R. Carballido-Lopez, Front. Biosci. 4S:1582–1606, 2012. With permission Frontiers in Bioscience.) plant cells (see Figure 19–65). As with FtsZ, MreB and Mbl filaments are highly dynamic, with half-lives of a few minutes, and nucleotide hydrolysis accompanies the polymerization process. Mutations disrupting MreB or Mbl expression cause extreme abnormalities in cell shape and defects in chromosome segregation (Figure 16–8B).
Relatives of MreB and Mbl have more specialized roles. A particularly intriguing bacterial actin homolog is ParM, which is encoded by a gene on certain bacterial plasmids that also carry genes responsible for antibiotic resistance and cause the spread of multidrug resistance in epidemics. Bacterial plasmids typically encode all the gene products that are necessary for their own segregation, presumably as a strategy to ensure their inheritance and propagation in bacterial hosts following plasmid replication. ParM assembles into filaments that associate at each end with a copy of the plasmid, and growth of the ParM filament pushes the replicated plasmid copies apart (Figure 16–9). This spindle-like structure apparently arises from the selective stabilization of filaments that bind to specialized proteins recruited to the origins of replication on the plasmids. A distant relative of both tubulin and FtsZ, called TubZ, has a similar function in other bacterial species.
Thus, self-association of nucleotide-binding proteins into dynamic filaments is used in all cells, and the actin and tubulin families are very ancient, predating the split between the eukaryotic and bacterial kingdoms.
At least one bacterial species, Caulobacter crescentus, appears to harbor a protein with significant structural similarity to the third major class of cytoskeletal filaments found in animal cells, the intermediate filaments. A protein called crescentin forms a filamentous structure that influences the unusual crescent shape of this species; when the gene encoding crescentin is deleted, the Caulobacter cells grow as straight rods (Figure 16–10).
ParM plasmid origin of ParM ParR monomers replication flaments proteins
Figure 16–9 Role of the actin homolog ParM in plasmid segregation in bacteria.
(A) Some bacterial drug-resistance plasmids (orange) encode an actin homolog, ParM, that will spontaneously nucleate to form small, dynamic filaments (green) throughout the bacterial cytoplasm. A second plasmid-encoded protein called ParR (blue) binds to specific DNA sequences in the plasmid and also stabilizes the dynamic ends of the ParM filaments. When the plasmid duplicates, both ends of the ParM filaments become stabilized, and the growing ParM filaments push the duplicated plasmids to opposite ends of the cell. (B) In these bacterial cells harboring a drug-resistance plasmid, the plasmids are labeled in red and the ParM protein in green. Left, a short ParM filament bundle connects the two daughter plasmids shortly after their duplication. Right, the fully assembled ParM filament has pushed the duplicated plasmids to the cell poles. (A, adapted from E.C. Garner,
C.S. Campbell and R.D. Mullins, Science 306:1021–1025, 2004; B, from J. Møller-Jensen et al., Mol. Cell 12:1477–1487, 2003. With permission from Elsevier.)