Treadmilling of a prokaryotic tubulin-like protein, TubZ, required for plasmid stability in Bacillus thuringiensis

Abstract

Prokaryotes rely on a distant tubulin homolog, FtsZ, for assembling the cytokinetic ring essential for cell division, but are otherwise generally thought to lack tubulin-like polymers that participate in processes such as DNA segregation. Here we characterize a protein (TubZ) from the Bacillus thuringiensis virulence plasmid pBtoxis, which is a member of the tubulin/FtsZ GTPase superfamily but is only distantly related to both FtsZ and tubulin. TubZ assembles dynamic, linear polymers that exhibit directional polymerization with plus and minus ends, movement by treadmilling, and a critical concentration for assembly. A point mutation (D269A) that alters a highly conserved catalytic residue within the T7 loop completely eliminates treadmilling and allows the formation of stable polymers at a much lower protein concentration than the wild-type protein.When expressed in trans, TubZ(D269A) coassembles with wild-type TubZ and significantly reduces the stability of pBtoxis, demonstrating a direct correlation between TubZ dynamics and plasmid maintenance. The tubZ gene is in an operon with tubR, which encodes a putative DNA-binding protein that regulates TubZ levels. Our results suggest that TubZ is representative of a novel class of prokaryotic cytoskeletal proteins important for plasmid stability that diverged long ago from the ancient tubulin/FtsZ ancestor.[Keywords: FtsZ; tubulin; cytoskeleton] Supplemental material is available at http://www.genesdev.org.

Dynamics of bacterial cytoskeletal elements

“…In B. thuringienis, TubZ forms filaments of several lm (2-5) length [Larsen et al, 2007]. Interestingly, the filaments are highly dynamic and show treadmillinglike extension at the plus ends (average of 1.48 6 0.63 lm/min) and depolymerisation at the minus ends.…”

Modeling Three-Dimensional Spatial Regulation of Bacterial Cell Division (Dissertation)

“…Protein filaments confined to spherical or rod-shaped cell membranes are known to arrange into helical (Shih et al, 2003), ring (Bi and Lutkenhaus, 1991) and line (Larsen et al, 2007) structures. In the cytoplasm, subunits can assemble into tubules, asters and stars (Mejillano et al, 2004;Haviv et al, 2006).…”

One-way ticket to the cell pole: Plasmid transport by the prokaryotic tubulin homolog TubZ

“…Although evolutionarily unrelated, both ParA and ParM proteins are polymerization-based engines that assemble into cytoskeletal structures involved in DNA transport, positioning, and segregation (3,4). Type III systems were identified more recently: this discovery has opened up unforeseen perspectives on prokaryotic cytoskeletal proteins mediating genome segregation (7)(8)(9). In PNAS, Ni et al shed light on the molecular mechanisms underpinning the function of type III partition cassettes by reporting the structures of TubR and TubZ proteins encoded by the pBtoxis plasmid from Bacillus thuringiensis (10).…”

“…Type III segregation modules are found on large plasmids of the Bacillus cereus group of bacteria (7,9). The partition cassette of plasmid pBtoxis harbors two genes: orf157 encoding an 11.6-kDa specific DNA-binding protein, known as TubR, and orf156 encoding a 54.4-kDa protein, designated as TubZ, which is a distant homolog of both tubulin and FtsZ (7,8).…”

“…The partition cassette of plasmid pBtoxis harbors two genes: orf157 encoding an 11.6-kDa specific DNA-binding protein, known as TubR, and orf156 encoding a 54.4-kDa protein, designated as TubZ, which is a distant homolog of both tubulin and FtsZ (7,8). Both orf157 and orf156 are necessary for plasmid maintenance (11).…”