The Structure and Function of Type III Secretion Systems

Abstract

Type III secretion systems (T3SSs) afford Gram-negative bacteria an intimate means of altering the biology of their eukaryotic hosts--the direct delivery of effector proteins from the bacterial cytoplasm to that of the eukaryote. This incredible biophysical feat is accomplished by nanosyringe "injectisomes," which form a conduit across the three plasma membranes, peptidoglycan layer, and extracellular space that form a barrier to the direct delivery of proteins from bacterium to host. The focus of this chapter is T3SS function at the structural level; we will summarize the core findings that have shaped our understanding of the structure and function of these systems and highlight recent developments in the field. In turn, we describe the T3SS secretory apparatus, consider its engagement with secretion substrates, and discuss the posttranslational regulation of secretory function. Lastly, we close with a discussion of the future prospects for the interrogation of structure-function relationships in the T3SS.

Figure 1. Gross architecture of the T3SS

Cryo-EM reconstruction of the Salmonella typhimurium injectisome basal body at subnanometer resolution reveals its overall architecture. (A) Surface representation of the highest resolution cryo-EM map (EMD 1875, contour level 0.0233) published by Marlovits and colleagues (27). Dashed lines indicate the positions of bacterial membranes in vivo. Abbreviations used: OR, outer ring; IR, inner ring; OM, outer membrane; IM, inner membrane. (B) An axial section through the map in (A). (C) Transverse sections through the map in (A) at the level of the neck (top) and IR1 (bottom).

Figure 2. Hybrid models of basal body structure

(A) Computational modeling of the neck (SctC, PDB 3J1V), IR1 (SctD, PDB 3J1X), and IR2 (SctD, PDB 3J1W) annuli of the Salmonella typhimurium basal body. No high-resolution structural information is available for the basal body above the neck. (B) In this model, complementary electrostatic surfaces support ring building, as shown for the SctD periplasmic domains. Note the modular domain architecture (enumerated 1, 2, 3) for SctD_periplasmic.

Figure 3. Chaperone-substrate interactions

Structural distinctions between effector-chaperone and translocator-chaperone complexes. (A) The structure of the Salmonella effector SipA chaperone-binding domain (CBD, red and yellow) in complex with the Class IB chaperone InvB (dark grey, light grey). PDB 2FM8 (47). The structurally conserved β-motif is highlighted in yellow. (B) The SipA β-motif is bound by a hydrophobic (grey) patch on the InvB surface (blue/grey). (C) Superposition of the CBDs from effectors from multiple species shows a common binding mode marked by the structurally conserved β-motif. The prototypical Class I chaperone SicP is shown in place of the various chaperones. PDB codes: YopN, 1XKP (153); YopE, 1L2W (170); YscM2, 1TTW (171); SptP-SicP, 1JYO (88); SipA, 2FM8 (47); HopA1, 4G6T (93). (D) The Yersinia translocator YopD CBD (red) lacks secondary structure and is bound by the concave cleft of the Class II chaperone SycD (grey). PDB 4AM9 (172).