Abstract
Actin-like bacterial cytoskeletal element MreB has been shown to be essential for the maintenance of rod cell shape in many bacteria. MreB forms rapidly remodelling helical filaments underneath the cell membrane in Bacillus subtilis and in other bacterial cells, and co-localizes with its two paralogs, Mbl and MreBH. We show that MreB localizes as dynamic bundles of filaments underneath the cell membrane in Drosophila S2 Schneider cells, which become highly stable when the ATPase motif in MreB is modified. In agreement with ATP-dependent filament formation, the depletion of ATP in the cells lead to rapid dissociation of MreB filaments. Extended induction of MreB resulted in the formation of membrane protrusions, showing that like actin, MreB can exert force against the cell membrane. Mbl also formed membrane associated filaments, while MreBH formed filaments within the cytosol. When co-expressed, MreB, Mbl and MreBH built up mixed filaments underneath the cell membrane. Membrane protein RodZ localized to endosomes in S2 cells, but localized to the cell membrane when co-expressed with Mbl, showing that bacterial MreB/Mbl structures can recruit a protein to the cell membrane. Thus, MreB paralogs form a self-organizing and dynamic filamentous scaffold underneath the membrane that is able to recruit other proteins to the cell surface.
Highlights
Prokaryotes show an amazing variety of different cell shapes, as do many eukaryotic single cell organisms
The loss of MreB in B. subtilis can be compensated by the addition of high concentrations of magnesium and sucrose to the medium [13], indicating that MreB may play a mechanical function in bacterial cells by stabilizing the cell membrane
When B. subtilis MreB was expressed in the heterologous system, a YFP-MreB fusion formed up to 7 mm long filaments as soon as 30 min after induction of transcription, all of which were exclusively localized underneath the cell membrane (Fig. 1A and 1B, and movie S1)
Summary
Prokaryotes show an amazing variety of different cell shapes, as do many eukaryotic single cell organisms. MreB localizes as helical filaments underneath the cell membrane [2,3,4,5,6,7], which in Bacillus subtilis and in Caulobacter crescentus are highly dynamic and appear to extend at one end and retract at the other end (and appear to move along helical tracks) by a ratchet like mechanism [8,9]. Their dynamic remodelling is important for the function in cell wall maintenance [10]. MreB and actin share only 14% sequence identity, which mostly comprises the conserved ATP binding pocket; the surfaces of MreB and actin do not share any significant sequence similarity [16], such that specific interactions of MreB with actin-interacting proteins are unlikely
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