Molecular architecture of bacterial type IV secretion systems

Abstract

Bacterial type IV secretion systems (T4SSs) are a versatile group of nanomachines that can horizontally transfer DNA through conjugation and deliver effector proteins into a wide range of target cells. The components of T4SSs in gram-negative bacteria are organized into several large subassemblies: an inner membrane complex, an outer membrane core complex, and, in some species, an extracellular pilus. Cryo-electron tomography has been used to define the structures of T4SSs in intact bacteria, and high-resolution structural models are now available for isolated core complexes from conjugation systems, the Xanthomonas citri T4SS, the Helicobacter pylori Cag T4SS, and the Legionella pneumophila Dot/Icm T4SS. In this review, we compare the molecular architectures of these T4SSs, focusing especially on the structures of core complexes. We discuss structural features that are shared by multiple T4SSs as well as evolutionary strategies used for T4SS diversification. Finally, we discuss how structural variations among T4SSs may confer specialized functional properties.

Fig 1. Cryo-ET analyses of T4SSs in situ.

Images are shown (left to right) for the pKM101 T4SS (A, EMDB 24098 and 24100), the F plasmid-encoded T4SS (B, EMDB 9344 and 9347), the H. pylori Cag T4SS (C, EMDB 0635 and 0634), and the L. pneumophila Dot/Icm T4SS (D, EMDB 7611 and 7612) [32,33,35,37]. Multiple views are shown for each T4SS. The “top” views are relative to the outer membrane (looking into the cell toward the inner membrane). The “bottom” views are relative to the inner membrane (looking out of the cell toward the outer membrane). T4SS structural features include a core complex (CC), stalk (shown in gray), collar (orange), and IMC (shown in green and yellow). The images also show the locations of the OM and IM (tan color). Within the pKM101 and F plasmid-encoded T4SSs, the core complex contains an OL (shown in blue) and an IL (shown in cyan). The H. pylori Cag and L. pneumophila Dot/Icm T4SS core complexes contain an OMC (blue) and a PR (cyan). Features within the IMC include O, M, and I rings. cryo-ET, cryo-electron tomography; I, inner; IL, inner layer; IM, inner membrane; IMC, inner membrane complex; M, middle; O, outer; OL, outer layer; OM, outer membrane; OMC, outer membrane cap; PR, periplasmic ring; T4SS, type IV secretion system.

Fig 2. Structure of core complexes from minimized T4SSs.

The structures of core complexes from the pKM101 (PDB 3JQO) and X. citri (PDB 6GYB) T4SSs are shown as surface representations [48,52]. The O-layers are shown in blue, and the I-layer of the X. citri core complex is shown in gray. The asymmetric unit for each T4SS is indicated in yellow and consists of VirB7, VirB9, and VirB10. The structures of the asymmetric unit for each system are shown in the inset. T4SS, type IV secretion system.

Fig 3. Overall architecture of core complexes in expanded T4SSs.

The structure of each expanded T4SS core complex was determined in 2 or 3 parts, corresponding to regions with different symmetries [56,57,60,61]. The F plasmid T4SS core complex is characterized by a CC or IR (yellow), and an outer (peripheral) ring comprised of an O-layer (blue) and I-layer (green) (EMDB 12962, 12963, and 13231). OMC and PR regions (blue and green) are distinguishable in the H. pylori Cag T4SS core complex (EMDB 20021 and 22081). The L. pneumophila Dot/Icm T4SS core complex contains 3 features termed a dome (yellow), disk (blue), and PR (green) (EMDB 24018, 24005, and 24006). The figure illustrates symmetry mismatch between adjacent regions. CC, central cone; IR, inner ring; OMC, outer membrane cap; PR, periplasmic ring; T4SS, type IV secretion system.

Fig 4. Components of core complex asymmetric units in expanded T4SSs.

(A) Listing of core complex components in 3 expanded T4SSs. (B) Asymmetric unit of the core complex in the F plasmid-encoded T4SS shown in surface representation (PDB 7OKO and 7OKN) [54,55]. (C) Asymmetric unit of the core complex in the H. pylori Cag T4SS shown in surface representation (PDB 6X6S and 6X6J) [56,57]. Components that are related to canonical T4SS proteins (CagT, CagX, and CagY) are shown in shades of blue/green, and species-specific components (CagM and Cag3) are depicted in shades of red. (D) Structures of the VirB7, VirB9, and VirB10-like proteins in the H. pylori Cag asymmetric unit. (E) Asymmetric unit of the core complex in the L. pneumophila Dot/Icm T4SS shown in surface representation (PDB 7MUC) [60,61]. Components that are related to canonical T4SS proteins (DotD and DotH) are shown in shades of blue/green, and species-specific components (DotC, DotF, DotK, Dis1, Dis2, and Dis3) are depicted in shades of red. (F) VirB7, VirB9, and VirB10-like proteins in the L. pneumophila Dot/Icm asymmetric unit. T4SS, type IV secretion system.