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.