Jensen and his team wanted to use the technique to observe how V. cholerae cells segregate two duplicate copies of their genetic material before dividing. Instead, they noticed relatively large tubelike structures spanning the entire width of the cells. And they had no idea what the structures were.
Jensen started sharing preliminary images of the mysterious structures in lectures around the country, asking if anyone knew what they might be. Finally, someone suggested that he talk to John Mekalanos of Harvard Medical School, who was involved in the original discovery of the type VI secretion system. After Martin Pilhofer, a postdoctoral scholar in Jensen's lab, comprehensively imaged the system and conducted additional investigations, Mekalanos's group became convinced that the tubelike structures might actually help the bacteria translocate proteins.
The Mekalanos lab made a version of V. cholerae lacking one of the proteins that makes up the tube structure. With that protein knocked out, the type VI secretion system disappeared. In another experiment, they attached fluorescent tags to the proteins and were actually able to watch the structures form and contract within living cells.
"When the tube contracts, that's when it shoots," says Pilhofer. "That result agrees well with what we had seen using the electron cryomicroscope, where we observed long tubular structures in two different conformationsextended and contracted. Whereas electron cryomicroscopy allowed us to observe the secretion apparatus at high resolution, the fluorescence study gave us more insight into the dynamics
|Contact: Deborah Williams-Hedges|
California Institute of Technology