Process of protein transport by the type III secretion system

Abstract

The type III secretion system (TTSS) of gram-negative bacteria is responsible for delivering bacterial proteins, termed effectors, from the bacterial cytosol directly into the interior of host cells. The TTSS is expressed predominantly by pathogenic bacteria and is usually used to introduce deleterious effectors into host cells. While biochemical activities of effectors vary widely, the TTSS apparatus used to deliver these effectors is conserved and shows functional complementarity for secretion and translocation. This review focuses on proteins that constitute the TTSS apparatus and on mechanisms that guide effectors to the TTSS apparatus for transport. The TTSS apparatus includes predicted integral inner membrane proteins that are conserved widely across TTSSs and in the basal body of the bacterial flagellum. It also includes proteins that are specific to the TTSS and contribute to ring-like structures in the inner membrane and includes secretin family members that form ring-like structures in the outer membrane. Most prominently situated on these coaxial, membrane-embedded rings is a needle-like or pilus-like structure that is implicated as a conduit for effector translocation into host cells. A short region of mRNA sequence or protein sequence in effectors acts as a signal sequence, directing proteins for transport through the TTSS. Additionally, a number of effectors require the action of specific TTSS chaperones for efficient and physiologically meaningful translocation into host cells. Numerous models explaining how effectors are transported into host cells have been proposed, but understanding of this process is incomplete and this topic remains an active area of inquiry.

FIG. 1.

The TTSS and the flagellum. (A) Schematic of the needle-containing TTSS apparatus (left), as found in bacterial pathogens of animals, and the flagellum (right). The TTSS needle is positioned on outer membrane and inner membrane rings, as is the flagellum. For the TTSS, proteins that are conserved in the flagellar apparatus are indicated without parentheses while those that are not conserved in the flagellar apparatus are enclosed in parentheses. Question marks indicate uncertain localization of proteins. Yersinia proteins are indicated for the TTSS, except for the inner membrane ring, for which Salmonella proteins are denoted. For the flagellar system, proteins that are conserved in the TTSS are indicated without parentheses and proteins that are not conserved in the TTSS are enclosed in parentheses. Effector proteins of the TTSS are thought to travel from the bacterial cytosol through the two rings and the needle and to cross into the host cell cytoplasm through pores formed in the host cell plasma membrane by TTSS translocator proteins. The TTSS needle is ∼100-fold shorter than the flagellum. (B) Schematic of inner membrane ring components, as exemplified by Salmonella PrgK and PrgH. Putative transmembrane crossings are shown as rectangles, and cytoplasmic or periplasmic domains are shown as circles. The jagged line for PrgK indicates lipid acylation. (C) Schematic of predicted inner membrane proteins, with representation similar to that in panel B and exemplified by Yersinia proteins.

FIG. 2.

Effectors and secreted proteins of the TTSS have a secretion signal (SS) encoded in their first ∼15 mRNA codons, amino acids, or both. Physiologically significant translocation of many effectors depends on the action of TTSS chaperones, which generally bind to a chaperone-binding region that follows the secretion signal; the chaperone is shown as a circle. Effector activities, either catalytic or host cell target binding, are encoded by domains that usually follow the chaperone-binding region. Some effectors apparently have no cognate chaperones and are translocated independently of chaperone action.

FIG. 3.

Structures of N-terminal putative secretion signals. (A) Ribbon representation of YopH-N (PDB 1HUF, 1K46, 1MOV). Residues 1 to 20, the N-terminal putative secretion signal, are in blue, and residues 20 to 70, the chaperone-binding region, are in red. A bound phosphotyrosine is shown in bonds representation. This and other molecular figures were made using Molscript (127). (B) Ribbon representation of YopM (PDB 1JL5). The N-terminal putative secretion signal is encoded within residues 1 to 40, with residues 1 to 33 being disordered and residues 34 to 40 forming part of an α-helix (blue). Residues 41 to 100 (red) promote translocation.

FIG. 4.

Ribbon representation of the TTSS chaperones Yersinia SycE, Salmonella SicP, Salmonella SigE, and E. coli CesT. Chaperones are dimers, and monomer subunits are shown in red and blue. The α1, α2, and α3 helices are indicated, and the locations of patch 1 and patch 2 are indicated for SycE. Domain swapping is corrected for in an approximate way for CesT. PDB identifications are as follows: SycE, 1JYA, 1KSZ, 1N5B; SicP, 1JYO; SigE, 1K3S; and CesT, 1K3E.

FIG. 5.

Phylogenetic analysis of TTSS chaperones, which indicates a basis for distinguishing between effector chaperones and translocator chaperones. The analysis was carried out using Clustal W (226). Boxed chaperones have been characterized to be structurally similar. GenBank accession numbers: Y. enterocolitica SycN, NP_863520; Y. enterocolitica SycT, NP_863508; Y. pestis SycE, AAC62588; P. aeruginosa Orf1, AAA66490; Y. enterocolitica SycH, NP_863547; E. coli CesT, P58233; Y. enterocolitica YscB, NP_863534; Y. enterocolitica Orf155, NP_052379; E. amylovara OrfA, AAF63399; P. aeruginosa SpcU, AAP82960; Y. enterocolitica YscG, NP_863539; S. enterica serovar Typhimurium SicP, AAF63399; S. enterica serovar Typhimurium SigE, NP_460063; S. flexneri IpgE, AAP78997; Y. enterocolitica YsaK, AAK84111; S. enterica flexneri Spa15, AAP79011; S. enterica serovar Typhimurium InvB, NP_461816; S. enterica serovar Typhimurium SscB, NP_460368; Y. enterocolitica SycB, AAM47500; S. flexneri IpgC, AAP78992; S. enterica serovar Typhimurium SicA, NP_461807; E. coli CesD, NP_312603; Y. enterocolitica SycD, NP_863513; P. aeruginosa PcrH, AAO91772; P. aeruginosa Pcr4, AAC45943; Y. enterocolitica YscY, NP_863518.

FIG. 6.

Chaperone-effector fragment complexes. (A) Ribbon representation of the SicP-SptP fragment (PDB 1JYO). The chaperone SicP is in gray, and the chaperone-binding region of the effector SptP is in red (β-strands), blue (α-helices), and green (coils). Domain swapping in this complex has been corrected for in an approximate way. (B) Ribbon representation of the SycE-YopE fragment (PDB 1L2W). The chaperone SycE is in gray, and the chaperone-binding region of the effector YopE is colored as in panel A. (C) Overlay of Cα traces of the SicP-SptP fragment (blue) and SycE-YopE fragment (red). Chaperones are shown in thin lines, and effectors are shown in thick lines. Two different SptP molecules are shown (in dark and light blue) to correct for domain swapping. This panel has been reproduced from Birtalan et al. (16) with permission from Elsevier.