A model built that is based on the "molecular snapshot" of pilus assembly at the bacterial outer membrane by the twin pores usher complex. 

David G. Thanassi, PhD, and co-investigators from StonyBrookUniversity, Brookhaven National Laboratory, WashingtonUniversity, and UniversityCollege 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. Their findings are reported in the current edition of 

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.”

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 Thanassi, of the StonyBrookUniversityCenter for Infectious Diseases, and associate professor in the 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.”

 article, titled “Fiber Formation across the Bacterial Outer Membrane by the Chaperone/Usher Pathway.” 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.”

Source: Stony BrookUniversityMedicalCenter