Near-atomic-resolution cryo-EM analysis of the Salmonella T3S injectisome basal body

Abstract

The type III secretion (T3S) injectisome is a specialized protein nanomachine that is critical for the pathogenicity of many Gram-negative bacteria, including purveyors of plague, typhoid fever, whooping cough, sexually transmitted infections and major nosocomial infections. This syringe-shaped 3.5-MDa macromolecular assembly spans both bacterial membranes and that of the infected host cell. The internal channel formed by the injectisome allows for the direct delivery of partially unfolded virulence effectors into the host cytoplasm. The structural foundation of the injectisome is the basal body, a molecular lock-nut structure composed predominantly of three proteins that form highly oligomerized concentric rings spanning the inner and outer membranes. Here we present the structure of the prototypical Salmonella enterica serovar Typhimurium pathogenicity island 1 basal body, determined using single-particle cryo-electron microscopy, with the inner-membrane-ring and outer-membrane-ring oligomers defined at 4.3 Å and 3.6 Å resolution, respectively. This work presents the first, to our knowledge, high-resolution structural characterization of the major components of the basal body in the assembled state, including that of the widespread class of outer-membrane portals known as secretins.

A comprehensive guide to pilus biogenesis in Gram-negative bacteria

“…Although, other secretin structures have been reported with other symmetries, including the type III secretion system secretin InvG from Salmonella typhimurium (C15) 87 and a type II secretion system secretin GspD from E. coli and Vibrio cholerae (C15) 88 . …”

Structure and Membrane Topography of the Vibrio-Type Secretin Complex from the Type 2 Secretion System of Enteropathogenic Escherichia coli

“…Members of the secretin protein family are also found in the needle complex of the type 3 secretion system (T3SS) (29)(30)(31). Despite showing some overall principles in common, the other components and the overall architectures of the T2SS and T3SS are quite distinct, as is the mechanism of substrate entry into the complex (Fig.…”

“…4C). In EPEC, the T3SS secretin is EscC, and the structure of the homologous protein InvG from Salmonella was recently solved (31). Comparative hydrophobicity analysis of the T3SS secretin complex revealed a major distinction from the T2SS.…”

Multiple Structures Disclose the Secretins' Secrets

“…Because of recent outstanding improvements in structural biology and particularly in cryo-electron microscopy (cryo-EM), acknowledged by the 2017 Nobel prize for chemistry, the pace at which larger secretion systems has dramatically increased. We can now, for example, visualize the whole T3SS (2,3) or exquisite details of the T4SS (4). In the latter case, visualization came with a surprise in that what composes the outer membrane channel has no resemblance to our standard belief; in particular, the portion of the protein which plugs the system into the outer membrane is not a ␤-barrel but a series of amphipathic ␣-helices.…”