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Many cellular processes are regulated by ubiquitin-mediated proteasomal degradation. Bacterial pathogens can regulate eukaryotic proteolysis through the delivery of proteins with de-ubiquitinating (DUB) activities. The obligate intracellular pathogen Chlamydia trachomatis secretes Cdu1 (ChlaDUB1), a dual deubiquitinase and Lys-acetyltransferase, that promotes Golgi remodeling and survival of infected host cells presumably by regulating the ubiquitination of host and bacterial proteins. Here we determined that Cdu1’s acetylase but not its DUB activity is important to protect Cdu1 from ubiquitin-mediated degradation. We further identified three C. trachomatis proteins on the pathogen-containing vacuole (InaC, IpaM, and CTL0480) that required Cdu1‘s acetylase activity for protection from degradation and determined that Cdu1 and these Cdu1-protected proteins are required for optimal egress of Chlamydia from host cells. These findings highlight a non-canonical mechanism of pathogen-mediated protection of virulence factors from degradation after their delivery into host cells and the coordinated regulation of secreted effector proteins.

During host cell invasion,Shigellaescapes to the cytosol and polymerizes actin for cell-to-cell spread. To restrict cell-to-cell spread, host cells employ cell-autonomous immune responses including antibacterial autophagy and septin cage entrapment. How septins interact with autophagy to targetShigellato destruction is poorly understood. Here, we employed a correlative light and cryo-soft X-ray tomography (cryo-SXT) pipeline to studyShigellaseptin cage entrapment in its near native state. Quantitative cryo-SXT showed thatShigellafragments mitochondria and enabled visualization of X-ray dense structures (∼30 nm resolution) surroundingShigellaentrapped in septin cages. Using Airyscan confocal microscopy, we observed Lysine 63 (K63)-linked ubiquitin chains decorating septin caged entrappedShigella. Remarkably, septins and K63 chains form separate bacterial microdomains, indicating they are recruited separately during antibacterial autophagy. Cryo-SXT and live cell imaging revealed an interaction between septins and LC3B-positive membranes during autophagy ofShigella. Together, these findings demonstrate how septin cagedShigellaare targeted to autophagy and provide fundamental insights into autophagy-cytoskeleton interactions.

Ester-linked ubiquitination of serine or threonine residues - or even lipids - has emerged as a new regulatory earmark in cell signalling. Petrova et al. (2021) now reveal that ubiquitin esterification by the atypical ubiquitin ligase HOIL-1, a component of the LUBAC complex, is critical for proper formation of linear ubiquitin chains and control of immune signalling in T cells and macrophages. Surprisingly, ester-linked ubiquitination can either promote or inhibit linear ubiquitin conjugation and cytokine production depending on the receptor and immune cell engaged. Comment on: https://doi.org/10.1111/febs.15896.

The versatility of ubiquitination to impose control over vast domains of eukaryotic biology is due, in part, to diversification through differently-linked poly-ubiquitin chains. Deciphering the signaling roles for some poly-ubiquitin chain types, including those linked via K6, has been stymied by a lack of stringent linkage specificity among the implicated regulatory proteins. Forged through strong evolutionary pressures, pathogenic bacteria have evolved intricate mechanisms to regulate host ubiquitin, and in some cases even with exquisite specificity for distinct poly-ubiquitin signals. Herein, we identify and characterize a deubiquitinase domain of the secreted effector protein LotA from Legionella pneumophila that specifically regulates K6-linked poly-ubiquitin during infection. We demonstrate the utility of LotA as a tool for studying K6 poly-ubiquitin. By determining apo and diUb-bound structures, we identify the mechanism of LotA activation and K6 poly-ubiquitin specificity, and identify a novel ubiquitin-binding domain utilized among bacterial deubiquitinases.

Ubiquitylation, a wide-spread post-translational protein modification in eukaryotes, marks cytosol‐invading bacteria as cargo for anti-bacterial autophagy.(1–3) The identity of the ubiquitylated substrate on bacteria has remained unknown. Here we show that the ubiquitin coat on cytosol-invading Salmonella is formed through the unprecedented ubiquitylation of a non-proteinaceous substrate, the lipid A moiety of bacterial lipopolysaccharide (LPS), by the E3 ubiquitin ligase RNF213. RNF213 is a risk factor for Moyamoya disease (MMD)(4,5), a progressive stenosis of the supraclinoid internal carotid artery that causes stroke, especially in children.(6,7) RNF213 restricts the proliferation of cytosolic Salmonella and is essential for the generation of the bacterial ubiquitin coat, both directly, through ubiquitylation of LPS, and indirectly, through recruitment of LUBAC, a downstream E3 ligase that adds M1-linked ubiquitin chains onto pre-existing ubiquitin coats.(8) In cells lacking RNF213 bacteria do not attract ubiquitin-dependent autophagy cargo receptors and fail to induce anti-bacterial autophagy. The ubiquitylation of LPS on cytosol-invading Salmonella requires the dynein-like core of RNF213 but not its RING domain. Instead, LPS ubiquitylation relies on an RZ finger in the E3 shell. We conclude that ubiquitylation extends beyond protein substrates, that LPS ubiquitylation triggers cell-autonomous immunity and we postulate that non- proteinaceous substances other than LPS may also become ubiquitylated.

A new study reveals that, in addition to its longstanding role in recruiting proteins to the proteasome, ubiquitination can also induce a structural destabilization that allows the target protein to be efficiently unraveled for degradation.

Changes in the cellular environment modulate protein energy landscapes to drive important biology, with consequences for signaling, allostery, and other vital processes. The effects of ubiquitination are particularly important because of their potential influence on degradation by the 26S proteasome. Moreover, proteasomal engagement requires unstructured initiation regions that many known proteasome substrates lack. To assess the energetic effects of ubiquitination and how these manifest at the proteasome, we developed a generalizable strategy to produce isopeptide-linked ubiquitin within structured regions of a protein. The effects on the energy landscape vary from negligible to dramatic, depending on the protein and site of ubiquitination. Ubiquitination at sensitive sites destabilizes the native structure and increases the rate of proteasomal degradation. Importantly, in well-folded proteins, ubiquitination can even induce the requisite unstructured regions needed for proteasomal engagement. Our results indicate a biophysical role of site-specific ubiquitination as a potential regulatory mechanism for energy-dependent substrate degradation.

The tumor-suppressor protein BRCA1 works with BARD1 to catalyze the transfer of ubiquitin onto protein substrates. The N-terminal regions of BRCA1 and BARD1 that contain their RING domains are responsible for dimerization and ubiquitin ligase activity. This activity is a common feature among hundreds of human RING domain-containing proteins. RING domains bind and activate E2 ubiquitin-conjugating enzymes to promote ubiquitin transfer to substrates. We show that the identity of residues at specific positions in the RING domain can tune activity levels up or down. We report substitutions that create a structurally intact BRCA1/BARD1 heterodimer that is inactive in vitro with all E2 enzymes. Other substitutions in BRCA1 or BARD1 RING domains result in hyperactivity, revealing that both proteins have evolved attenuated activity. Loss of attenuation results in decreased product specificity, providing a rationale for why nature has tuned BRCA1 activity. The ability to tune BRCA1 provides powerful tools for understanding its biological functions and provides a basis to assess mechanisms for rescuing the activity of cancer-associated variations. Beyond the applicability to BRCA1, we show the identity of residues at tuning positions that can be used to predict and modulate the activity of an unrelated RING E3 ligase. These findings provide valuable insights into understanding the mechanism and function of RING E3 ligases like BRCA1.

Summary The Salmonella enterica effector SteD depletes mature MHC class II (mMHCII) molecules from the surface of infected antigen-presenting cells through ubiquitination of the cytoplasmic tail of the mMHCII β chain. Here, through a genome-wide mutant screen of human antigen-presenting cells, we show that the NEDD4 family HECT E3 ubiquitin ligase WWP2 and a tumor-suppressing transmembrane protein of unknown biochemical function, TMEM127, are required for SteD-dependent ubiquitination of mMHCII. Although evidently not involved in normal regulation of mMHCII, TMEM127 was essential for SteD to suppress both mMHCII antigen presentation in mouse dendritic cells and MHCII-dependent CD4+ T cell activation. We found that TMEM127 contains a canonical PPxY motif, which was required for binding to WWP2. SteD bound to TMEM127 and enabled TMEM127 to interact with and induce ubiquitination of mature MHCII. Furthermore, SteD also underwent TMEM127- and WWP2-dependent ubiquitination, which both contributed to its degradation and augmented its activity on mMHCII., Graphical Abstract, Highlights • TMEM127 and WWP2 are required for MHCII depletion by Salmonella SteD • Tumor suppressor TMEM127 is an adaptor for oncoprotein E3 ligase WWP2 • SteD interacts with TMEM127 to induce ubiquitination and degradation of mature MHCII • SteD also undergoes TMEM127- and WWP2-dependent ubiquitination, Salmonella inhibits the adaptive immune response by reducing cell surface levels of mature MHC class II. Alix et al. reveal that the Salmonella effector SteD co-opts TMEM127, a tumor suppressor protein of previously unknown function. TMEM127 binds the E3 ubiquitin ligase WWP2 enabling ubiquitination and degradation of MHCII and SteD.

Protein ubiquitination is a highly orchestrated process that controls diverse aspects of human biology. Dysregulation of this process can lead to various disease states including cancer, neurodegeneration, and autoimmunity. It is the correction of these dysregulated pathways, as well as the enticing ability to manipulate protein stability, that have instigated intense research into the therapeutic control of protein ubiquitination. A major bottleneck in the development and validation of small molecule modulators is the availability of a suitable high-throughput assay for enzyme activity. Herein, we present a new assay, which we term UbiReal, that uses fluorescence polarization to monitor all stages of Ub conjugation and deconjugation in real time. We use the assay to validate a chemical inhibitor of the E1 ubiquitin-activating enzyme, as well as to assess the activities and specificities of E2s, E3s, and deubiquitinases. The sensitivity and accessibility of this approach make it an excellent candidate for high-throughput screens of activity modulators, as well as a valuable tool for basic research into the mechanisms of ubiquitin regulation.

The post-translational modification of proteins with ubiquitin is a dynamic multifaceted process affecting all aspects of eukaryotic cellular biology. The complexity of ubiquitin modifications arises from their ability to form architecturally distinct polyubiquitin chains1-3. Despite our understanding of the importance of these signals, we currently lack tools and methods to study them. Here we describe an approach, termed Ub-clipping, which provides unprecedented insight into ubiquitin chain architecture. This protocol is related to our recent Nature paper titled, “Insights into ubiquitin chain architecture using Ub-clipping”. This technology takes advantage of an engineered viral protease, Lbpro*, which ‘clips’ ubiquitin such that the information on the site of modification is retained and the remaining ubiquitin and substrate polypeptides are kept intact. The goal of this protocol is to allow researchers to efficiently adapt our new technology to their proteomic workflows. We anticipate this method will continue to shed light on the architecture of ubiquitin signals, and therefore further our understanding of the ubiquitin code across a broad spectrum of biological systems.

Ubiquitin signaling requires tight control of all aspects of protein ubiquitination, including the timing, locale, extent, and type of modification. Dysregulation of any of these signaling features can lead to severe human disease. One key mode of regulation is through the controlled removal of the ubiquitin signal by dedicated families of proteases, termed deubiquitinases. In light of their key roles in signal regulation, deubiquitinases have become a recent focus for therapeutic intervention as a means to regulate protein abundance. This work and recent discoveries of novel deubiquitinases in humans, viruses, and bacteria, provide the impetus for this chapter on methods for evaluating the activities and structures of deubiquitinases. An array of available deubiquitinase substrates for biochemical characterization are presented and their limitations as standalone tools are discussed. Methods for the determination and analysis of deubiquitinase structure are also presented, with a focus on visualizing recognition of the ubiquitin substrate.

Summary Pathogenic bacteria rely on secreted effector proteins to manipulate host signaling pathways, often in creative ways. CE clan proteases, specific hydrolases for ubiquitin-like modifications (SUMO and NEDD8) in eukaryotes, reportedly serve as bacterial effector proteins with deSUMOylase, deubiquitinase, or, even, acetyltransferase activities. Here, we characterize bacterial CE protease activities, revealing K63-linkage-specific deubiquitinases in human pathogens, such as Salmonella, Escherichia, and Shigella, as well as ubiquitin/ubiquitin-like cross-reactive enzymes in Chlamydia, Rickettsia, and Xanthomonas. Five crystal structures, including ubiquitin/ubiquitin-like complexes, explain substrate specificities and redefine relationships across the CE clan. Importantly, this work identifies novel family members and provides key discoveries among previously reported effectors, such as the unexpected deubiquitinase activity in Xanthomonas XopD, contributed by an unstructured ubiquitin binding region. Furthermore, accessory domains regulate properties such as subcellular localization, as exemplified by a ubiquitin-binding domain in Salmonella Typhimurium SseL. Our work both highlights and explains the functional adaptations observed among diverse CE clan proteins., Graphical Abstract, Highlights • Bacterial CE proteases exhibit distinct ubiquitin/ubiquitin-like specificities • Substrate specificity is acquired through variability in three common regions • Structural and functional data redefine CE clan relationships across kingdoms • CE effectors are fitted with accessory domains that modulate function, Focusing on examples from pathogenic bacteria, Pruneda et al. examine a family of proteases that displays remarkably distinct specificities toward ubiquitin and ubiquitin-like modifications. Leveraging structural and functional data, the authors derive mechanisms through which substrate specificity is achieved and redefine relationships within the enzyme family across kingdoms of life.

Deubiquitinating enzymes (DUBs) are proteases that fulfill crucial roles in the ubiquitin (Ub) system, by deconjugation of Ub from its targets and disassembly of polyUb chains. The specificity of a DUB towards one of the polyUb chain linkages largely determines the ultimate signaling function. We present a novel set of diubiquitin FRET probes, comprising all seven isopeptide linkages, for the absolute quantification of chain cleavage specificity of DUBs by means of Michaelis–Menten kinetics. Each probe is equipped with a FRET pair consisting of Rhodamine110 and tetramethylrhodamine to allow the fully synthetic preparation of the probes by SPPS and NCL. Our synthetic strategy includes the introduction of N,N′‐Boc‐protected 5‐carboxyrhodamine as a convenient building block in peptide chemistry. We demonstrate the value of our probes by quantifying the linkage specificities of a panel of nine DUBs in a high‐throughput manner.

Protein ubiquitination is a multi-functional post-translational modification that affects all cellular processes. Its versatility arises from architecturally complex polyubiquitin chains, in which individual ubiquitin moieties may be ubiquitinated on one or multiple residues, and/or modified by phosphorylation and acetylation1-3. Advances in mass spectrometry have enabled the mapping of individual ubiquitin modifications that generate the ubiquitin code; however, the architecture of polyubiquitin signals has remained largely inaccessible. Here we introduce Ub-clipping as a methodology by which to understand polyubiquitin signals and architectures. Ub-clipping uses an engineered viral protease, Lbpro∗, to incompletely remove ubiquitin from substrates and leave the signature C-terminal GlyGly dipeptide attached to the modified residue; this simplifies the direct assessment of protein ubiquitination on substrates and within polyubiquitin. Monoubiquitin generated by Lbpro∗ retains GlyGly-modified residues, enabling the quantification of multiply GlyGly-modified branch-point ubiquitin. Notably, we find that a large amount (10-20%) of ubiquitin in polymers seems to exist as branched chains. Moreover, Ub-clipping enables the assessment of co-existing ubiquitin modifications. The analysis of depolarized mitochondria reveals that PINK1/parkin-mediated mitophagy predominantly exploits mono- and short-chain polyubiquitin, in which phosphorylated ubiquitin moieties are not further modified. Ub-clipping can therefore provide insight into the combinatorial complexity and architecture of the ubiquitin code.

Host cells assemble linear ubiquitin chains to activate immune signalling during bacterial infection. A new study reveals that Legionella pneumophila — the causative agent of Legionnaires’ disease — produces an enzyme that specifically disassembles these linear ubiquitin modifications to restrict immune responses.

Protein ubiquitination regulates many cellular processes via attachment of structurally and functionally distinct ubiquitin (Ub) chains. Several atypical chain types have remained poorly characterized because the enzymes mediating their assembly and receptors with specific binding properties have been elusive. We found that the human HECT E3 ligases UBE3C and AREL1 assemble K48/K29- and K11/K33-linked Ub chains, respectively, and can be used in combination with DUBs to generate K29- and K33-linked chains for biochemical and structural analyses. Solution studies indicate that both chains adopt open and dynamic conformations. We further show that the N-terminal Npl4-like zinc finger (NZF1) domain of the K29/K33-specific deubiquitinase TRABID specifically binds K29/K33-linked diUb, and a crystal structure of this complex explains TRABID specificity and suggests a model for chain binding by TRABID. Our work uncovers linkage-specific components in the Ub system for atypical K29- and K33-linked Ub chains, providing tools to further understand these unstudied posttranslational modifications.

The Ser/Thr protein kinase PINK1 phosphorylates the well‐folded, globular protein ubiquitin (Ub) at a relatively protected site, Ser65. We previously showed that Ser65 phosphorylation results in a conformational change in which Ub adopts a dynamic equilibrium between the known, common Ub conformation and a distinct, second conformation wherein the last β‐strand is retracted to extend the Ser65 loop and shorten the C‐terminal tail. We show using chemical exchange saturation transfer (CEST) nuclear magnetic resonance experiments that a similar, C‐terminally retracted (Ub‐CR) conformation also exists at low population in wild‐type Ub. Point mutations in the moving β5 and neighbouring β‐strands shift the Ub/Ub‐CR equilibrium. This enabled functional studies of the two states, and we show that while the Ub‐CR conformation is defective for conjugation, it demonstrates improved binding to PINK1 through its extended Ser65 loop, and is a superior PINK1 substrate. Together our data suggest that PINK1 utilises a lowly populated yet more suitable Ub‐CR conformation of Ub for efficient phosphorylation. Our findings could be relevant for many kinases that phosphorylate residues in folded protein domains.

RING finger domain and RING finger-like ubiquitin ligases (E3s), such as U-box proteins, constitute the vast majority of known E3s. RING-type E3s function together with ubiquitin-conjugating enzymes (E2s) to mediate ubiquitination and are implicated in numerous cellular processes. In part because of their importance in human physiology and disease, these proteins and their cellular functions represent an intense area of study. Here we review recent advances in RING-type E3 recognition of substrates, their cellular regulation, and their varied architecture. Additionally, recent structural insights into RING-type E3 function, with a focus on important interactions with E2s and ubiquitin, are reviewed. This article is part of a Special Issue entitled: Ubiquitin–Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.

The tumor‐suppressor protein BRCA1 works with BARD1 to catalyze the transfer of ubiquitin onto protein substrates. The N‐terminal regions of BRCA1 and BARD1 that contain their RING domains are responsible for dimerization and ubiquitin ligase activity. This activity is a common feature among hundreds of human RING domain‐containing proteins. RING domains bind and activate E2 ubiquitin‐conjugating enzymes to promote ubiquitin transfer to substrates. We show that the identity of residues at specific positions in the RING domain can tune activity levels up or down. We report substitutions that create a structurally intact BRCA1/BARD1 heterodimer that is inactive in vitro with all E2 enzymes. Other substitutions in BRCA1 or BARD1 RING domains result in hyperactivity, revealing that both proteins have evolved attenuated activity. Loss of attenuation results in decreased product specificity, providing a rationale for why nature has tuned BRCA1 activity. The ability to tune BRCA1 provides powerful tools for understanding its biological functions and provides a basis to assess mechanisms for rescuing the activity of cancer‐associated variations. Beyond the applicability to BRCA1, we show the identity of residues at tuning positions that can be used to predict and modulate the activity of an unrelated RING E3 ligase. These findings provide valuable insights into understanding the mechanism and function of RING E3 ligases like BRCA1.

There are few insecticidal options for potato tuber moth (PTM), Phthorimaea operculella Zeller (Lepidoptera: Gelechiidae), control late in the growing season. We evaluated the PTM granulovirus (PoGV) and Bacillus thuringiensis Berliner ssp. kurstaki (Btk) for season-long control of PTM on potato foliage in 2006 and 2007. Compared to untreated controls, 10 weekly applications of PoGV (10 13 occlusion bodies/ha) reduced PTM populations in replicated 1-m 3 field cages by 86‐96% on pre-harvest foliage and 90‐97% on tubers added to cages shortly before harvest. Infection rates of 82‐95% of L4 larvae by PoGV were noted within individual larval cohorts. Equivalently timed Btk treatments (1.12 kg product/ha) were significantly less effective at population suppression, with a 36‐76% reduction in larvae recovered from tubers added to cages. A PoGV/Btk alternation was significantly more effective than Btk alone and as effective as PoGV in 2007, but not in 2006. There was some evidence that reduced rate PoGV treatments (10% rate or 50% application frequency) were less effective than the standard program. There were no treatment effects on percentage of tubers growing in the ground that were infested at harvest, which remained comparatively low at ≤ 8.1%. Bioassays were conducted to evaluate the residual activities of foliar deposits. Early-season applications were highly effective for the first 24 h ( ≥ 93% mortality) with a steady decline in activity over 10 days. A second application, applied later in the season, showed similar patterns, although in this case Btk was less persistent than PoGV, whereas both agents provided significant larval mortality compared with controls over 14 days. Both PoGV and Btk provide alternatives to manage field infestations of PTM prior to harvest, thus reducing the risk of tuber infestations in storage.

Pathogenic bacteria introduce effector proteins directly into the cytosol of eukaryotic cells to promote invasion and colonization. OspG, a Shigella spp. effector kinase, plays a role in this process by helping to suppress the host inflammatory response. OspG has been reported to bind host E2 ubiquitin-conjugating enzymes activated with ubiquitin (E2~Ub), a key enzyme complex in ubiquitin transfer pathways. A co-crystal structure of the OspG/UbcH5c~Ub complex reveals that complex formation has important ramifications for the activity of both OspG and the UbcH5c~Ub conjugate. OspG is a minimal kinase domain containing only essential elements required for catalysis. UbcH5c~Ub binding stabilizes an active conformation of the kinase, greatly enhancing OspG kinase activity. In contrast, interaction with OspG stabilizes an extended, less reactive form of UbcH5c~Ub. Recognizing conserved E2 features, OspG can interact with at least ten distinct human E2s~Ub. Mouse oral infection studies indicate that E2~Ub conjugates act as novel regulators of OspG effector kinase function in eukaryotic host cells.

Although ubiquitination plays a critical role in virtually all cellular processes, mechanistic details of ubiquitin (Ub) transfer are still being defined. To identify the molecular determinants within E3 ligases that modulate activity, we scored each member of a library of nearly 100,000 protein variants of the murine ubiquitination factor E4B (Ube4b) U-box domain for auto-ubiquitination activity in the presence of the E2 UbcH5c. This assay identified mutations that enhance activity both in vitro and in cellular p53 degradation assays. The activity-enhancing mutations fall into two distinct mechanistic classes: One increases the U-box:E2-binding affinity, and the other allosterically stimulates the formation of catalytically active conformations of the E2∼Ub conjugate. The same mutations enhance E3 activity in the presence of another E2, Ube2w, implying a common allosteric mechanism, and therefore the general applicability of our observations to other E3s. A comparison of the E3 activity with the two different E2s identified an additional variant that exhibits E3:E2 specificity. Our results highlight the general utility of high-throughput mutagenesis in delineating the molecular basis of enzyme activity.

Type II secretion systems (T2SSs) are critical for secretion of many proteins from Gram-negative bacteria. In the T2SS, the outer membrane secretin GspD forms a multimeric pore for translocation of secreted proteins. GspD and the inner membrane protein GspC interact with each other via periplasmic domains. Three different crystal structures of the homology region domain of GspC (GspCHR) in complex with either two or three domains of the N-terminal region of GspD from enterotoxigenic Escherichia coli show that GspCHR adopts an all-β topology. N-terminal β-strands of GspC and the N0 domain of GspD are major components of the interface between these inner and outer membrane proteins from the T2SS. The biological relevance of the observed GspC–GspD interface is shown by analysis of variant proteins in two-hybrid studies and by the effect of mutations in homologous genes on extracellular secretion and subcellular distribution of GspC in Vibrio cholerae. Substitutions of interface residues of GspD have a dramatic effect on the focal distribution of GspC in V. cholerae. These studies indicate that the GspCHR–GspDN0 interactions observed in the crystal structure are essential for T2SS function. Possible implications of our structures for the stoichiometry of the T2SS and exoprotein secretion are discussed., Author Summary Many bacterial pathogens affecting humans, animals and plants export diverse proteins across the cell membranes into the medium surrounding the bacteria. Some of these secreted proteins are involved in pathogenesis. One example is cholera toxin secreted by the bacterium Vibrio cholerae, a causative agent of cholera. The sophisticated type II secretion system is responsible for moving this toxin, and several other proteins, across the outer membrane. Here, we studied the interaction between the outer membrane pore of the type II secretion system, the secretin GspD, and the inner membrane protein GspC. We have solved three crystal structures of complexes between the interacting domains and identified critical contacts in the GspC–GspD interface. We also showed the importance of these contacts for assembly of the secretion system and for secretion of proteins by V. cholerae. Our studies provide a major piece in the puzzle of how the type II secretion system is assembled and how it functions. One day this knowledge might allow us to design compounds which interfere with this secretion process. Such compounds would be useful in the battle against bacteria affecting human health.