In situ architecture, function, and evolution of a contractile injection system

Abstract

Contractile injection systems mediate bacterial cell-cell interactions by a bacteriophage tail-like structure. In contrast to extracellular systems, the type 6 secretion system (T6SS) is defined by intracellular localization and attachment to the cytoplasmic membrane. Here we used cryo-focused ion beam milling, electron cryotomography, and functional assays to study a T6SS in The in situ architecture revealed three modules, including a contractile sheath-tube, a baseplate, and an anchor. All modules showed conformational changes upon firing. Lateral baseplate interactions coordinated T6SSs in hexagonal arrays. The system mediated interactions with host membranes and may participate in phagosome escape. Evolutionary sequence analyses predicted that T6SSs are more widespread than previously thought. Our insights form the basis for understanding T6SS key concepts and exploring T6SS diversity.

Serine/threonine kinase PpkA coordinates the interplay between T6SS2 activation and quorum sensing in the marine pathogen Vibrio alginolyticus

“…Some bacteria harbour more than one T6SS and these systems can carry out distinct functions (Cascales, ). These multi‐component secretion machines can mediate contact dependent killing of prokaryotic as well as eukaryotic cells (Hood et al ., ; Silverman et al ., ; Böck et al ., ). Besides killing adjacent bacterial cells, recent investigations have revealed that T6SS play multiple non‐killing functions, e.g., mediating intracellular pH homeostasis (Zhang et al ., ), Zn 2+ transport (Wang et al ., ), manganese/iron scavenging and resistance to reactive oxygen species (Lin et al ., ; Si et al ., ; Wan et al ., ), facilitating bacterial stress adaptation to changing environmental conditions.…”

“…Canonical T6SS consist of 13 core subunits and several additional components, which are less conserved (Boyer et al ., ). Each T6S apparatus has three main modules: a contractile sheath‐tube, a baseplate and an anchor attached to the cytoplasmic membrane (Böck et al ., ). The contractile sheath‐tube extends into the cytoplasm and consists of a polymer of TssBC (VipA/VipB) building blocks (Basler and Mekalanos, ; Kudryashev et al ., ).…”

“…VgrG (TssI) and some PAAR family proteins form the spike at the tip of the Hcp tube and can be decorated with various effector proteins (Shneider et al ., ). The anchor module is a large complex composed of TssJ, TssM and TssL (DotU), which fixes the baseplate, the sheath tube assembly platform, and allows passage of the Hcp tube/VgrG spike (Zoued et al ., ; Böck et al ., ).…”

Cryo‐ EM reconstruction of Type VI secretion system baseplate and sheath distal end

“…The previous cryo‐ET studies of V. cholerae , M. xanthus, and A. asiaticus showed that T6SS sheath–tube complex is connected to the cell envelope through a cone‐shaped density (Basler et al , 2012; Böck et al , 2017; Chang et al , 2017). In our baseplate reconstruction, the periphery of the wedge likely composed of TssE/F/G is decorated by six copies of a trimeric protein TssK.…”

“…The whole T6SS was first observed at low resolution using cryo‐electron tomography (cryo‐ET) in Vibrio cholerae (Basler et al , 2012) and more recently in Myxococcus xanthus (Chang et al , 2017) and Amoebophilus asiaticus (Böck et al , 2017). The system is composed of a long sheath surrounding an inner tube attached to a baseplate and anchored to a membrane complex that spans both membranes (Brunet et al , 2014, 2015; Durand et al , 2015).…”