Coupling and regulation mechanisms of the flavin-dependent halogenase PyrH observed by infrared difference spectroscopy.

Flavin-dependent halogenases are central enzymes in the production of halogenated secondary metabolites in various organisms and they constitute highly promising biocatalysts for regioselective halogenation. The mechanism of these monooxygenases includes formation of hypohalous acid from a reaction of fully reduced flavin with oxygen and halide. The hypohalous acid then diffuses via a tunnel to the substrate-binding site for halogenation of tryptophan and other substrates. Oxidized flavin needs to be reduced for regeneration of the enzyme, which can be performed in vitro by a photoreduction with blue light. Here, we employed this photoreduction to study characteristic structural changes associated with the transition from oxidized to fully reduced flavin in PyrH from Streptomyces rugosporus as a model for tryptophan-5-halogenases. The effect of the presence of bromide and chloride or the absence of any halides on the UV-vis spectrum of the enzyme demonstrated a halide-dependent structure of the flavin-binding pocket. Light-induced FTIR difference spectroscopy was applied and the signals assigned by selective isotope labeling of the protein moiety. The identified structural changes in α-helix and ß-sheet elements were strongly dependent on the presence of bromide, chloride, the substrate tryptophan, and the product 5-chloro-tryptophan, respectively. We identified a clear allosteric coupling in solution at ambient conditions between cofactor-binding site and substrate-binding site that is active in both directions, despite their separation by a tunnel. We suggest that this coupling constitutes a fine-tuned mechanism for the promotion of the enzymatic reaction of flavin-dependent halogenases in dependence of halide and substrate availability.

Increasing the Stability of Flavin-Dependent Halogenases by Disulfide Engineering.

Flavin-dependent halogenases allow halogenation of electron-rich aromatic compounds under mild reaction conditions even at electronically unfavored positions with high regioselectivity. In order to expand the application of halogenases, the enzymes need to be improved in terms of stability and efficiency. A previous study with the tryptophan 6-halogenase Thal demonstrated that thermostable Thal variants tend to form dimers in solution while the wild type is present as a monomer. Based on this a dimeric Thal variant was generated that is covalently linked by disulfide bonds. Introducing two cysteine residues at the dimer interface resulted in the variant Thal CC with significantly increased thermostability (▵T50 =15.7 K) and stability over time at elevated temperature compared to the wild type. By introducing the homologous mutations into the tryptophan 5-halogenase PyrH, we were able to show that the stabilization by covalent dimerization can also be transferred to other halogenases. Moreover, it was possible to further increase the thermostability of PyrH by inserting cysteine mutations at alternative sites of the dimer interface.

Protein target highlights in CASP15: Analysis of models by structure providers.

We present an in-depth analysis of selected CASP15 targets, focusing on their biological and functional significance. The authors of the structures identify and discuss key protein features and evaluate how effectively these aspects were captured in the submitted predictions. While the overall ability to predict three-dimensional protein structures continues to impress, reproducing uncommon features not previously observed in experimental structures is still a challenge. Furthermore, instances with conformational flexibility and large multimeric complexes highlight the need for novel scoring strategies to better emphasize biologically relevant structural regions. Looking ahead, closer integration of computational and experimental techniques will play a key role in determining the next challenges to be unraveled in the field of structural molecular biology.

Enzymatic Late-Stage Halogenation of Peptides.

The late-stage site-selective derivatisation of peptides has many potential applications in structure-activity relationship studies and postsynthetic modification or conjugation of bioactive compounds. The development of orthogonal methods for C-H functionalisation is crucial for such peptide derivatisation. Among them, biocatalytic methods are increasingly attracting attention. Tryptophan halogenases emerged as valuable catalysts to functionalise tryptophan (Trp), while direct enzyme-catalysed halogenation of synthetic peptides is yet unprecedented. Here, it is reported that the Trp 6-halogenase Thal accepts a wide range of amides and peptides containing a Trp moiety. Increasing the sequence length and reaction optimisation made bromination of pentapeptides feasible with good turnovers and a broad sequence scope, while regioselectivity turned out to be sequence dependent. Comparison of X-ray single crystal structures of Thal in complex with d-Trp and a dipeptide revealed a significantly altered binding mode for the peptide. The viability of this bioorthogonal approach was exemplified by halogenation of a cyclic RGD peptide.

Structure-based switch of regioselectivity in the flavin-dependent tryptophan 6-halogenase Thal.

Flavin-dependent halogenases increasingly attract attention as biocatalysts in organic synthesis, facilitating environmentally friendly halogenation strategies that require only FADH2, oxygen, and halide salts. Different flavin-dependent tryptophan halogenases regioselectively chlorinate or brominate trypto-phan's indole moiety at C5, C6, or C7. Here, we present the first substrate-bound structure of a tryptophan 6-halogenase, namely Thal, also known as ThdH, from the bacterium Streptomyces albogriseolus at 2.55 Šresolution. The structure revealed that the C6 of tryptophan is positioned next to the ϵ-amino group of a conserved lysine, confirming the hypothesis that proximity to the catalytic residue determines the site of electrophilic aromatic substitution. Although Thal is more similar in sequence and structure to the tryptophan 7-halogenase RebH than to the tryptophan 5-halogenase PyrH, the indole binding pose in the Thal active site more closely resembled that of PyrH than that of RebH. The difference in indole orientation between Thal and RebH appeared to be largely governed by residues positioning the Trp backbone atoms. The sequences of Thal and RebH lining the substrate binding site differ in only few residues. Therefore, we exchanged five amino acids in the Thal active site with the corresponding counterparts in RebH, generating the quintuple variant Thal-RebH5. Overall conversion of l-Trp by the Thal-RebH5 variant resembled that of WT Thal, but its regioselectivity of chlorination and bromination was almost completely switched from C6 to C7 as in RebH. We conclude that structure-based protein engineering with targeted substitution of a few residues is an efficient approach to tailoring flavin-dependent halogenases.

Competitive Binding Study Revealing the Influence of Fluorophore Labels on Biomolecular Interactions.

Fluorescence methods are important tools in modern biology. Direct labeling of biomolecules with a fluorophore might, however, change interaction surfaces. Here, we introduce a competitive binding assay in combination with fluorescence correlation spectroscopy that reports binding affinities of both labeled and unlabeled biomolecules to their binding target. We investigated how fluorophore labels at different positions of a DNA oligonucleotide affect hybridization to a complementary oligonucleotide and found dissociation constants varying within 2 orders of magnitude. We next demonstrated that placing a fluorophore label at position Leu280 in the protein ligand internalin B does not alter the binding affinity to the MET receptor tyrosine kinase, compared to unlabeled internalin B. Our approach is simple to implement and can be applied to investigate the influence of fluorophore labels in a large variety of biomolecular interactions.

MET-activating Residues in the B-repeat of the Listeria monocytogenes Invasion Protein InlB.

The facultative intracellular pathogen Listeria monocytogenes causes listeriosis, a rare but life-threatening disease. Host cell entry begins with activation of the human receptor tyrosine kinase MET through the bacterial invasion protein InlB, which contains an internalin domain, a B-repeat, and three GW domains. The internalin domain is known to bind MET, but no interaction partner is known for the B-repeat. Adding the B-repeat to the internalin domain potentiates MET activation and is required to stimulate Madin-Darby canine kidney (MDCK) cell scatter. Therefore, it has been hypothesized that the B-repeat may bind a co-receptor on host cells. To test this hypothesis, we mutated residues that might be important for binding an interaction partner. We identified two adjacent residues in strand ß2 of the ß-grasp fold whose mutation abrogated induction of MDCK cell scatter. Biophysical analysis indicated that these mutations do not alter protein structure. We then tested these mutants in human HT-29 cells that, in contrast to the MDCK cells, were responsive to the internalin domain alone. These assays revealed a dominant negative effect, reducing the activity of a construct of the internalin domain and mutated B-repeat below that of the individual internalin domain. Phosphorylation assays of MET and its downstream targets AKT and ERK confirmed the dominant negative effect. Attempts to identify a host cell receptor for the B-repeat were not successful. We conclude that there is limited support for a co-receptor hypothesis and instead suggest that the B-repeat contributes to MET activation through low affinity homodimerization.

Live cell imaging shows hepatocyte growth factor-induced Met dimerization.

The canonical model of receptor tyrosine kinase (RTK) activation assumes that ligand-induced dimerization of inactive receptor monomers is a prerequisite for autophosphorylation. For several RTK families, recent results of fluorescence microscopy provided evidence for preformed receptor dimers that may or may not require ligand binding for kinase activity. Here we report, for the first time, the application of advanced quantitative fluorescence microscopy techniques to study changes in the oligomerization state of the RTK Met in response to stimulation by its endogenous ligand hepatocyte growth factor (HGF). We used inducible C-terminal fusions between Met and enhanced green fluorescent protein (EGFP) or red fluorescent protein (RFP) in combination with fluorescence resonance energy transfer (FRET)-based fluorescence-lifetime imaging microscopy (FLIM) and fluorescence correlation spectroscopy (FCS). A small fraction of HGF-independent Met dimers appeared to be present in cells even at low receptor density. At high receptor density, both the fraction of Met dimers and the level of Met autophosphorylation increased in the absence of HGF. Stimulation with HGF at low receptor density significantly increased the fraction of Met dimers on live cells. We found no indications of Met oligomers larger than dimers. Our findings thus confirm a model of Met activation through HGF-induced dimerization and at the same time they support previous reports of Met dimers in unstimulated cells. The tools established in this work will be useful to further characterize the mechanism of Met activation and to define the contribution of co-receptors.

Structure of Halorhodopsin from Halobacterium salinarum in a new crystal form that imposes little restraint on the E-F loop.

Halorhodopsin from the halophilic archaeon Halobacterium salinarum is a membrane located light-driven chloride pump. Upon illumination Halorhodopsin undergoes a reversible photocycle initiated by the all-trans to 13-cis isomerization of the covalently bound retinal chromophore. The photocycle consists of several spectroscopically distinct intermediates. The structural basis of the chloride transport mechanism remains elusive, presumably because packing contacts have so far precluded protein conformational changes in the available crystals. With the intention to structurally characterize late photocycle intermediates by X-ray crystallography we crystallized Halorhodopsin in a new crystal form using the vesicle fusion method. In the new crystal form lateral contacts are mediated by helices A and G. Helices E and F that were suggested to perform large movements during the photocycle are almost unrestrained by packing contacts. This feature might permit the displacement of these helices without disrupting the crystal lattice. Therefore, this new crystal form might be an excellent system for the structural characterization of late Halorhodopsin photocycle intermediates by trapping or by time resolved experiments, especially at XFELs.

Structural basis of MET receptor dimerization by the bacterial invasion protein InlB and the HGF/SF splice variant NK1.

The structural basis of ligand-induced dimerization of the receptor tyrosine kinase MET by its natural ligand hepatocyte growth factor/scatter factor (HGF/SF) is not well understood. However, interesting insight into the molecular mechanism of MET dimerization has emerged from crystal structures of MET in complex with a bacterial agonist, the invasion protein internalin B (InlB) from pathogenic Listeria monocytogenes. MET activation by InlB promotes uptake of bacteria into host cells. Structural and biophysical data suggest that InlB is monomeric on its own but dimerizes upon binding to the membrane-anchored MET receptor promoting the formation of a signaling active 2:2 complex. The dimerization interface is small and unusually located on the convex side of the curved InlB leucine-rich repeat (LRR) domain. As InlB does not dimerize in solution, the dimerization site could only be identified by studying packing contacts of InlB in various crystal forms and had to be proven by scrutinizing its biological relevance in cellular assays. InlB dimerization is thus an example of a low-affinity contact that appears irrelevant in solution but becomes physiologically significant in the context of 2-dimensional diffusion restricted to the membrane plane. The resulting 2:2 InlB:MET complex has an InlB dimer at its center with one MET molecule bound peripherally to each InlB. This model of ligand-mediated MET dimerization may serve as a blue-print to understand MET activation by NK1, a naturally occurring HGF/SF splice variant and MET agonist. Crystal structures of NK1 repeatedly show a NK1 dimer, in which residues implicated in MET-binding are located on the outside. Thus, MET dimerization by NK1 may also be ligand-mediated with a NK1 dimer at the center of the 2:2 complex with one MET molecule bound peripherally to each NK1. This article is part of a Special Issue entitled: Emerging recognition and activation mechanisms of receptor tyrosine kinases.

Structure of a three-dimensional domain-swapped dimer of the Helicobacter pylori type IV secretion system pilus protein CagL.

A new crystal form of the Helicobacter pylori type IV secretion system (T4SS) pilus protein CagL is described here. In contrast to two previously reported monomeric structures, CagL forms a three-dimensional domain-swapped dimer. CagL dimers can arise during refolding from inclusion bodies or can form spontaneously from purified monomeric CagL in the crystallization conditions. Monomeric CagL forms a three-helix bundle, with which the N-terminal helix is only loosely associated. In the new crystal form, the N-terminal helix is missing. The domain swap is owing to exchange of the C-terminal helix between the two protomers of a dimer. A loop-to-helix transition results in a long helix of 108 amino acids comprising the penultimate and the last helix of the monomer. The RGD motif of dimeric CagL adopts an α-helical conformation. In contrast to the previously reported structures, the conserved and functionally important C-terminal hexapeptide is resolved. It extends beyond the three-helix bundle as an exposed helical appendage. This new crystal form contributes to the molecular understanding of CagL by highlighting rigid and flexible regions in the protein and by providing the first view of the C-terminus. Based on the structural features, a previously unrecognized homology between CagL and CagI is discussed.

Crystal structure of an engineered YopM-InlB hybrid protein.

BACKGROUND: The multi-domain protein InlB (internalin B) from Listeria monocytogenes is an agonist of the human receptor tyrosine kinase MET. Only the internalin domain directly interacts with MET. The internalin domain consists of seven central leucine-rich repeats (LRRs) flanked by an N-terminal helical cap domain and a C-terminal immunoglobulin-like structure. A potential function of the N-terminal cap in receptor binding could so far not be demonstrated by deleting the cap, since the cap is also implicated in nucleating folding of the LRR domain. RESULTS: We generated an InlB variant (YopM-InlB) in which the InlB cap domain was replaced by the unrelated N-terminal capping structure of the LRR protein YopM from Yersinia enterocolitica. The crystal structure of the engineered protein shows that it folds properly. Because the first LRR is structurally closely linked to the cap domain, we exchanged LRR1 along with the cap domain. This resulted in unexpected structural changes extending to LRR2 and LRR3, which are deeply involved in MET binding. As a consequence, the binding of YopM-InlB to MET was substantially weaker than that of wild type InlB. The engineered protein was about one order of magnitude less active in colony scatter assays than wild type InlB. CONCLUSIONS: We obtained a well-behaved InlB variant with an altered N-terminal capping structure through protein design. The reduced affinity for MET precludes a straightforward interpretation of the results from cell-based assays. Still, the engineered hybrid protein induced cell scatter, suggesting that the cap is required for folding and stability of InlB but is not essential for interactions that assemble the signalling-active receptor complex. The cap swap approach described here is clearly applicable to other L. monocytogenes internalins and other LRR proteins such as YopM and may yield useful structure/function correlates within this protein family.

Receptor-ligand interactions: binding affinities studied by single-molecule and super-resolution microscopy on intact cells.

Protein­ligand interactions play an important role in many biological processes. Notably, membrane receptors are the starting point for a huge variety of cellular signal transduction pathways. Quantifying the binding affinity of a ligand for its transmembrane receptor is of great importance as it provides information on the potency of the ligand. We developed a new experimental procedure to determine binding affinities of ligands for their membrane receptors directly on intact single cells using super-resolution imaging. Dissociation constants were determined by titrating fluorophore-labelled ligand against cells expressing the target protein and applying single-molecule imaging.

Crystal structure and enzyme engineering of the broad substrate spectrum l-amino acid oxidase 4 from the fungus Hebeloma cylindrosporum.

l-Amino acid oxidases (LAAOs) catalyze the oxidative deamination of l-amino acids to α-keto acids. Recombinant production of LAAOs with broad substrate spectrum remains a formidable challenge. We previously achieved this for the highly active and thermostable LAAO4 of Hebeloma cylindrosporum (HcLAAO4). Here, we crystallized a proteolytically truncated surface entropy reduction variant of HcLAAO4 and solved its structure in substrate-free form and in complex with diverse substrates. The ability to support the aliphatic portion of a substrate's side chain by an overall hydrophobic active site is responsible for the broad substrate spectrum of HcLAAO4, including l-amino acids with big aromatic, acidic and basic side chains. Based on the structural findings, we generated an E288H variant with increased activity toward pharmaceutical building blocks of high interest.

Structural basis of regioselective tryptophan dibromination by the single-component flavin-dependent halogenase AetF.

The flavin-dependent halogenase (FDH) AetF successively brominates tryptophan at C5 and C7 to generate 5,7-dibromotryptophan. In contrast to the well studied two-component tryptophan halogenases, AetF is a single-component flavoprotein monooxygenase. Here, crystal structures of AetF alone and in complex with various substrates are presented, representing the first experimental structures of a single-component FDH. Rotational pseudosymmetry and pseudomerohedral twinning complicated the phasing of one structure. AetF is structurally related to flavin-dependent monooxygenases. It contains two dinucleotide-binding domains for binding the ADP moiety with unusual sequences that deviate from the consensus sequences GXGXXG and GXGXXA. A large domain tightly binds the cofactor flavin adenine dinucleotide (FAD), while the small domain responsible for binding the nicotinamide adenine dinucleotide (NADP) is unoccupied. About half of the protein forms additional structural elements containing the tryptophan binding site. FAD and tryptophan are about 16 Šapart. A tunnel between them presumably allows diffusion of the active halogenating agent hypohalous acid from FAD to the substrate. Tryptophan and 5-bromotryptophan bind to the same site but with a different binding pose. A flip of the indole moiety identically positions C5 of tryptophan and C7 of 5-bromotryptophan next to the tunnel and to catalytic residues, providing a simple explanation for the regioselectivity of the two successive halogenations. AetF can also bind 7-bromotryptophan in the same orientation as tryptophan. This opens the way for the biocatalytic production of differentially dihalogenated tryptophan derivatives. The structural conservation of a catalytic lysine suggests a way to identify novel single-component FDHs.

The type III secretion chaperone SctY may shield the hydrophobic export gate-binding C-terminus of its substrate SctX.

Gram-negative bacteria such as Aeromonas and Yersinia spp. have developed mechanisms to inhibit the immune defense of their host. Effector proteins are directly injected into the host cytoplasm from the bacterial cytosol via type III secretion systems (T3SSs), where they modulate the cytoskeleton and signaling of the cell. Assembly of, and secretion via, T3SSs is tightly regulated by a number of bacterial proteins, including SctX (AscX in Aeromonas), the secretion of which is essential for T3SS function. Here, crystal structures of AscX in complex with SctY chaperones from Yersinia or Photorhabdus spp. carrying homologous T3SSs are described. There are crystal pathologies in all cases, with one crystal form diffracting anisotropically and the other two exhibiting strong pseudotranslation. The new structures reveal that the positioning of the substrate is very similar on different chaperones. However, the two C-terminal SctX helices that cap the N-terminal tetratricopeptide repeat of SctY shift and tilt depending on the identity of the chaperone. Moreover, the C-terminus of the α3 helix of AscX exhibits an unprecedented kink in two of the structures. In previous structures, the C-terminus of SctX protrudes beyond the chaperone as a straight helix: a conformation that is required for binding to the nonameric export gate SctV but that is unfavorable for binary SctX-SctY complexes due to the hydrophobicity of helix α3 of SctX. A kink in helix α3 may allow the chaperone to shield the hydrophobic C-terminus of SctX in solution.

Differential Effects of Rhodococcus equi Virulence-Associated Proteins on Macrophages and Artificial Lipid Membranes.

Virulence-associated protein A (VapA) of Rhodococcus equi is a pathogenicity factor required for the multiplication of virulent R. equi strains within spacious macrophage vacuoles. The production of VapA is characteristic for R. equi isolates from pneumonic foals. VapB and VapN proteins in R. equi isolates from infected pig (VapB) and cattle (VapN) have amino acid sequences very similar to VapA and consequently have been assumed to be its functional correlates. Using model membrane experiments, phagosome pH acidification analysis, lysosome size measurements, protein partitioning, and degradation assays, we provide support for the view that VapA and VapN promote intracellular multiplication of R. equi by neutralizing the pH of the R. equi-containing vacuole. VapB does not neutralize vacuole pH, is not as membrane active as VapA, and does not support intracellular multiplication. This study also shows that the size of the sometimes enormous R. equi-containing vacuoles or the partitioning of purified Vaps into organic phases are not features that have predictive value for virulence of R. equi, whereas the ability of Vaps to increase phagosome pH is coupled to virulence. IMPORTANCE Rhodococcus equi is a major cause of life-threatening pneumonia in foals and occasionally in immunocompromised persons. Virulence-associated protein A (VapA) promotes R. equi multiplication in lung macrophages, which are the major host cells during foal infection. In this study, we compare cellular, biochemical, and biophysical phenotypes associated with VapA to those of VapB (typically produced by isolates from pigs) or VapN (isolates from cattle). Our data support the hypothesis that only some Vaps support multiplication in macrophages by pH neutralization of the phagosomes that R. equi inhabit.

Fold and function of the InlB B-repeat.

Host cell invasion by the facultative intracellular pathogen Listeria monocytogenes requires the invasion protein InlB in many cell types. InlB consists of an N-terminal internalin domain that binds the host cell receptor tyrosine kinase Met and C-terminal GW domains that bind to glycosaminoglycans (GAGs). Met binding and activation is required for host cell invasion, while the interaction between GW domains and GAGs enhances this effect. Soluble InlB elicits the same cellular phenotypes as the natural Met ligand hepatocyte growth factor/scatter factor (HGF/SF), e.g. cell scatter. So far, little is known about the central part of InlB, the B-repeat. Here we present a structural and functional characterization of the InlB B-repeat. The crystal structure reveals a variation of the ß-grasp fold that is most similar to small ubiquitin-like modifiers (SUMOs). However, structural similarity also suggests a potential evolutionary relation to bacterial mucin-binding proteins. The B-repeat defines the prototype structure of a hitherto uncharacterized domain present in over a thousand bacterial proteins. Generally, this domain probably acts as a spacer or a receptor-binding domain in extracellular multi-domain proteins. In cellular assays the B-repeat acts synergistically with the internalin domain conferring to it the ability to stimulate cell motility. Thus, the B-repeat probably binds a further host cell receptor and thereby enhances signaling downstream of Met.

Crystal structure of the Yersinia enterocolitica type III secretion chaperone SycD in complex with a peptide of the minor translocator YopD.

BACKGROUND: Type III secretion systems are used by Gram-negative bacteria as "macromolecular syringes" to inject effector proteins into eukaryotic cells. Two hydrophobic proteins called translocators form the necessary pore in the host cell membrane. Both translocators depend on binding to a single chaperone in the bacterial cytoplasm to ensure their stability and efficient transport through the secretion needle. It was suggested that the conserved chaperones bind the more divergent translocators via a hexapeptide motif that is found in both translocators and conserved between species. RESULTS: We crystallized a synthetic decapeptide from the Yersinia enterocolitica minor type III secretion translocator YopD bound to its cognate chaperone SycD and determined the complex structure at 2.5 Å resolution. The structure of peptide-bound SycD is almost identical to that of apo SycD with an all helical fold consisting of three tetratricopeptide repeats (TPRs) and an additional C-terminal helix. Peptide-bound SycD formed a kinked head-to-head dimer that had previously been observed for the apo form of SycD. The homodimer interface comprises both helices of the first tetratricopeptide repeat. The YopD peptide bound in extended conformation into a mainly hydrophobic groove on the concave side of SycD. TPRs 1 and 2 of SycD form three hydrophobic pockets that accommodated the conserved hydrophobic residues at position 1, 3 and 6 of the translocator hexapeptide sequence. Two tyrosines that are highly conserved among translocator chaperones contribute to the hydrophobic patches but also form hydrogen bonds to the peptide backbone. CONCLUSIONS: The interaction between SycD and YopD is very similar to the binding of the Pseudomonas minor translocator PopD to its chaperone PcrH and the Shigella major translocator IpaB to its chaperone IpgC. This confirms the prediction made by Kolbe and co-workers that a hexapeptide with hydrophobic residues at three positions is a conserved chaperone binding motif. Because the hydrophobic groove on the concave side of translocator chaperones is involved in binding of the major and the minor translocator, simultaneous binding of both translocators to a single type III secretion class II chaperone appears unlikely.

Direct interaction of a chaperone-bound type III secretion substrate with the export gate.

Several gram-negative bacteria employ type III secretion systems (T3SS) to inject effector proteins into eukaryotic host cells directly from the bacterial cytoplasm. The export gate SctV (YscV in Yersinia) binds substrate:chaperone complexes such as YscX:YscY, which are essential for formation of a functional T3SS. Here, we present structures of the YscX:YscY complex alone and bound to nonameric YscV. YscX binds its chaperone YscY at two distinct sites, resembling the heterotrimeric complex of the T3SS needle subunit with its chaperone and co-chaperone. In the ternary complex the YscX N-terminus, which mediates YscX secretion, occupies a binding site within one YscV that is also used by flagellar chaperones, suggesting the interaction's importance for substrate recognition. The YscX C-terminus inserts between protomers of the YscV ring where the stalk protein binds to couple YscV to the T3SS ATPase. This primary YscV-YscX interaction is essential for the formation of a secretion-competent T3SS.