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Scientists define structures Salmonella use to invade cells
Edward R. Winstead

When bacteria enter another organism, they inject protein into host cells. This requires the guidance of 'chaperone' molecules that bind the secreted protein and guide it through the tip of a needle-like appendage. The chaperone is an escort that discourages unintended contacts with other molecules and preserves the structural integrity of the protein. A new study reports on the superstructures created when chaperone molecules bind secreted proteins from Salmonella bacteria.

Protease footprinting reveals a chaperone-binding sequence. View larger

Using Salmonella as a model for investigating bacterial delivery systems, C. Erec Stebbins and Jorge Galán of Yale School of Medicine in New Haven, Connecticut, characterized the three-dimensional structures that Salmonella assume when secreting protein. The structural analysis revealed grooves and crevices in the chaperone molecules that allow intricate connections with fragments of the secreted protein. This information could be useful to researchers developing ways to block the delivery of protein by harmful bacteria.

The Salmonella system for delivering proteins is one of the more complex protein-secretion systems known. It comprises about 20 proteins and a family of chaperone molecules. "Each chaperone is specific in most cases for one or, in a few cases, two secreted proteins," the researchers write in Nature.

The authors of a commentary entitled "Bacteria thread the needle" observe that molecular chaperones, like human chaperones, "have the job of preventing their charges (in this case proteins) from making unacceptable non-productive interactions, while encouraging them to encounter and interact with acceptable partners."

Detail from cartoon of chaperone-binding complex. View larger

The commentators, Craig L. Smith and Scott J. Hultgren of Washington University School of Medicine in St. Louis, write that the new study "reveals fundamental clues about chaperone function and mechanism of action, and will influence our understanding of the basic principles of protein folding and pathogenesis."

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Smith, C.L. & Hultgren, S.J. Bacteria thread the needle. Nature 414, 29-30 (November 1, 2001).
Stebbins, C.E. & Galán, J.E. Maintenance of an unfolded polypeptide by a cognate chaperone in bacterial type III secretion. Nature 414, 77-81 (November 1, 2001).

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