3.4 Vibrioid Atlas of Bacterial and Archaeal Cell Structure Home
Source: Lynch et al. (2017) Structure: PDB 5U3C

CTP synthase

CTP synthase is a ubiquitous metabolic protein found throughout all domains of life that helps make the building blocks of RNA and DNA. It also polymerizes into filaments. In eukaryotes, the filament structure activates the enzyme. In bacteria, the filament structure (shown here for Escherichia coli [31]) inhibits the enzyme’s metabolic function. Polymerization of enzymes is fairly common, providing an elegant way to quickly regulate the activity of a protein that may not always be needed, but would be costly or slow to degrade and synthesize again. You will see another example in Chapter 4. In the case of CTP synthase, the cytoskeletal role likely arose secondarily; once you have a long filament lying around, why not use it as a scaffold?

Vibrioid

What if you want to curve your rod-shaped cell into a comma? Vibrioid shape (named for the genus Vibrio, where it is common) may help cells swim faster. To make a vibrioid cell wall, you can imagine the contractor simply telling the workers to incorporate more material on one side of the rod relative to the other. In Caulobacter crescentus like this one, this is the function of two cytoskeletal proteins. The first, called (for an obvious reason) Crescentin, inhibits cell wall synthesis. It is kept in check by the second, called CTP synthase. (As its name implies, CTP synthase has another, metabolic, function in the cell (⇩).) The balance between the two makes sure the cell curves, but not too much. Such checks and balances are a common theme in nature. Both cytoskeletal proteins localize to one side of the cell, resulting in more cell wall growth on the opposite side, and a curved cell. Here you can see a bundle of CTP synthase filaments on the inner curvature of the cell. The form of Crescentin is more elusive; we only see obvious filaments when it is artificially overexpressed, so its exact structure in the cell remains unclear.

This system is only one way of making a curved cell. There must be others since many vibrioid species (including Vibrio!) do not use Crescentin. As you will see throughout this book, there is no shortage of biological questions still to be figured out.

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