"One of the remarkable things about this system," says Jensen, "is that the chemoreceptors are exquisitely sensitive to changes in the concentrations of positive and negative stimuli."
Scientists believe that this sensitivity is due to the way the hundreds of chemoreceptors cluster together in the bacterial cell. "It is known that if one receptor binds a stimulus molecule, it turns on other receptors around it as well to amplify the signal," Jensen explains. "The whole system also adapts to changing conditions, dynamically adjusting the range of concentrations that it responds to."
To fully understand just what is happening in these cells, Jensen says, it is thus important to figure out the ways in which these receptors interact with one another, which in turn depends on understanding precisely how they are situated in relation to one another.
In other words, scientists need to be able to "see" the internal architecture of the bacterial cell and, in particular, how its chemoreceptors are arrayed.
Jensen and his team were able to get just such a glimpse at the chemoreceptor architecture at the macromolecular level, thanks to a state-of-the-art electron cryomicroscope that was purchased with a gift from the Gordon and Betty Moore Foundation.
"The electron cryomicroscope allows us to see the arrangement of individual proteins inside cells in a lifelike state," says Jensen. "To do this, living cells are quickly frozen so that all the proteins are frozen in placein the same places they were in the living state."
The high-tech microscope allowed the researchers to take 3-D images of intact cells through a technique called electron cryotomography. The researchers looked at some 700 tomographic imagesor tomogramsof bacterial cells, says Caltech postdoctoral scholar Ariane Briegel, the first author on the PNAS paper, and an HHMI associate. "This is the first time such
|Contact: Lori Oliwenstein|
California Institute of Technology