The Structure and Function of Type III Secretion Systems
- Authors: Ryan Q. Notti1,2, C. Erec Stebbins3
- Editor: Indira T. Kudva4
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VIEW AFFILIATIONS HIDE AFFILIATIONSAffiliations: 1: Laboratory of Structural Microbiology, Rockefeller University, New York, NY 10065; 2: Tri-Institutional Medical Scientist Training Program, Weill Cornell Medical College, New York, NY, 10021; 3: Laboratory of Structural Microbiology, Rockefeller University, New York, NY 10065; 4: National Animal Disease Center, Agricultural Research Service, U.S. Department of Agriculture, Ames, IA.
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Received 12 January 2015 Accepted 15 April 2015 Published 12 February 2016
- Correspondence: C. Erec Stebbins, [email protected]

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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.
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Citation: Notti R, Stebbins C. 2016. The Structure and Function of Type III Secretion Systems. Microbiol Spectrum 4(1):VMBF-0004-2015. doi:10.1128/microbiolspec.VMBF-0004-2015.




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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.

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Figures

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FIGURE 1
Gross architecture of the T3SS. Cryo-EM reconstruction of the Salmonella enterica serovar 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 Schraidt and Marlovits ( 27 ). Dashed lines indicate the positions of bacterial membranes in vivo. Abbreviations: 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).

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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 enterica serovar 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 SctDperiplasmic.

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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 gray, light gray). PDB 2FM8 ( 47 ). The structurally conserved β-motif is highlighted in yellow. (B) The SipA β-motif is bound by a hydrophobic (gray) patch on the InvB surface (blue/gray). (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 (gray). Protein Data bank ID number 4AM9 (PDB 4AM9) ( 172 ).
Tables

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TABLE 1
A unified nomenclature for the homologous core components of the T3SS a
Supplemental Material
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