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SignificanceAside from its undisputed role in the import of newly synthesized proteins into the endoplasmic reticulum (ER), the Sec61 translocon was proposed to ensure the reverse transport of misfolded proteins to the cytosol. Based on this model, Sec61 was also proposed to be the channel exporting internalized antigens from endosomes to the cytosol, for degradation and cross-presentation. Establishing Sec61’s contribution to these connected trafficking pathways has nevertheless proven difficult, due to a technical incapacity to blunt its activity acutely. Here, we took advantage of a recently identified Sec61 blocker to determine whether or not Sec61 can mediate retrograde protein transport. Both ER-to-cytosol and endosome-to-cytosol protein export were intact in mycolactone-treated cells, which argues against Sec61 operating as a retrotranslocon. Although antigen cross-presentation in dendritic cells (DCs) is critical to the initiation of most cytotoxic immune responses, the intracellular mechanisms and traffic pathways involved are still unclear. One of the most critical steps in this process, the export of internalized antigen to the cytosol, has been suggested to be mediated by Sec61. Sec61 is the channel that translocates signal peptide-bearing nascent polypeptides into the endoplasmic reticulum (ER), and it was also proposed to mediate protein retrotranslocation during ER-associated degradation (a process called ERAD). Here, we used a newly identified Sec61 blocker, mycolactone, to analyze Sec61’s contribution to antigen cross-presentation, ERAD, and transport of internalized antigens into the cytosol. As shown previously in other cell types, mycolactone prevented protein import into the ER of DCs. Mycolactone-mediated Sec61 blockade also potently suppressed both antigen cross-presentation and direct presentation of synthetic peptides to CD8+ T cells. In contrast, it did not affect protein export from the ER lumen or from endosomes into the cytosol, suggesting that the inhibition of cross-presentation was not related to either of these trafficking pathways. Proteomic profiling of mycolactone-exposed DCs showed that expression of mediators of antigen presentation, including MHC class I and β2 microglobulin, were highly susceptible to mycolactone treatment, indicating that Sec61 blockade affects antigen cross-presentation indirectly. Together, our data suggest that the defective translocation and subsequent degradation of Sec61 substrates is the cause of altered antigen cross-presentation in Sec61-blocked DCs.

Self-complementing split fluorescent proteins (FPs) have been widely used for protein labeling, visualization of subcellular protein localization, and detection of cell–cell contact. To expand this toolset, we have developed a screening strategy for the direct engineering of self-complementing split FPs. Via this strategy, we have generated a yellow–green split-mNeonGreen2 1–10/11 that improves the ratio of complemented signal to the background of FP 1–10 -expressing cells compared to the commonly used split GFP 1–10/11 ; as well as a 10-fold brighter red-colored split-sfCherry2 1–10/11 . Based on split sfCherry2, we have engineered a photoactivatable variant that enables single-molecule localization-based super-resolution microscopy. We have demonstrated dual-color endogenous protein tagging with sfCherry2 11 and GFP 11 , revealing that endoplasmic reticulum translocon complex Sec61B has reduced abundance in certain peripheral tubules. These new split FPs not only offer multiple colors for imaging interaction networks of endogenous proteins, but also hold the potential to provide orthogonal handles for biochemical isolation of native protein complexes. Split fluorescent proteins (FPs) have been widely used to visualise proteins in cells. Here the authors develop a screen for engineering new split FPs, and report a yellow-green split-mNeonGreen2 with reduced background, a red split-sfCherry2 for multicolour labeling, and its photoactivatable variant for super-resolution use.

The Sec61 complex forms a protein-conducting channel in the endoplasmic reticulum membrane that is required for secretion of soluble proteins and production of many membrane proteins. Several natural and synthetic small molecules specifically inhibit Sec61, generating cellular effects that are useful for therapeutic purposes, but their inhibitory mechanisms remain unclear. Here we present near-atomic-resolution structures of human Sec61 inhibited by a comprehensive panel of structurally distinct small molecules—cotransin, decatransin, apratoxin, ipomoeassin, mycolactone, cyclotriazadisulfonamide and eeyarestatin. All inhibitors bind to a common lipid-exposed pocket formed by the partially open lateral gate and plug domain of Sec61. Mutations conferring resistance to the inhibitors are clustered at this binding pocket. The structures indicate that Sec61 inhibitors stabilize the plug domain in a closed state, thereby preventing the protein-translocation pore from opening. Our study provides the atomic details of Sec61–inhibitor interactions and the structural framework for further pharmacological studies and drug design. Itskanov and Wang et al. determined high-resolution structures of the human Sec61 channel inhibited by several structurally distinct small molecules and revealed the common inhibitor-binding site in Sec61 and molecular interactions in atomic detail.

During the biogenesis of most eukaryotic integral membrane proteins (IMPs), transmembrane domains are inserted into the endoplasmic reticulum membrane by a dedicated insertase or the SEC61 translocon. The SRP-independent (SND) pathway is the least understood route into the membrane, despite catering for a broad range of IMP types. Here, we show that Chaetomium thermophilum SND3 is a membrane insertase with an atypical fold. We further present a cryo-electron microscopy structure of a ribosome-associated SND3 translocon complex involved in co-translational IMP insertion. The structure reveals that the SND3 translocon additionally comprises the complete SEC61 translocon, CCDC47 and TRAPɑ. Here, the SEC61β N-terminus works together with CCDC47 to prevent substrate access to the translocon. Instead, molecular dynamics simulations show that SND3 disrupts the lipid bilayer to promote IMP insertion via its membrane-embedded hydrophilic groove. Structural and sequence comparisons indicate that the SND3 translocon is a distinct multipass translocon in fungi, euglenozoan parasites and other eukaryotic taxa. The fungal SND pathway inserts a wide range of proteins into the ER membrane. Here, SND3 is identified as a membrane insertase within a distinct SEC61 translocon complex, implying a role in co-translational insertion of multipass membrane proteins.

Many proteins must translocate through the protein-conducting Sec61 channel in the eukaryotic endoplasmic reticulum membrane or the SecY channel in the prokaryotic plasma membrane 1 , 2 . Proteins with highly hydrophobic signal sequences are first recognized by the signal recognition particle (SRP) 3 , 4 and then moved co-translationally through the Sec61 or SecY channel by the associated translating ribosome. Substrates with less hydrophobic signal sequences bypass the SRP and are moved through the channel post-translationally 5 , 6 . In eukaryotic cells, post-translational translocation is mediated by the association of the Sec61 channel with another membrane protein complex, the Sec62–Sec63 complex 7 – 9 , and substrates are moved through the channel by the luminal BiP ATPase 9 . How the Sec62–Sec63 complex activates the Sec61 channel for post-translational translocation is not known. Here we report the electron cryo-microscopy structure of the Sec complex from Saccharomyces cerevisiae , consisting of the Sec61 channel and the Sec62, Sec63, Sec71 and Sec72 proteins. Sec63 causes wide opening of the lateral gate of the Sec61 channel, priming it for the passage of low-hydrophobicity signal sequences into the lipid phase, without displacing the channel’s plug domain. Lateral channel opening is triggered by Sec63 interacting both with cytosolic loops in the C-terminal half of Sec61 and transmembrane segments in the N-terminal half of the Sec61 channel. The cytosolic Brl domain of Sec63 blocks ribosome binding to the channel and recruits Sec71 and Sec72, positioning them for the capture of polypeptides associated with cytosolic Hsp70 10 . Our structure shows how the Sec61 channel is activated for post-translational protein translocation. The cryo-EM structure of the post-translational protein translocation machinery of the endoplasmic reticulum membrane shows that Sec63 opens the channel, enabling insertion of low-hydrophobicity signal sequences into the lipid phase.

Preventing the biogenesis of disease-relevant proteins is an attractive therapeutic strategy, but attempts to target essential protein biogenesis factors have been hampered by excessive toxicity. Here we describe KZR-8445, a cyclic depsipeptide that targets the Sec61 translocon and selectively disrupts secretory and membrane protein biogenesis in a signal peptide-dependent manner. KZR-8445 potently inhibits the secretion of pro-inflammatory cytokines in primary immune cells and is highly efficacious in a mouse model of rheumatoid arthritis. A cryogenic electron microscopy structure reveals that KZR-8445 occupies the fully opened Se61 lateral gate and blocks access to the lumenal plug domain. KZR-8445 binding stabilizes the lateral gate helices in a manner that traps select signal peptides in the Sec61 channel and prevents their movement into the lipid bilayer. Our results establish a framework for the structure-guided discovery of novel therapeutics that selectively modulate Sec61-mediated protein biogenesis. A selective inhibitor of Sec61 blocks protein entry into the secretory pathway and has therapeutic efficacy in rheumatoid arthritis. A cryo-EM structure of the inhibited Sec61 provides a model for client-selective protein translocation inhibition.

Stalled ribosomes at the endoplasmic reticulum (ER) are covalently modified with the ubiquitin-like protein UFM1 on the 60S ribosomal subunit protein RPL26 (also known as uL24) 1 , 2 . This modification, which is known as UFMylation, is orchestrated by the UFM1 ribosome E3 ligase (UREL) complex, comprising UFL1, UFBP1 and CDK5RAP3 (ref. 3 ). However, the catalytic mechanism of UREL and the functional consequences of UFMylation are unclear. Here we present cryo-electron microscopy structures of UREL bound to 60S ribosomes, revealing the basis of its substrate specificity. UREL wraps around the 60S subunit to form a C-shaped clamp architecture that blocks the tRNA-binding sites at one end, and the peptide exit tunnel at the other. A UFL1 loop inserts into and remodels the peptidyl transferase centre. These features of UREL suggest a crucial function for UFMylation in the release and recycling of stalled or terminated ribosomes from the ER membrane. In the absence of functional UREL, 60S–SEC61 translocon complexes accumulate at the ER membrane, demonstrating that UFMylation is necessary for releasing SEC61 from 60S subunits. Notably, this release is facilitated by a functional switch of UREL from a ‘writer’ to a ‘reader’ module that recognizes its product—UFMylated 60S ribosomes. Collectively, we identify a fundamental role for UREL in dissociating 60S subunits from the SEC61 translocon and the basis for UFMylation in regulating protein homeostasis at the ER. Attachment of the ubiquitin-like modifier UFM1 to 60S ribosomes has a critical function in the release and recycling of stalled or terminated ribosomes from the endoplasmic reticulum membrane.

α-Helical integral membrane proteins comprise approximately 25% of the proteome in all organisms. The membrane proteome is highly diverse, varying in the number, topology, spacing and properties of transmembrane domains. This diversity imposes different constraints on the insertion of different regions of a membrane protein into the lipid bilayer. Here, we present a cohesive framework to explain membrane protein biogenesis, in which different parts of a nascent substrate are triaged between Oxa1 and SecY family members for insertion. In this model, Oxa1 family proteins insert transmembrane domains flanked by short translocated segments, whereas the SecY channel is required for insertion of transmembrane domains flanked by long translocated segments. Our unifying model rationalizes evolutionary, genetic, biochemical and structural data across organisms and provides a foundation for future mechanistic studies of membrane protein biogenesis. In this Perspective, the authors propose a framework to explain membrane protein biogenesis, wherein different parts of a nascent substrate are triaged between Oxa1 and SecY family members for insertion.

The essential process of protein secretion is achieved by the ubiquitous Sec machinery. In prokaryotes, the drive for translocation comes from ATP hydrolysis by the cytosolic motor-protein SecA, in concert with the proton motive force (PMF). However, the mechanism through which ATP hydrolysis by SecA is coupled to directional movement through SecYEG is unclear. Here, we combine all-atom molecular dynamics (MD) simulations with single molecule FRET and biochemical assays. We show that ATP binding by SecA causes opening of the SecY-channel at long range, while substrates at the SecY-channel entrance feed back to regulate nucleotide exchange by SecA. This two-way communication suggests a new, unifying 'Brownian ratchet' mechanism, whereby ATP binding and hydrolysis bias the direction of polypeptide diffusion. The model represents a solution to the problem of transporting inherently variable substrates such as polypeptides, and may underlie mechanisms of other motors that translocate proteins and nucleic acids. A protective membrane surrounds all cells, and controls what goes in and out of the cell. Many proteins that are made inside the cell need to be exported in order to do their job. In most organisms, a specialised transport motor that sits inside the membrane, known as 'Sec', carries out this export process. Sec recognises proteins that need to be exported and pushes them across the membrane and out of the cell. The energy required to do this comes from the cell's universal power source, a molecule called ATP. Previous studies have shown what Sec looks like, but not how it pushes proteins from one side of the membrane to the other. Currently, the most popular theory for how Sec works is that it grabs hold of part of the protein and pushes it through a gate in the membrane. It then lets go and goes back to grab and push the next bit of the protein. Allen, Corey, Oatley et al. have now used a combination of experimental and computational methods to look at how the different parts of Sec move around as it uses ATP. The reasoning behind using these methods was that it's easier to understand how a motor works by watching it in action rather than just looking at a still picture. Using these methods, Allen, Corey, Oatley et al. show that the biggest movement in Sec as it uses ATP is in the membrane gate itself, which opens and closes. This suggests that Sec acts like a turnstile: proteins can freely move one way across the membrane, but are prevented from moving back in again. This mechanism has not been described before and may apply to other transport systems. Further investigations will be needed to understand exactly how Sec recognises cargo and starts the transport process, and to explore the specific features of a protein that activate the turnstile. It also remains to be discovered how this transport process differs in other, non-bacterial cells. This could potentially help us develop new drugs that specifically block the bacterial Sec system without affecting human cells.

The SEC61 translocon complex has emerged as a multifunctional therapeutic target linking protein secretion, calcium homeostasis, and immune regulation in kidney transplantation. Beyond canonical protein translocation, SEC61 regulates antigen cross-presentation, cytokine secretion (IL-2, IFN-γ, TNF-α), surface activation molecules (CD62L), and functions as an endoplasmic reticulum calcium-leak channel that modulates unfolded protein response activation under specific physiological and stress conditions. During ischemia–reperfusion injury, ATP depletion impairs SERCA-mediated calcium reuptake while SEC61-mediated calcium efflux persists, triggering ER stress and tubular injury. Selective pharmacological SEC61 inhibition has been proposed to confer multiple immunomodulatory and cytoprotective effects - including reduced antigen cross-presentation, suppression of high-burden secretory lymphocytes, limited T cell migration, and intrinsic antiviral activity through blockade of envelope glycoprotein biogenesis—based on mechanistic and preclinical evidence, although these effects remain to be validated in transplant-specific models. Emerging phase I oncology data with client-selective inhibitors demonstrate the feasibility of pharmacologic SEC61 modulation in humans, although the safety, dosing, and patient population in transplantation may differ substantially from oncology settings. This review examines SEC61’s multifaceted roles in transplant immunobiology and its therapeutic potential as a novel immunomodulatory target in kidney transplantation.