ScienceDaily (June 6, 2008) — David G. Thanassi, Ph.D., and co-investigators from Stony Brook University, Brookhaven National Laboratory, Washington University, and University College in London, are the first to capture a view of proteins during translocation across the bacterial outer membrane. This “molecular snapshot” may enlighten scientists to the process of protein secretion across membranes, a problem faced by all cells, and provide a foundation to understanding certain bacterial virulence factors that allow bacteria to cause disease.

The investigators used X-ray crystallography to determine the structure of a protein – called the usher – that is part of the chaperone/usher pathway and serves as a molecular scaffold for the assembly of adhesive pili by pathogenic bacteria such as E. coli. Pili are hair-like fibers that form a class of virulence factors that allow bacteria to attach to host cells and cause disease.

In addition to the crystal structure analysis of the usher, the researchers used electron microscopy imaging to capture a “molecular snapshot” of the pilus fiber during the act of secretion through the usher to the cell surface. These structures show the pilus assembly machinery in action during the bacterial outer membrane translocation process. The usher contains two channels or “twin pores.”

Dr. Thanassi said the team discovered that only one of these pores is used for secretion while the other remains closed. This discovery – and others the team anticipates to find – provides insight into the secretion of virulence factors across the outer membrane of Gram-negative bacteria.

“This ‘molecular snapshot’ is a new way to look at and analyze virulence factors assembled by bacteria,” says Dr. Thanassi of the Stony Brook University Center for Infectious Diseases, and Associate Professor, Department of Molecular Genetics and Microbiology. “The snapshot provides us with a window to viewing protein secretion across membranes and understanding how disease-causing bacteria assemble virulence factors on their surface. This method may help us to target virulence factors that allow bacteria to cause disease, which would ultimately lead to a new approach to antibiotics.”

 In an accompanying commentary to the article in the journal, scientists from the Swedish Institute for Infectious Disease Control and the Karolinska Institute in Sweden stated that the “new structures provide the first detailed view of a translocase in action [and] at astounding resolution.” The scientists further commented, “the present work combined with several other studies positions the chaperone usher pathway as the most mechanistically understood translocation process at a structural level.”

Dr. Thanassi’s co-authors on the study include: Huilin Li, Ph.D., Department of Biochemistry & Cell Biology, Stony Brook University (SBU), and Biology Department, Brookhaven National Laboratory (BNL); Han Remaut, Ph.D., Institute of Structural Molecular Biology, University College in London; Chanyan Tang, Ph.D., Biology Department, BNL; Nadine S. Henderson, Center for Infectious Diseases, SBU; Jerome S. Pinker, Department of Molecular Microbiology, Washington University; Tao Wang, Ph.D., Biology Department, BNL; Scott J. Hultgren, Ph.D., Department of Molecular Microbiology, Washington University; and Gabriel Waksman, Ph.D., Institute of Structural Molecular Biology, University College in London.

Journal reference:

1. REMAUT et al. Fiber Formation across the Bacterial Outer Membrane by the Chaperone/Usher Pathway. Cell, 2008; 133 (4): 640 DOI: 10.1016/j.cell.2008.03.033

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A model built that is based on the "molecular snapshot" of pilus assembly at the bacterial outer membrane by the twin pores usher complex. (Credit: Image courtesy of Stony Brook University Medical Center)

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