Eukaryotic cells are nicely organized - all the important bits are housed in the nucleus or associated with other organelles. But, what about bacteria? How do they keep everything organized without organelles?
For many years, people assumed that bacteria were just little bags filled with genetic material, proteins, and other molecules. The other assumption was that that all of these components just sloshed about willy-nilly without any strict organizational scheme.
It turns out, however, that the innards of bacterial cells are surprisingly well-organized. A short review on the subject was recently authored by Sutharsan Govindarajan and Orna Amster-Choder in Current Opinion in Microbiology.
The cell poles of rod-shaped bacteria are one hub at which proteins accumulate and organize themselves. The poles are particularly important for localizing sensory systems. In E. coli, the receptors for chemotaxis and phosphotransferase (carbon uptake) systems, for example, organize themselves at the cell poles. The fact that both of these systems localize to the same part of the cells means it’s easy for them to communicate with each other - the two systems have to work together to make sure the bacteria find the right nutrients.
In addition to the poles, the protein MreB helps confer an organizational structure to the bacterial cell. MreB is an actin homolog that has been likened to a bacterial cytoskeletal protein, shuttling proteins to and fro.
MreB is attached to the cytoplasmic face of the inner membrane. One of MreB’s jobs in E. coli is to relocate penicllin binding proteins from one part of the cell to another so that they can remodel the cell wall. In Caulobacter crescentus, however, MreB localizes the protein PilT to the cell poles.
Finally, the authors discuss how RNA is organized in the bacterial cell. These same authors showed that, in E. coli, some mRNAs localized to the same region of the cell as their protein products (a process that was shown to be independent of translation). Specifically, mRNAs coding for membrane proteins accumulated around the cell’s circumference, those coding for cytoplasmic proteins assembled in a helical pattern, and mRNAs for polar proteins assembled at (you guessed it), the poles.
Many questions remain. For example, how do proteins know to go to the cell pole? How many other proteins does MreB interact with? And finally, how do specific mRNAs know where to localize in the cell?