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The Destroyers

University of Washington team uses NMR to uncover reactions in Salmonella

This reaction scheme shows how bacteria create a ubiquitin chain inside a host cell. The chain, which marks a protein for destruction by the host cell, is grown using a see-saw approach, where the chain is passed between two active sites on ubiquitin ligase.

Virulence proteins from Salmonella typhimurium can hijack a healthy host cell’s signaling pathways and use those pathways to enhance infection. One signaling pathway involves modification of key host proteins with ubiquitin, which can trigger the cell to inactivate or destroy its own proteins used for defense against invading pathogens. Several of the virulence proteins in this Gram-negative bacteria can subvert ubiquitin signaling pathways by functioning as enzymes that attach ubiquitin directly onto key proteins in the host cell. Once tagged, the proteins can be destroyed by the cell.

The researchers were able to gain molecular-level understanding of bacterial proteins involved in these processes using the 600-, 800-, and 900-MHz nuclear magnetic resonance (NMR) spectroscopy systems at the Department of Energy’s EMSL, along with the 500-MHz NMR and mass spectrometers at the University of Washington.

Scientific impact: This study enables a fundamental understanding of the novel mechanism by which S. typhimurium E3 ubiquitin-ligase SspH2 recognizes features of host protein and uses ubiquitin transfer reactions to rapidly build ubiquitin chains. This mechanism may allow the invading bacteria to efficiently tag host proteins for destruction and to compete with the host cell’s own enzymes. This research is part of EMSL’s ongoing work to characterize surfaces and interfaces in large, multi-component complexes with unprecedented spatial resolution.

Societal impact: New information about how pathogenic bacteria invade human
cells and evade cellular defenses may provide health researchers a new approach
for developing methods to treat bacterial infections.

Reference: Levin I, C Eakin, MP Blanc, RE Klevit, SI Miller, and PS Brzovic. 2010. “Identification of an Unconventional E3 Binding Surface on the UbcH5 ∼ Ub Conjugate Recognized by a Pathogenic Bacterial E3 Ligase.” Proceedings of the National Academy of Sciences 107(7):2848-2853.

Acknowledgments: This work was funded by the National Science Foundation and the National Institutes of Health.

User Proposal: 25658

Released: April 21, 2010