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Maintenance of an unfolded polypeptide by a cognate chaperone in bacterial type III secretion

Nature volume 414, pages 77–81 (2001)Cite this article

Abstract

Many bacterial pathogens use a type III protein secretion system to deliver virulence effector proteins directly into the host cell cytosol, where they modulate cellular processes1,2. A requirement for the effective translocation of several such effector proteins is the binding of specific cytosolic chaperones, which typically interact with discrete domains in the virulence factors3,4,5. We report here the crystal structure at 1.9 Å resolution of the chaperone-binding domain of the Salmonella effector protein SptP with its cognate chaperone SicP. The structure reveals that this domain is maintained in an extended, unfolded conformation that is wound around three successive chaperone molecules. Short segments from two different SptP molecules are juxtaposed by the chaperones, where they dimerize across a hydrophobic interface. These results imply that the chaperones associated with the type III secretion system maintain their substrates in a secretion-competent state that is capable of engaging the secretion machinery to travel through the type III apparatus in an unfolded or partially folded manner.

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Figure 1: Protease footprinting delineates the chaperone-binding sequence of SptP.
Figure 2: Each molecule of SptP binds as an unfolded polypeptide to three SicP chaperones.
Figure 3: Different portions of SptP bind identical motifs in SicP primarily through hydrophobic interactions.
Figure 4: The tetrameric SicP arrangement presents an extensively hydrophobic surface that is buried on SptP binding.
Figure 5: The hydrophobic dimerization of SptP and the extensive homodimeric SicP interface hold the complex together.

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Acknowledgements

We thank the following people for access to and assistance with crystallographic equipment: at Yale University School of Medicine, P. Pepin of the Macromolecular X-ray Crystallography Facility, and S. Kunchaparty and A.-M. Quinn of the Research Computing Group for workstation access; at the CHESS, D. Szebenyi and the MacChess staff; B. Sweet, A. Saxena and the staff of Brookhaven beamline X12C. We also acknowledge N. Papavasiliou, P. Jeffrey, S. Fugmann and members of the Galán laboratory for discussions and critical reading of this manuscript before submission. C.E.S. was supported by a fellowship of the Cancer Research Fund of the Damon Runyon-Walter Winchell Foundation. This work was funded by Public Health Services grants to J.E.G.

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Author notes

  1. C. Erec Stebbins

    Present address: Laboratory of Structural Microbiology, The Rockefeller University, Box 52, 1230 York Avenue, New York, New York, 10021, USA

  2. Correspondence and requests for materials should be addressed to J.E.G. Coordinates have been deposited in the Protein Data Bank under accession number RCSB014340.

Authors and Affiliations

  1. Section of Microbial Pathogenesis, Boyer Center for Molecular Medicine, Yale School of Medicine, New Haven, 06536, Connecticut, USA

    C. Erec Stebbins & Jorge E. Galán

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Correspondence to Jorge E. Galán.

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Stebbins, C., Galán, J. Maintenance of an unfolded polypeptide by a cognate chaperone in bacterial type III secretion. Nature 414, 77–81 (2001). https://doi.org/10.1038/35102073

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