“The fact that we found this cytoskeletal element in bacteria was really exciting,” says Jacobs-Wagner. It meant that eukaryotes and bacteria constructed their cytoskeletons from the same parts, hinting that structure performs similar functions in both kinds of cells.

One of those functions, Jacobs-Wagner's team demonstrated in the same study, is shaping the cell. After tagging crescentin, they found that it was in the right position to help bend cells of the water-dwelling bacterium Caulobacter. The protein forms a spiral structure along the inner surface of the curved cell. When the researchers removed the protein, the cell straightened out. In a paper published in EMBO Journal in 2009, Jacobs-Wagner and colleagues clarified how crescentin works—it spurs some parts of the bacterial cell wall to grow faster than others.

Jensen's lab is one of the few in the world using ECT to scrutinize bacteria. He's trained the microscope on everything from the chemical receptors that allow a bacterium to sense food—in essence, the cell's nose—to the base of the flagellum, the bacterial propeller. His work has linked structural subtleties of bacteria to cellular functions such as movement and division.

In a 2006 study published in Science, for example, his group teamed with HHMI investigator Dianne Newman, also of Caltech at the time, to take a close look at how bacteria arrange their organelles. Bacteria lack most of the organelles found in eukaryotes, such as mitochondria and chloroplasts, but some species harbor organelles called magnetosomes. These bags of iron-containing crystals serve as a compass and usually line up in chains along the length of the cell.

ECT revealed that one type of cytoskeletal filament made up of a protein called MamK, an equivalent of actin in eukaryotic cells, flanks the magnetosome chains and helps shepherd them into formation. If the protein is absent, magnetosomes scatter, and the bacteria lose their sense of direction. “It's one of the first cases in which we visualized a filament in a bacterial cell and saw that it was being used to position an organelle,” says Jensen. Cytoskeletal filaments perform the same job in eukaryotes.

Jensen's results have also suggested a new explanation for how a cell cuts itself in two during division. The researchers homed in on the protein FtsZ, the bacterial equivalent of the eukaryotic cytoskeletal protein tubulin. Previous studies suggested that FtsZ proteins wrapped around the midsection of the cell. When a bacterium was ready to divide, researchers assumed that the protein ring tightened like a belt and eventually pinched the cell in half. But ECT images showed that the filaments were too short and too disorganized to form the continuous loops necessary for belt tightening. In a 2007 paper, Jensen and colleagues proposed that the filaments gradually constrict the cell wall by repeatedly straightening and bending, an alternative they call iterative pinching.

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