Only in the past two decades has it become widely accepted that bacteria are internally organized and possess a complex multi-faceted cytoskeleton similar to eukaryotic cells. The proteins that make up the bacterial cytoskeleton can largely be grouped into families based on homology to the major types of eukaryotic cytoskeletal filaments: tubulin-like, actin-like, and intermediate filament-like. Additionally, bacteria are host to some filament systems that lack recognizable homology to eukaryotic cytoskeletons.
The first bacterial cytoskeletal element discovered was FtsZ, a tubulin homolog found in most bacteria that assembles into a ring-like complex at the site of cell division [1
]. Like its eukaryotic relative, FtsZ polymerizes in a GTP-dependent manner, and the structure of the monomers bears significant similarity to α- and β-tubulins [2
]. Proteins in the actin-like family include MreB and ParM, which maintain cell shape in non-spherical bacteria and carry out DNA partitioning, respectively [6
]. These proteins are not strong amino acid sequence matches for actin, but do possess strong structural conservation with actin [9
]. A recent bioinformatic study identified 35 different families of bacterial actin homologs, suggesting that there may be significant divergence in the sequences, structures, and functions of the members of the newly identified bacterial actin superfamily [10*
]. Finally, a prokaryotic homolog of intermediate filaments, crescentin, was discovered in Caulobacter crescentus
. In this species, crescentin helps maintain a slight curve in the overall cell shape [11
], while other Coiled-Coil Rich Proteins (CCRPs) have been implicated as intermediate-filament-like proteins with diverse localizations and functions in other species [12
]. More recently, several types of bacterial proteins unrelated to actin, tubulin, and intermediate filaments have been found to form filamentous structures. These newly appreciated filaments include bacterial-specific families such as the Walker A Cytoskeletal ATPases (WACA), which include the DNA segregation protein ParA and the division site placement protein MinD, and the bactofilin family of polymerizing proteins [13
In addition to the “classical” cytoskeletal elements, there is a growing list of proteins that are conserved throughout bacteria, archaea, and eukaryotes and have previously characterized functions, but upon further investigation turn out to self-assemble in filamentous polymers. This new class primarily consists of enzymes that would not necessarily be expected to polymerize based on their cellular functions. Several recent reviews have focused on the non-enzymatic members of the bacterial cytoskeleton and the evolutionary relationships between bacterial and eukaryotic cytoskeletal elements [14
]. Therefore, here we have decided to focus on the emerging class of filament-forming enzymes. We focus on the evidence that these proteins form filaments both in vivo
and in vitro
, the functional significance of polymerization for enzyme function, and the ability of cells to co-opt polymerizing enzymes for secondary functions. We also discuss how such dual-role enzymes may have served as the evolutionary precursors for the modern cytoskeleton.