Bacteria contain compartments

Dec. 21, 2012

A new study reveals that bacterial cells can contain internal compartments, indicating that the single-celled organisms are more complex than previously thought. The finding by researchers from Princeton University and colleagues in Germany was published in the Dec. 7 issue of Cell.

Caulobacter crescentus grows a long stalk that it uses to attach to surfaces. Researchers at Princeton and in Germany have found that the bacterium grows internal protein barriers (green, left) that prevent diffusion between the cell body and the stalk. When genetic mutations block the formation of the barriers (right), proteins can diffuse freely between the cell body and stalk. Photo credit: Cell.  The researchers discovered the compartments in certain crescent-shaped bacteria, which normally live in freshwater streams and lakes. When the organism -- known as Caulobacter crescentus -- is ready to settle down and reproduce, it grows a stalk to attach itself to a rock at the bottom. The researchers discovered barriers that prevent cellular ingredients from moving freely between the stalk and cell body.

"In more complex organisms, we are accustomed to finding membrane-bound compartments such as the nucleus, where DNA is stored," said Zemer Gitai, associate professor of molecular biology and leader of the Princeton team, which identified the presence of the barriers. "We've shown that these bacteria have some of the same characteristics as more complex organisms."

The compartments could provide a growth advantage by allowing the bacteria to make only the proteins that are needed in the cell body, without having those proteins float down into the stalk where they are not needed, Gitai said.

Gitai's team at Princeton included Eric Klein, a postdoctoral researcher in the department of molecular biology at Princeton. The Princeton researchers worked with scientists from Germany, led by Martin Thanbichler at the LOEWE Center for Synthetic Microbiology, and others at the Max Planck Institute for Terrestrial Microbiology.

The Princeton investigators visualized the barriers by marking the proteins inside the cell body and stalk with fluorescent tags that give off a red color when exposed to green light. When the light is very intense, the tags lose the ability to give off the red color, a phenomenon called “bleaching.”

To visualize the diffusion barrier in Caulobacter crescentus, cells expressing a red-fluorescent tag were exposed to light in the region indicated by yellow circles, either in the cell body (first frame of Panel A) or the stalk (first frame of Panel B). The light caused tagged proteins to first fluoresce red and then lose all color, or "bleach," over time. When light was shone on the cell body (Panel A), the cell body bleached but the stalk retained its fluorescence, and when light was shone on the stalk (Panel B), the stalk bleached but the cell body was still able to fluoresce, indicating the presence of a barrier separating these domains. Image credit: Cell.Klein directed a thin beam of green light onto only one spot in the cell body over a period of several seconds. He observed that eventually all of the proteins in the cell body drifted into the light, emitted the fluroescent red color, and bleached. However, the proteins in the stalk were unable to float into the lit area, so they retained their ability to turn red. From these results, the researchers inferred the presence of a barrier between the cell body and the stalk.

Meanwhile, in Germany, Susan Schlimpert of the Max Planck Institute was investigating the nature of proteins found in the stalk. Schlimpert and her colleagues looked for genes that were active when the stalk was growing, and identified four types of proteins that they named “Stalk proteins A, B, C, and D.” When the researchers inactivated the genes that code for these stalk proteins, the barrier did not form.

The researchers do not know how many types of bacteria contain similar protein-mediated structures, but such barriers do occur in other types of cells that contain both a main body and a long thin projection. For example, the cells that line the human gastrointestinal tract have a cell body and long thin projections called cilia, each of which contains a protein barrier at the base.

Funding for the project came from the National Institute of General Medical Sciences, which supported Eric Klein's postdoctoral fellowship. Zemer Gitai was funded by the National Institutes of Health (NIH) and the Human Frontier Science Program. Additional funds came from the Max Planck Society.


General Protein Diffusion Barriers Create Compartments within Bacterial Cells. Cell, Volume 151, Issue 6, 7 December 2012, Pages 1270-82.