Structure of a type III secretion needle at 7-Å resolution provides insights into its assembly and signaling mechanisms

Abstract

Type III secretion systems of Gram-negative bacteria form injection devices that deliver effector proteins into eukaryotic cells during infection. They span both bacterial membranes and the extracellular space to connect with the host cell plasma membrane. Their extracellular portion is a needle-like, hollow tube that serves as a secretion conduit for effector proteins. The needle of Shigella flexneri is approximately 50-nm long and 7-nm thick and is made by the helical assembly of one protein, MxiH. We provide a 7-Å resolution 3D image reconstruction of the Shigella needle by electron cryomicroscopy, which resolves α-helices and a β-hairpin that has never been observed in the crystal and solution structures of needle proteins, including MxiH. An atomic model of the needle based on the 3D-density map, in comparison with that of the bacterial-flagellar filament, provides insights into how such a thin tubular structure is stably assembled by intricate intermolecular interactions. The map also illuminates how the needle-length control protein functions as a ruler within the central channel during export of MxiH for assembly at the distal end of the needle, and how the secretion-activation signal may be transduced through a conformational change of the needle upon host-cell contact.helical image analysis | Shigella pathogenesis | bacterial protein secretion

Oligomeric states of the Shigella translocator protein IpaB provide structural insights into formation of the type III secretion translocon

“…7 and Table I). Similar values have been determined for the MxiH needle channel 5,30 as well as for the channel of the IpaD pentamer 9 and support the idea that the IpaB oligomer represents a membrane active structure with pore-like properties. Perhaps this is the scaffold for the translocon pore that forms at the interface between the Shigella T3SA and the host cell membrane.…”

Atomic Structure and Handedness of the Building Block of a Biological Assembly

“…This tetramer contains all three of the different unique interfaces ( Figure 1C) and can therefore be considered as the "asymmetric unit" of the assembly. The determined long-range restraints are equally distributed along 34,35 The total number of restraints collected in this study largely exceeds the input data used by us previously to determine the 3D model of the T3SS needle 18 (∼3 times more restraints). Considering the size of the PrgI subunit (80 residues), the sum of restraints collected by means of glucoseand glycerol-based labeling schemes represents a number of more than 10 nonredundant, meaningful long-range restraints per residue, which is in the range of input data used in standard solution NMR protein structure determination.…”

Structure d’une nanomachine bactérienne

“…L'extrémité amino-terminale de PrgI forme un segment rigide de conformation étendue participant à l'interface supramoléculaire. Cette découverte est en contradiction avec le modèle de l'aiguille discuté précédemment [4], où l'extrémité amino-terminale avait été modélisée en hélice α. Dans notre étude, l'architecture de l'aiguille diffère également au niveau de l'extrémité amino-terminale qui se situe sur la face extérieure de l'aiguille et de l'extrémité carboxy-terminale sur la face intérieure, parant ainsi le pore interne, alors que le modèle basé sur la cryomicroscopie présentait une configuration inverse (extrémité carboxy-terminale sur la face externe). Afin de confirmer notre structure RMN, nous avons attaché un tag moléculaire à l'extrémité amino-terminale que nous avons localisé grâce à la microscopie immunoélectronique.…”