Motility is not everything. Another major force that shapes cells is metabolism. Nutrients are often scarce, and increasing your cell’s ability to absorb them can give it a boost in the competitive game of life. So how can you do that? Remember that a sphere maximizes volume relative to surface area. To maximize surface area (for nutrient uptake) relative to volume, you would instead want something spikier. Some bacteria extend prosthecae (“add-ons” or appendages) for this purpose. Some, like Caulobacter crescentus, use a single prostheca, which is also called a stalk. Stalks are commonly located at the pole of the cell, where, as you’ll see in Chapter 8.4, they can help cells attach to a surface and hang on even in turbulent flow. Other species have a stalk on either end. Still others, like this Verrucomicrobium spinosum, form astral shapes with prosthecae jutting out in all directions.
Prosthecae offer an architectural challenge: thin extensions are delicate. Prosthecate cells use cytoskeletal proteins to form and stabilize their stalks, although exactly how this works remains unclear. One of these cytoskeletal proteins is Bactofilin (⇩), which is similar to the proteins that make intermediate filaments in eukaryotes. C. crescentus use Bactofilin polymers to help make their stalks. Prosthecobacter contain a different cytoskeletal element–microtubules–in their stalks, the function of which remains unclear (⇩).
Bactofilins are found in many species of bacteria and archaea, suggesting that they perform diverse (and currently unknown) functions. They polymerize into very stable filaments with a triangular beta-helical structure, like this one from Thermus thermophilus [32]. Bactofilin filaments lack two hallmarks of actin- and tubulin-based cytoskeletal elements: polarity and dynamic assembly/disassembly. In this way, they are similar to intermediate filaments in eukaryotic cytoskeletons.
Some bacterial species with prosthecae express structures similar to eukaryotic microtubules, made from two proteins called BtubA and BtubB to reflect their homology to eukaryotic tubulins. Eukaryotic microtubules are hollow tubes formed by 13 protofilaments; bacterial microtubules are smaller, with only ~5 protofilaments. Cells commonly contain a bundle of microtubules in their prosthecae, like this Prosthecobacter vanneervenii cell, which has a bundle of four.
Prosthecobacter belong to an evolutionarily unique group of species that share characteristics unusual in the rest of the bacterial phylogenetic tree. We refer to the collective group as the PVC superphylum (because it contains Planctomycetes, Verrucomicrobia, and Chlamydiae). Having homologs of eukaryotic microtubule proteins is one of these unique characteristics; so far, Btubs have only been identified in species of Prosthecobacter. They seem to have come from a horizontal gene transfer from a eukaryotic cell (meaning that microtubules evolved first in eukaryotes and were later borrowed by the bacteria).
