination, and in some cases, the cellular machinery that targets and transfers them into or across the membrane. Unlike secretory proteins, which are localized to the external compartment, each polytopic membrane protein molecule must be partitioned among the cytoplasm, the membrane and the external milieu. How does the ribosome-translocon complex cope with the different domains of polytopic membrane proteins?