Department of structural infection biology ([CSSB] STIB)
head of the department: Prof. Kolbe
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Cryo-EM structure of the Shigella type III needle complex.The Type III Secretion Systems (T3SS) needle complex is a conserved syringe-shaped protein translocation nanomachine with a mass of about 3.5 MDa essential for the survival and virulence of many Gram-negative bacterial pathogens. This system is composed of a membrane-embedded basal body and an extracellular needle that deliver effector proteins into host cells. High-resolution structures of the T3SS from different organisms and infection stages are needed to understand the underlying molecular mechanisms of effector translocation. Here, we present the cryo-electron microscopy structure of the isolated Shigella T3SS needle complex. The inner membrane (IM) region of the basal body adopts 24-fold rotational symmetry and forms a channel system that connects the bacterial periplasm with the export apparatus cage. The secretin oligomer adopts a heterogeneous architecture with 16- and 15-fold cyclic symmetry in the periplasmic N-terminal connector and C-terminal outer membrane ring, respectively. Two out of three IM subunits bind the secretin connector via a β-sheet augmentation. The cryo-EM map also reveals the helical architecture of the export apparatus core, the inner rod, the needle and their intervening interfaces.
Role of flagellar hydrogen bonding in Salmonella motility and flagellar polymorphic transition.Bacterial flagellar filaments are assembled by tens of thousands flagellin subunits, forming 11 helically arranged protofilaments. Each protofilament can take either of the two bistable forms L-type or R-type, having slightly different conformations and inter-protofilaments interactions. By mixing different ratios of L-type and R-type protofilaments, flagella adopt multiple filament polymorphs and promote bacterial motility. In this study, we investigated the hydrogen bonding networks at the flagellin crystal packing interface in Salmonella enterica serovar typhimurium (S. typhimurium) by site-directed mutagenesis of each hydrogen bonded residue. We identified three flagellin mutants D108A, N133A and D152A that were non-motile despite their fully assembled flagella. Mutants D108A and D152A trapped their flagellar filament into inflexible right-handed polymorphs, which resemble the previously predicted 3L/8R and 4L/7R helical forms in Calladine's model but have never been reported in vivo. Mutant N133A produces floppy flagella that transform flagellar polymorphs in a disordered manner, preventing the formation of flagellar bundles. Further, we found that the hydrogen bonding interactions around these residues are conserved and coupled to flagellin L/R transition. Therefore, we demonstrate that the hydrogen bonding networks formed around flagellin residues D108, N133 and D152 greatly contribute to flagellar bending, flexibility, polymorphisms and bacterial motility.
Structural analysis of ligand-bound states of the Salmonella type III secretion system ATPase InvC.Translocation of virulence effector proteins through the type III secretion system (T3SS) is essential for the virulence of many medically relevant Gram‐negative bacteria. The T3SS ATPases are conserved components that specifically recognize chaperone–effector complexes and energize effector secretion through the system. It is thought that functional T3SS ATPases assemble into a cylindrical structure maintained by their N‐terminal domains. Using size‐exclusion chromatography coupled to multi‐angle light scattering and native mass spectrometry, we show that in the absence of the N‐terminal oligomerization domain the Salmonella T3SS ATPase InvC can form monomers and dimers in solution. We also present for the first time a 2.05 å resolution crystal structure of InvC lacking the oligomerization domain (InvCΔ79) and map the amino acids suggested for ATPase intersubunit interaction, binding to other T3SS proteins and chaperone–effector recognition. Furthermore, we validate the InvC ATP‐binding site by co‐crystallization of InvCΔ79 with ATPγS (2.65 å) and ADP (2.80 å). Upon ATP‐analogue recognition, these structures reveal remodeling of the ATP‐binding site and conformational changes of two loops located outside of the catalytic site. Both loops face the central pore of the predicted InvC cylinder and are essential for the function of the T3SS ATPase. Our results present a fine functional and structural correlation of InvC and provide further details of the homo‐oligomerization process and ATP‐dependent conformational changes underlying the T3SS ATPase activity.
Molecular Organization of Soluble Type III Secretion System Sorting Platform Complexes.Many medically relevant Gram-negative bacteria use the type III secretion system (T3SS) to translocate effector proteins into the host for their invasion and intracellular survival. A multi-protein complex located at the cytosolic interface of the T3SS is proposed to act as a sorting platform by selecting and targeting substrates for secretion through the system. However, the precise stoichiometry and 3D organization of the sorting platform components are unknown. Here we reconstitute soluble complexes of the Salmonella Typhimurium sorting platform proteins including the ATPase InvC, the regulator OrgB, the protein SpaO and a recently identified subunit SpaOC, which we show to be essential for the solubility of SpaO. We establish domain-domain interactions, determine for the first time the stoichiometry of each subunit within the complexes by native mass spectrometry and gain insight into their organization using small-angle X-ray scattering. Importantly, we find that in solution the assembly of SpaO/SpaOC/OrgB/InvC adopts an extended L-shaped conformation resembling the sorting platform pods seen in in situ cryo-electron tomography, proposing that this complex is the core building block that can be conceivably assembled into higher oligomers to form the T3SS sorting platform. The determined molecular arrangements of the soluble complexes of the sorting platform provide important insights into its architecture and assembly.
Cryo-electron Microscopy Study of the Genome Release of the Dicistrovirus Israeli Acute Bee Paralysis Virus.Viruses of the family Dicistroviridae can cause substantial economic damage by infecting agriculturally important insects. Israeli acute bee paralysis virus (IAPV) causes honeybee colony collapse disorder in the United States. High-resolution molecular details of the genome delivery mechanism of dicistroviruses are unknown. Here we present a cryo-electron microscopy analysis of IAPV virions induced to release their genomes in vitro We determined structures of full IAPV virions primed to release their genomes to a resolution of 3.3 Å and of empty capsids to a resolution of 3.9 Å. We show that IAPV does not form expanded A particles before genome release as in the case of related enteroviruses of the family Picornaviridae The structural changes observed in the empty IAPV particles include detachment of the VP4 minor capsid proteins from the inner face of the capsid and partial loss of the structure of the N-terminal arms of the VP2 capsid proteins. Unlike the case for many picornaviruses, the empty particles of IAPV are not expanded relative to the native virions and do not contain pores in their capsids that might serve as channels for genome release. Therefore, rearrangement of a unique region of the capsid is probably required for IAPV genome release.