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The Role of Specialized Secretion Chaperones in Type III Delivery

Type III secretion systems (T3SS) are central to the virulence of a wide variety of animal and plant pathogens and commensals, including such human scourges as typhoid fever, the plague, and bacillary dysentery. T3SS are used to translocate bacterial proteins into host cells. The virulence factor substrates of T3SS are biochemically diverse, manipulating host cell biological systems such as cytoskeletal structure, signal transduction, cell cycle progression, and programmed cell death, allowing bacteria to precisely modulate host tissues and systems for the benefit of the pathogen.

The pathogenic T3SS substrates are multi-domain proteins that are typically subdivided into two distinct regions: an N-terminal domain that contains secretion and translocation signals that function within the bacterium, and one or more C-terminal domains that harbor the host cell effector activities. The first 15-20 amino acids are known to be required for secretion, although controversy exists concerning their nature as a peptide and/or mRNA signal. Following the secretion signal is a small 50-100 amino acid domain that is responsible for binding secretion chaperones in the bacterium and targeting the virulence factors to the pathogenic secretion system.

Structure characterization of these chaperones and their complexes with virulence factors has revealed two important concepts: (i) that the secretion chaperones share a very similar fold and general mode of binding virulence factors, and therefore constitute a diversified but highly related family, and (ii) that the secretion chaperones bind their cognate virulence factors by interacting with an extended, non-globular N-terminal peptide that wraps around the chaperone dimer. The details of this interaction include the burying of large hydrophobic surfaces on the chaperones by the virulence factor-extended peptides as well as the interesting observation that the three-dimensional path of the non-globular peptide is generally similar.


Gram negative bacteria possessing T3SS express thousands of proteins, and yet only a handful are secreted through these systems. Despite the high degree of structural and functional conservation in the chaperones, there has been little progress toward synthesizing a unified view of how these substrates are targeted to their chaperones or to the pathogenic T3SS. In particular, no common amino acid motifs have been identified to play such roles, although, in the context of the chaperone-CBD complexes, generalized similarities in the hydrophobic interactions between chaperones and their substrates have been observed.

We have identified a structural motif that may direct virulence factors to their cognate chaperones in a diverse range of pathogenic bacteria. Disruption of this structural motif leads to a destabilization of several chaperone-substrate complexes from different species, as well as an impairment of secretion in Salmonella. We favor an “adaptor-targeting” model for the delivery of secretion substrates to the pathogenic T3SS. In this model, the β motif of the CBD plays the role of a conserved adaptor element that targets the virulence substrate to the chaperone. The chaperone alone, or in concert with the far N-terminal signal peptide and other potential signals in the CBD, then acts as a secondary adaptor that docks the substrate to the secretion apparatus. Such a system would convey an informational signal that distinguishes the T3SS substrates from all other proteins in the bacterium but also provides many avenues for the regulation of virulence factor delivery.


M. Lilic, M. Vujanac, and C.E. Stebbins (2006). "A Common Structural Motif in the binding of Virulence Factors to Bacterial Secretion Chaperones." Mol. Cell. 21(5):653-64.
[Abstract] [pdf] [pdb SipA N-terminal domain] [pdb SipA-InvB complex]

C.E. Stebbins. (2005) "Structural microbiology at the pathogen-host interface." Cell Microbiol. 2005 Sep;7(9):1227-36. [Abstract] [pdf]

C.E. Stebbins. (2005). "Type III Secretion Machinery and Effectors." In Structural Biology of Bacterial Pathogenesis, G. Waksman, M. Caparon, and S. Hultgren, eds. (Washington, ASM Press), 149-177.

C.E. Stebbins and J.E. Galán. (2003). "Priming virulence factors for delivery into the host." Nat Rev Mol Cell Biol, 4, 738-742.
[Abstract] [pdf]

C.E. Stebbins and J.E. Galán. (2001). "Maintenance of an unfolded polypeptide by a cognate chaperone in bacterial type III secretion." Nature, 414, 77-81.
[Abstract] [pdf] [pdb]


Drs. Mirjana Lilic and Milos Vujanac were the lead scientists on the SipA-InvB, β motif  project.