A structural perspective on tail-anchored protein biogenesis by the GET pathway

Combination of hydrophobicity and codon usage bias determines sorting of model K+ channel protein to either mitochondria or endoplasmic reticulum

When the K+ channel-like protein Kesv from Ectocarpus siliculosus virus 1 is heterologously expressed in mammalian cells, it is sorted to the mitochondria. This targeting can be redirected to the endoplasmic reticulum (ER) by altering the codon usage in distinct regions of the gene or by inserting a triplet of hydrophobic amino acids (AAs) into the protein's C-terminal transmembrane domain (ct-TMD). Systematic variations in the flavor of the inserted AAs and/or its codon usage show that a positive charge in the inserted AA triplet alone serves as strong signal for mitochondria sorting. In cases of neutral AA triplets, mitochondria sorting are favored by a combination of hydrophilic AAs and rarely used codons; sorting to the ER exhibits the inverse dependency. This propensity for ER sorting is particularly high when a common codon follows a rarer one in the AA triplet; mitochondria sorting in contrast is supported by codon uniformity. Since parameters like positive charge, hydrophobic AAs, and common codons are known to facilitate elongation of nascent proteins in the ribosome the data suggest a mechanism in which local changes in elongation velocity and co-translational folding in the ct-TMD influence intracellular protein sorting.

Identification of Bacillus subtilis YidC substrates using a MifM-instructed translation arrest-based reporter

YidC is a member of the YidC/Oxa1/Alb3 protein family that is crucial for membrane protein biogenesis in the bacterial plasma membrane. While YidC facilitates the folding and complex assembly of membrane proteins along with the Sec translocon, it also functions as a Sec-independent membrane protein insertase in the YidC-only pathway. However, little is known about how membrane proteins are recognized and sorted by these pathways, especially in Gram-positive bacteria, for which only a small number of YidC substrates have been identified to date. In this study, we aimed to identify Bacillus subtilis membrane proteins whose membrane insertion depends on SpoIIIJ, the primary YidC homolog in B. subtilis. We took advantage of the translation arrest sequence of MifM, which can monitor YidC-dependent membrane insertion. Our systematic screening identified eight membrane proteins as candidate SpoIIIJ substrates. Results of our genetic study also suggest that the conserved arginine in the hydrophilic groove of SpoIIIJ is crucial for the membrane insertion of the substrates identified here. However, in contrast to MifM, a previously identified YidC substrate, the importance of the negatively charged residue on the substrates for membrane insertion varied depending on the substrate. These results suggest that B. subtilis YidC uses substrate-specific interactions to facilitate membrane insertion.

The Get1/2 insertase forms a channel to mediate the insertion of tail-anchored proteins into the ER

Tail-anchored (TA) proteins contain a single C-terminal transmembrane domain (TMD) that is captured by the cytosolic Get3 in yeast (TRC40 in humans). Get3 delivers TA proteins to the Get1/2 complex for insertion into the endoplasmic reticulum (ER) membrane. How Get1/2 mediates insertion of TMDs of TA proteins into the membrane is poorly understood. Using bulk fluorescence and microfluidics assays, we show that Get1/2 forms an aqueous channel in reconstituted bilayers. We estimate the channel diameter to be ∼2.5 nm wide, corresponding to the circumference of two Get1/2 complexes. We find that the Get3 binding can seal the Get1/2 channel, which dynamically opens and closes. Our mutation analysis further shows that the Get1/2 channel activity is required to release TA proteins from Get3 for insertion into the membrane. Hence, we propose that the Get1/2 channel functions as an insertase for insertion of TMDs and as a translocase for translocation of C-terminal hydrophilic segments.

Cryo-EM insights into tail-anchored membrane protein biogenesis in eukaryotes

Tail-anchored (TA) proteins are a biologically significant class of membrane proteins, which require specialised cellular pathways to insert their single C-terminal transmembrane domain into the correct membrane. Cryo-electron microscopy has recently provided new insights into the organelle-specific machineries for TA protein biogenesis. Structures of targeting and insertase complexes within the canonical guided entry of TA proteins (GET) pathway indicate how substrates are faithfully chaperoned into the endoplasmic reticulum (ER) membrane in metazoans. The core of the GET insertase is conserved within structures of the ER membrane protein complex (EMC), which acts in parallel to insert a different subset of TA proteins. Furthermore, structures of the dislocases Spf1 and Msp1 show how they remove mislocalised TA proteins from the ER and outer mitochondrial membranes respectively.

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

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

Deciphering the molecular organization of GET pathway chaperones through native mass spectrometry

Get3/4/5 chaperone complex is responsible for targeting C-terminal tail-anchored membrane proteins to the endoplasmic reticulum. Despite the availability of several crystal structures of independent proteins and partial structures of subcomplexes, different models of oligomeric states and structural organization have been proposed for the protein complexes involved. Here, using native mass spectrometry (Native-MS), coupled with intact dissociation, we show that Get4/5 exclusively forms a tetramer using both Get5/5 and a novel Get4/4 dimerization interface. Addition of Get3 to this leads to a hexameric (Get3)2-(Get4)2-(Get5)2 complex with closed-ring cyclic architecture. We further validate our claims through molecular modeling and mutational abrogation of the proposed interfaces. Native-MS has become a principal tool to determine the state of oligomeric organization of proteins. The work demonstrates that for multiprotein complexes, native-MS, coupled with molecular modeling and mutational perturbation, can provide an alternative route to render a detailed view of both the oligomeric states as well as the molecular interfaces involved. This is especially useful for large multiprotein complexes with large unstructured domains that make it recalcitrant to conventional structure determination approaches.

Targeting of Proteins for Translocation at the Endoplasmic Reticulum

The endoplasmic reticulum represents the gateway to the secretory pathway. Here, proteins destined for secretion, as well as soluble and membrane proteins that reside in the endomembrane system and plasma membrane, are triaged from proteins that will remain in the cytosol or be targeted to other cellular organelles. This process requires the faithful recognition of specific targeting signals and subsequent delivery mechanisms to then target them to the translocases present at the ER membrane, which can either translocate them into the ER lumen or insert them into the lipid bilayer. This review focuses on the current understanding of the first step in this process representing the targeting phase. Targeting is typically mediated by cleavable N-terminal hydrophobic signal sequences or internal membrane anchor sequences; these can either be captured co-translationally at the ribosome or recognised post-translationally and then delivered to the ER translocases. Location and features of the targeting sequence dictate which of several overlapping targeting pathway substrates will be used. Mutations in the targeting machinery or targeting signals can be linked to diseases.

Protein Targeting Into the Thylakoid Membrane Through Different Pathways

In higher plants, chloroplasts are essential semi-autonomous organelles with complex compartments. As part of these sub-organellar compartments, the sheet-like thylakoid membranes contain abundant light-absorbing chlorophylls bound to the light-harvesting proteins and to some of the reaction center proteins. About half of the thylakoid membrane proteins are encoded by nuclear genes and synthesized in the cytosol as precursors before being imported into the chloroplast. After translocation across the chloroplast envelope by the Toc/Tic system, these proteins are subsequently inserted into or translocated across the thylakoid membranes through distinct pathways. The other half of thylakoid proteins are encoded by the chloroplast genome, synthesized in the stroma and integrated into the thylakoid through a cotranslational process. Much progress has been made in identification and functional characterization of new factors involved in protein targeting into the thylakoids, and new insights into this process have been gained. In this review, we introduce the distinct transport systems mediating the translocation of substrate proteins from chloroplast stroma to the thylakoid membrane, and present the recent advances in the identification of novel components mediating these pathways. Finally, we raise some unanswered questions involved in the targeting of chloroplast proteins into the thylakoid membrane, along with perspectives for future research.

Cryo-EM structures of the endoplasmic reticulum membrane complex

The transmembrane α-helices of membrane proteins are in general highly hydrophobic, and they enter the lipid bilayer through a lateral gate in the Sec61 translocon. However, some transmembrane α-helices are less hydrophobic and form membrane channels or substrate-binding pockets. Insertion of these amphipathic transmembrane α-helices into the membrane requires the specific membrane-embedded insertase called the endoplasmic reticulum membrane complex (EMC), which is a multi-subunit chaperone distinct from the GET insertase complex. Four recent cryo-electron microscopy studies on the eukaryotic EMC have revealed their remarkable architectural conservation from yeast to humans; a general consensus on the substrate transmembrane helix-binding pocket; and the evolutionary link to the prokaryotic insertases of the tail-anchored membrane proteins. These structures provide a solid framework for future mechanistic investigation.

The Molecular Biodiversity of Protein Targeting and Protein Transport Related to the Endoplasmic Reticulum

Looking at the variety of the thousands of different polypeptides that have been focused on in the research on the endoplasmic reticulum from the last five decades taught us one humble lesson: no one size fits all. Cells use an impressive array of components to enable the safe transport of protein cargo from the cytosolic ribosomes to the endoplasmic reticulum. Safety during the transit is warranted by the interplay of cytosolic chaperones, membrane receptors, and protein translocases that together form functional networks and serve as protein targeting and translocation routes. While two targeting routes to the endoplasmic reticulum, SRP (signal recognition particle) and GET (guided entry of tail-anchored proteins), prefer targeting determinants at the N- and C-terminus of the cargo polypeptide, respectively, the recently discovered SND (SRP-independent) route seems to preferentially cater for cargos with non-generic targeting signals that are less hydrophobic or more distant from the termini. With an emphasis on targeting routes and protein translocases, we will discuss those functional networks that drive efficient protein topogenesis and shed light on their redundant and dynamic nature in health and disease.

Folding and Insertion of Transmembrane Helices at the ER

In eukaryotic cells, the endoplasmic reticulum (ER) is the entry point for newly synthesized proteins that are subsequently distributed to organelles of the endomembrane system. Some of these proteins are completely translocated into the lumen of the ER while others integrate stretches of amino acids into the greasy 30 Å wide interior of the ER membrane bilayer. It is generally accepted that to exist in this non-aqueous environment the majority of membrane integrated amino acids are primarily non-polar/hydrophobic and adopt an α-helical conformation. These stretches are typically around 20 amino acids long and are known as transmembrane (TM) helices. In this review, we will consider how transmembrane helices achieve membrane integration. We will address questions such as: Where do the stretches of amino acids fold into a helical conformation? What is/are the route/routes that these stretches take from synthesis at the ribosome to integration through the ER translocon? How do these stretches ‘know’ to integrate and in which orientation? How do marginally hydrophobic stretches of amino acids integrate and survive as transmembrane helices?

Inhibitors of the Sec61 Complex and Novel High Throughput Screening Strategies to Target the Protein Translocation Pathway

Proteins targeted to the secretory pathway start their intracellular journey by being transported across biological membranes such as the endoplasmic reticulum (ER). A central component in this protein translocation process across the ER is the Sec61 translocon complex, which is only intracellularly expressed and does not have any enzymatic activity. In addition, Sec61 translocon complexes are difficult to purify and to reconstitute. Screening for small molecule inhibitors impairing its function has thus been notoriously difficult. However, such translocation inhibitors may not only be valuable tools for cell biology, but may also represent novel anticancer drugs, given that cancer cells heavily depend on efficient protein translocation into the ER to support their fast growth. In this review, different inhibitors of protein translocation will be discussed, and their specific mode of action will be compared. In addition, recently published screening strategies for small molecule inhibitors targeting the whole SRP-Sec61 targeting/translocation pathway will be summarized. Of note, slightly modified assays may be used in the future to screen for substances affecting SecYEG, the bacterial ortholog of the Sec61 complex, in order to identify novel antibiotic drugs.

A homolog of GuidedEntry of Tail-anchored proteins3 functions in membrane-specific protein targeting in chloroplasts of Arabidopsis

The chloroplasts and mitochondria of photosynthetic eukaryotes contain proteins that are closely related to cytosolic Guided Entry of Tail-anchored proteins3 (Get3). Get3 is a targeting factor that efficiently escorts tail-anchored (TA) proteins to the ER. Because other components of the cytosolic-targeting pathway appear to be absent in organelles, previous investigators have asserted that organellar Get3 homologs are unlikely to act as targeting factors. However, we show here both that the Arabidopsis thaliana chloroplast homolog designated as GET3B is structurally similar to cytosolic Get3 proteins and that it selectively binds a thylakoid-localized TA protein. Based on genetic interactions between a get3b mutation and mutations affecting the chloroplast signal recognition particle-targeting pathway, as well as changes in the abundance of photosynthesis-related proteins in mutant plants, we propose that GET3B acts primarily to direct proteins to the thylakoids. Furthermore, through molecular complementation experiments, we show that function of GET3B depends on its ability to hydrolyze ATP, and this is consistent with action as a targeting factor. We propose that GET3B and related organellar Get3 homologs play a role that is analogous to that of cytosolic Get3 proteins, and that GET3B acts as a targeting factor in the chloroplast stroma to deliver TA proteins in a membrane-specific manner.

Capture and delivery of tail-anchored proteins to the endoplasmic reticulum

Tail-anchored (TA) proteins fulfill diverse cellular functions within different organellar membranes. Their characteristic C-terminal transmembrane segment renders TA proteins inherently prone to aggregation and necessitates their posttranslational targeting. The guided entry of TA proteins (GET in yeast)/transmembrane recognition complex (TRC in humans) pathway represents a major route for TA proteins to the endoplasmic reticulum (ER). Here, we review important new insights into the capture of nascent TA proteins at the ribosome by the GET pathway pretargeting complex and the mechanism of their delivery into the ER membrane by the GET receptor insertase. Interestingly, several alternative routes by which TA proteins can be targeted to the ER have emerged, raising intriguing questions about how selectivity is achieved during TA protein capture. Furthermore, mistargeting of TA proteins is a fundamental cellular problem, and we discuss the recently discovered quality control machineries in the ER and outer mitochondrial membrane for displacing mislocalized TA proteins.

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

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

Ribosome-bound Get4/5 facilitates the capture of tail-anchored proteins by Sgt2 in yeast

The guided entry of tail-anchored proteins (GET) pathway assists in the posttranslational delivery of tail-anchored proteins, containing a single C-terminal transmembrane domain, to the ER. Here we uncover how the yeast GET pathway component Get4/5 facilitates capture of tail-anchored proteins by Sgt2, which interacts with tail-anchors and hands them over to the targeting component Get3. Get4/5 binds directly and with high affinity to ribosomes, positions Sgt2 close to the ribosomal tunnel exit, and facilitates the capture of tail-anchored proteins by Sgt2. The contact sites of Get4/5 on the ribosome overlap with those of SRP, the factor mediating cotranslational ER-targeting. Exposure of internal transmembrane domains at the tunnel exit induces high-affinity ribosome binding of SRP, which in turn prevents ribosome binding of Get4/5. In this way, the position of a transmembrane domain within nascent ER-targeted proteins mediates partitioning into either the GET or SRP pathway directly at the ribosomal tunnel exit.

The guided entry of tail-anchored proteins (GET) pathway assists in the delivery of such proteins to the ER. Here, the authors reveal that the pathway components Get4/5 probe a region near the ribosomal exit tunnel. Upon emergence of a client protein, Get4/5 recruits Sgt2 and initiates the targeting phase of the pathway.

Highlighting membrane protein structure and function: A celebration of the Protein Data Bank

Biological membranes define the boundaries of cells and compartmentalize the chemical and physical processes required for life. Many biological processes are carried out by proteins embedded in or associated with such membranes. Determination of membrane protein (MP) structures at atomic or near-atomic resolution plays a vital role in elucidating their structural and functional impact in biology. This endeavor has determined 1198 unique MP structures as of early 2021. The value of these structures is expanded greatly by deposition of their three-dimensional (3D) coordinates into the Protein Data Bank (PDB) after the first atomic MP structure was elucidated in 1985. Since then, free access to MP structures facilitates broader and deeper understanding of MPs, which provides crucial new insights into their biological functions. Here we highlight the structural and functional biology of representative MPs and landmarks in the evolution of new technologies, with insights into key developments influenced by the PDB in magnifying their impact.

Structural and mechanistic basis of the EMC-dependent biogenesis of distinct transmembrane clients

Membrane protein biogenesis in the endoplasmic reticulum (ER) is complex and failure-prone. The ER membrane protein complex (EMC), comprising eight conserved subunits, has emerged as a central player in this process. Yet, we have limited understanding of how EMC enables insertion and integrity of diverse clients, from tail-anchored to polytopic transmembrane proteins. Here, yeast and human EMC cryo-EM structures reveal conserved intricate assemblies and human-specific features associated with pathologies. Structure-based functional studies distinguish between two separable EMC activities, as an insertase regulating tail-anchored protein levels and a broader role in polytopic membrane protein biogenesis. These depend on mechanistically coupled yet spatially distinct regions including two lipid-accessible membrane cavities which confer client-specific regulation, and a non-insertase EMC function mediated by the EMC lumenal domain. Our studies illuminate the structural and mechanistic basis of EMC's multifunctionality and point to its role in differentially regulating the biogenesis of distinct client protein classes.

Generation of Arabidopsis lines with a red fluorescent marker for endoplasmic reticulum using a tail-anchored protein cytochrome b5 -B

The endoplasmic reticulum (ER) is a multifunctional organelle that performs multiple cellular activities in eukaryotes. Visualizing ER using fluorescent proteins is a powerful method of analyzing its dynamics and to understand its functions. However, red fluorescent proteins with both an N-terminal signal peptide (SP) and a C-terminal ER retention tetrapeptide (HDEL) often cause mislocalization to vacuoles or extracellular spaces when they are constitutively expressed in Arabidopsis. To obtain a red fluorescent ER marker, we selected Arabidopsis cytochrome b5 -B (Cb5-B), a tail-anchored (TA) protein on the ER membrane. Its localization is determined by the transmembrane domain (TMD) and tail domain at the C-terminus. We fused the TMD and the tail domain of Cb5-B to the C-terminus of a red fluorescent protein, tdTomato (tdTomato-CTT). When tdTomato-CTT was constitutively expressed under the ubiquitin10 promoter in Arabidopsis, the fluorescent signal was exclusively detected at the ER by means of the reliable ER marker SP-GFP-HDEL. Therefore, tdTomato-CTT can accurately visualize the ER in stable Arabidopsis lines. Additionally, transient assays showed that tdTomato-CTT can also be used as an ER marker in onion, rice, and Nicotiana benthamiana. We believe that TA proteins could be used to generate various organellar membrane markers in plants.

The natural history of Get3-like chaperones

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

Prototypic SNARE Proteins Are Encoded in the Genomes of Heimdallarchaeota, Potentially Bridging the Gap between the Prokaryotes and Eukaryotes

A defining feature of eukaryotic cells is the presence of numerous membrane-bound organelles that subdivide the intracellular space into distinct compartments. How the eukaryotic cell acquired its internal complexity is still poorly understood. Material exchange among most organelles occurs via vesicles that bud off from a source and specifically fuse with a target compartment. Central players in the vesicle fusion process are the soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. These small tail-anchored (TA) membrane proteins zipper into elongated four-helix bundles that pull membranes together. SNARE proteins are highly conserved among eukaryotes but are thought to be absent in prokaryotes. Here, we identified SNARE-like factors in the genomes of uncultured organisms of Asgard archaea of the Heimdallarchaeota clade, which are thought to be the closest living relatives of eukaryotes. Biochemical experiments show that the archaeal SNARE-like proteins can interact with eukaryotic SNARE proteins. We did not detect SNAREs in α-proteobacteria, the closest relatives of mitochondria, but identified several genes encoding for SNARE proteins in γ-proteobacteria of the order Legionellales, pathogens that live inside eukaryotic cells. Very probably, their SNAREs stem from lateral gene transfer from eukaryotes. Together, this suggests that the diverse set of eukaryotic SNAREs evolved from an archaeal precursor. However, whether Heimdallarchaeota actually have a simplified endomembrane system will only be seen when we succeed studying these organisms under the microscope.

The Ways of Tails: the GET Pathway and more

Due to their topology tail-anchored (TA) proteins must target to the membrane independently of the co-translational route defined by the signal sequence recognition particle (SRP), its receptor and the translocon Sec61. More than a decade of work has extensively characterized a highly conserved pathway, the yeast GET or mammalian TRC40 pathway, which is capable of countering the biogenetic challenge posed by the C-terminal TA anchor. In this review we briefly summarize current models of this targeting route and focus on emerging aspects such as the intricate interplay with the proteostatic network of cells and with other targeting pathways. Importantly, we consider the lessons provided by the in vivo analysis of the pathway in different model organisms and by the consideration of its full client spectrum in more recent studies. This analysis of the state of the field highlights directions in which the current models may be experimentally probed and conceptually extended.

Synthetic Protein Scaffolding at Biological Membranes

Protein scaffolding is a natural phenomenon whereby proteins colocalize into macromolecular complexes via specific protein-protein interactions. In the case of metabolic enzymes, protein scaffolding drives metabolic flux through specific pathways by colocalizing enzyme active sites. Synthetic protein scaffolding is increasingly used as a mechanism to improve product specificity and yields in metabolic engineering projects. To date, synthetic scaffolding has focused primarily on soluble enzyme systems, but many metabolic pathways for high-value secondary metabolites depend on membrane-bound enzymes. The compositional diversity of biological membranes and general challenges associated with modifying membrane proteins complicate scaffolding with membrane-requiring enzymes. Several recent studies have introduced new approaches to protein scaffolding at membrane surfaces, with notable success in improving product yields from specific metabolic pathways.

The big BAM theory: An open and closed case?

The β-barrel assembly machinery (BAM) is responsible for the biogenesis of outer membrane proteins (OMPs) into the outer membranes of Gram-negative bacteria. These OMPs have a membrane-embedded domain consisting of a β-barrel fold which can vary from 8 to 36 β-strands, with each serving a diverse role in the cell such as nutrient uptake and virulence. BAM was first identified nearly two decades ago, but only recently has the molecular structure of the full complex been reported. Together with many years of functional characterization, we have a significantly clearer depiction of BAM's structure, the intra-complex interactions, conformational changes that BAM may undergo during OMP biogenesis, and the role chaperones may play. But still, despite advances over the past two decades, the mechanism for BAM-mediated OMP biogenesis remains elusive. Over the years, several theories have been proposed that have varying degrees of support from the literature, but none has of yet been conclusive enough to be widely accepted as the sole mechanism. We will present a brief history of BAM, the recent work on the structures of BAM, and a critical analysis of the current theories for how it may function.

The WRB Subunit of the Get3 Receptor is Required for the Correct Integration of its Partner CAML into the ER

Calcium-modulating cyclophilin ligand (CAML), together with Tryptophan rich basic protein (WRB, Get1 in yeast), constitutes the mammalian receptor for the Transmembrane Recognition Complex subunit of 40 kDa (TRC40, Get3 in yeast), a cytosolic ATPase with a central role in the post-translational targeting pathway of tail-anchored (TA) proteins to the endoplasmic reticulum (ER) membrane. CAML has also been implicated in other cell-specific processes, notably in immune cell survival, and has been found in molar excess over WRB in different cell types. Notwithstanding the stoichiometric imbalance, WRB and CAML depend strictly on each other for expression. Here, we investigated the mechanism by which WRB impacts CAML levels. We demonstrate that CAML, generated in the presence of sufficient WRB levels, is inserted into the ER membrane with three transmembrane segments (TMs) in its C-terminal region. By contrast, without sufficient levels of WRB, CAML fails to adopt this topology, and is instead incompletely integrated to generate two aberrant topoforms; these congregate in ER-associated clusters and are degraded by the proteasome. Our results suggest that WRB, a member of the recently proposed Oxa1 superfamily, acts catalytically to assist the topogenesis of CAML and may have wider functions in membrane biogenesis than previously appreciated.

Sorting out how Msp1 maintains mitochondrial membrane proteostasis

Robust membrane proteostasis networks are essential for cells to withstand proteotoxic stress arising from environmental insult and intrinsic errors in protein production (Labbadia and Morimoto, 2015; Hegde and Zavodszky, 2019). Failures in mitochondrial membrane proteostasis are associated with cancer, aging, and a range of cardiovascular and neurodegenerative diseases (Wallace et al., 2010; Martin, 2012; Gustafsson and Gottlieb, 2007). As a result, mitochondria possess numerous pathways to maintain proteostasis (Avci and Lemberg, 2015; Shi et al., 2016; Weidberg and Amon, 2018; Shpilka and Haynes, 2018; Quirós et al., 2016; Sorrentino et al., 2017). Mitochondrial Sorting of Proteins 1 (Msp1) is a membrane anchored AAA ATPase that extracts proteins from the outer mitochondrial membrane (OMM) (Chen et al., 2014; Okreglak and Walter, 2014). In the past few years, several papers have addressed various aspects of Msp1 function. Here, we summarize these recent advances to build a basic model for how Msp1 maintains mitochondrial membrane proteostasis while also highlighting outstanding questions in the field.

Folding and Misfolding of Human Membrane Proteins in Health and Disease: From Single Molecules to Cellular Proteostasis

Advances over the past 25 years have revealed much about how the structural properties of membranes and associated proteins are linked to the thermodynamics and kinetics of membrane protein (MP) folding. At the same time biochemical progress has outlined how cellular proteostasis networks mediate MP folding and manage misfolding in the cell. When combined with results from genomic sequencing, these studies have established paradigms for how MP folding and misfolding are linked to the molecular etiologies of a variety of diseases. This emerging framework has paved the way for the development of a new class of small molecule "pharmacological chaperones" that bind to and stabilize misfolded MP variants, some of which are now in clinical use. In this review, we comprehensively outline current perspectives on the folding and misfolding of integral MPs as well as the mechanisms of cellular MP quality control. Based on these perspectives, we highlight new opportunities for innovations that bridge our molecular understanding of the energetics of MP folding with the nuanced complexity of biological systems. Given the many linkages between MP misfolding and human disease, we also examine some of the exciting opportunities to leverage these advances to address emerging challenges in the development of therapeutics and precision medicine.