Membrane protein chaperones: a new twist in the tail?

Mechanism of NACHO-mediated assembly of pentameric ligand-gated ion channels

Pentameric ligand-gated ion channels (pLGICs) are cell surface receptors of crucial importance for animal physiology1-4. This diverse protein family mediates the ionotropic signals triggered by major neurotransmitters and includes γ-aminobutyric acid receptors (GABAARs) and acetylcholine receptors (nAChRs). Receptor function is fine-tuned by a myriad of endogenous and pharmacological modulators3. A functional pLGIC is built from five homologous, sometimes identical, subunits, each containing a β-scaffold extracellular domain (ECD), a four-helix transmembrane domain (TMD) and intracellular loops of variable length. Although considerable progress has been made in understanding pLGICs in structural and functional terms, the molecular mechanisms that enable their assembly at the endoplasmic reticulum (ER)5 in a vast range of potential subunit configurations6 remain unknown. Here, we identified candidate pLGICs assembly factors selectively associated with an unassembled GABAAR subunit. Focusing on one of the candidates, we determined the cryo-EM structure of an assembly intermediate containing two α1 subunits of GABAAR each bound to an ER-resident membrane protein NACHO. The structure showed how NACHO shields the principal (+) transmembrane interface of α1 subunits containing an immature extracellular conformation. Crosslinking and structure-prediction revealed an adjacent surface on NACHO for β2 subunit interactions to guide stepwise oligimerisation. Mutations of either subunit-interacting surface on NACHO also impaired the formation of homopentameric α7 nAChRs, pointing to a generic framework for pLGIC assembly. Our work provides the foundation for understanding the regulatory principles underlying pLGIC structural diversity.

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

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

Mutation of wrb, a Component of the Guided Entry of Tail-Anchored Protein Pathway, Disrupts Photoreceptor Synapse Structure and Function

PURPOSE

Tail-anchored (TA) proteins contain a single hydrophobic domain at the C-terminus and are posttranslationally inserted into the ER membrane via the GET (guided entry of tail-anchored proteins) pathway. The role of the GET pathway in photoreceptors is unexplored. The goal of this study was to characterize the zebrafish pinball wizard mutant, which disrupts Wrb, a core component of the GET pathway.

METHODS

Electroretinography, optokinetic response measurements (OKR), immunohistochemistry, and electron microscopy analyses were employed to assess ribbon synapse function, protein expression, and ultrastructure in 5-day-old zebrafish larvae. Expression of wrb was investigated with real-time qRT-PCR and in situ hybridization.

RESULTS

Mutation of wrb abolished the OKR and greatly diminished the ERG b-wave, but not the a-wave. Ribeye and SV2 were partially mislocalized in both photoreceptors and hair cells of wrb mutants. Fewer contacts were seen between photoreceptors and bipolar cells in wrb-/- mutants. Expression of wrb was observed throughout the nervous system and Wrb localized to the ER and synaptic region of photoreceptors. Morpholino knockdown of the cytosolic ATPase trc40, which targets TA proteins to the ER, also diminished the OKR. Overexpression of wrb fully restored contrast sensitivity in mutants, while overexpression of mutant wrbR73A, which cannot bind Trc40, did not.

CONCLUSIONS

Proteins Wrb and Trc40 are required for synaptic transmission between photoreceptors and bipolar cells, indicating that TA protein insertion by the TRC pathway is a critical step in ribbon synapse assembly and function.

Protein targeting and transport as a necessary consequence of increased cellular complexity

With increasing intracellular complexity, a new cell-biological problem that is the allocation of cytoplasmically synthesized proteins to their final destinations within the cell emerged. A special challenge is thereby the translocation of proteins into or across cellular membranes. The underlying mechanisms are only in parts well understood, but it can be assumed that the course of cellular evolution had a deep impact on the design of the required molecular machines. In this article, we aim to summarize the current knowledge and concepts of the evolutionary development of protein trafficking as a necessary premise and consequence of increased cellular complexity.

Regulation of chaperone effects on a yeast prion by cochaperone Sgt2

Yeast prions, based on self-seeded highly ordered fibrous aggregates (amyloids), serve as a model for human amyloid diseases. Propagation of yeast prions depends on the balance between chaperones of the Hsp100 and Hsp70 families. The yeast prion [PSI(+)] can be eliminated by an excess of the chaperone Hsp104. This effect is reversed by an excess of the chaperone Hsp70-Ssa. Here we show that the actions of Hsp104 and Ssa on [PSI(+)] are modulated by the small glutamine-rich tetratricopeptide cochaperone Sgt2. Sgt2 is conserved from yeast to humans, has previously been implicated in the guided entry of tail-anchored proteins (GET) trafficking pathway, and is known to interact with Hsps, cytosolic Get proteins, and tail-anchored proteins. We demonstrate that Sgt2 increases the ability of excess Ssa to counteract [PSI(+)] curing by excess Hsp104. Deletion of SGT2 also restores trafficking of a tail-anchored protein in cells with a disrupted GET pathway. One region of Sgt2 interacts both with the prion domain of Sup35 and with tail-anchored proteins. Sgt2 levels are increased in response to the presence of a prion when major Hsps are not induced. Our data implicate Sgt2 as an amyloid "sensor" and a regulator of chaperone targeting to different types of aggregation-prone proteins.

EBV protein BNLF2a exploits host tail-anchored protein integration machinery to inhibit TAP

EBV, the prototypic human γ(1)-herpesvirus, persists for life in infected individuals, despite the presence of vigorous antiviral immunity. CTLs play an important role in the protection against viral infections, which they detect through recognition of virus-encoded peptides presented in the context of HLA class I molecules at the cell surface. The viral peptides are generated in the cytosol and are transported into the endoplasmic reticulum (ER) by TAP. The EBV-encoded lytic-phase protein BNLF2a acts as a powerful inhibitor of TAP. Consequently, loading of antigenic peptides onto HLA class I molecules is hampered, and recognition of BNLF2a-expressing cells by cytotoxic T cells is avoided. In this study, we characterize BNLF2a as a tail-anchored (TA) protein and elucidate its mode of action. Its hydrophilic N-terminal domain is located in the cytosol, whereas its hydrophobic C-terminal domain is inserted into membranes posttranslationally. TAP has no role in membrane insertion of BNLF2a. Instead, Asna1 (also named TRC40), a cellular protein involved in posttranslational membrane insertion of TA proteins, is responsible for integration of BNLF2a into the ER membrane. Asna1 is thereby required for efficient BNLF2a-mediated HLA class I downregulation. To optimally accomplish immune evasion, BNLF2a is composed of two specialized domains: its C-terminal tail anchor ensures membrane integration and ER retention, whereas its cytosolic N terminus accomplishes inhibition of TAP function. These results illustrate how EBV exploits a cellular pathway for TA protein biogenesis to achieve immune evasion, and they highlight the exquisite adaptation of this virus to its host.

Targeting proteins to the plant nuclear envelope

The nuclear envelope and the nuclear pore are important structures that both separate and selectively connect the nucleoplasm and the cytoplasm. The requirements for specific targeting of proteins to the plant nuclear envelope and nuclear pore are poorly understood. How are transmembrane-domain proteins sorted to the nuclear envelope and nuclear pore membranes? What protein–protein interactions are involved in associating other proteins to the nuclear pore? Are there plant-specific aspects to these processes? We are using the case of the nuclear pore-associated Ran-cycle component RanGAP (Ran GTPase-activating protein) to address these fundamental questions. Plant RanGAP is targeted to the nuclear pore by a plant-specific mechanism involving two families of nuclear pore-associated proteins [WIP (WPP-domain-interacting protein) and WIT (WPP-domain-interacting tail-anchored protein)] not found outside the land plant lineage. One protein family (WIP or WIT) is sufficient for RanGAP targeting in differentiated root cells, whereas both families are necessary in meristematic cells. A C-terminal predicted transmembrane domain is sufficient for targeting WIP proteins to the nuclear envelope. Nuclear-envelope targeting of WIT proteins requires a coiled-coil domain and is facilitated by HSC70 (heat-shock cognate 70 stress protein) chaperones and a class of plant-specific proteins resembling the RanGAP-targeting domain (WPP proteins). Taken together, this sheds the first light on the requirements and interdependences of nuclear envelope and nuclear pore targeting in land plants.

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

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

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

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

Preliminary X-ray crystallographic studies of yeast Get3

Tail-anchored (TA) proteins contain a single transmembrane domain (TMD) at the C-terminus. The post-translational insertion of TA proteins into the ER membrane requires the cooperation of the Golgi ER-trafficking (GET) complex, which contains Get1, Get2 and Get3. Get3 is a cytosolic ATPase which can recognize and bind the TMD of the TA proteins. Get1 and Get2 are ER transmembrane proteins which can recruit and form a complex with TA-bound Get3. The GET complex carries out an energy-dependent process that facilitates the insertion of the TA-protein TMD into the ER membrane. In order to investigate the mechanism by which the GET complex functions to promote protein insertion into the ER membrane, yeast Get3 has been crystallized. The crystals diffracted to 2.7 A resolution using a synchrotron X-ray source. The crystals belonged to space group P2(1)2(1)2, with unit-cell parameters a = 220.26, b = 112.95, c = 48.27 A. There is one Get3 dimer in the asymmetric unit, which corresponds to a solvent content of approximately 65%.

Delivering proteins for export from the cytosol

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

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

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

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

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

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

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