Abstract
SummaryBacterial actin MreB is one of the key components of the bacterial cytoskeleton. It assembles into short filaments that lie just underneath the membrane and organize the cell wall synthesis machinery. Here we show that MreB from both T. maritima and E. coli binds directly to cell membranes. This function is essential for cell shape determination in E. coli and is proposed to be a general property of many, if not all, MreBs. We demonstrate that membrane binding is mediated by a membrane insertion loop in TmMreB and by an N-terminal amphipathic helix in EcMreB and show that purified TmMreB assembles into double filaments on a membrane surface that can induce curvature. This, the first example of a membrane-binding actin filament, prompts a fundamental rethink of the structure and dynamics of MreB filaments within cells.
Highlights
MreB filaments appear to form long spirals along the length of rod-shaped cells (Carballido-Lopez and Errington, 2003; Figge et al, 2004; Gitai et al, 2004; Jones et al, 2001; Slovak et al, 2005; Vats and Rothfield, 2007), but two recent reports suggest that in Bacillus subtilis these are composed of short, dynamic filaments that are driven by progression of the cell wall synthesis machinery (Domınguez-Escobar et al, 2011; Garner et al, 2011)
We show that MreBs from both T. maritima and E. coli interact directly with membranes and that this is mediated by a membrane insertion loop in TmMreB and an N-terminal amphipathic helix in EcMreB
We show that TmMreB assembles into filament doublets on a membrane surface, and that these can induce negative curvature in purified vesicles
Summary
TmMreB filaments are seen in the crystal structure (van den Ent et al, 2001) and closely resemble those in F-actin.MreB filaments appear to form long spirals along the length of rod-shaped cells (Carballido-Lopez and Errington, 2003; Figge et al, 2004; Gitai et al, 2004; Jones et al, 2001; Slovak et al, 2005; Vats and Rothfield, 2007), but two recent reports suggest that in Bacillus subtilis these are composed of short, dynamic filaments that are driven by progression of the cell wall synthesis machinery (Domınguez-Escobar et al, 2011; Garner et al, 2011). TmMreB filaments are seen in the crystal structure (van den Ent et al, 2001) and closely resemble those in F-actin. The assertion that cellular MreB filaments may not exceed 200 nm in length is supported by a recent electron tomography study that systematically searched and did not find long filaments in frozen cells (Swulius et al, 2011). We show that TmMreB assembles into filament doublets on a membrane surface, and that these can induce negative curvature in purified vesicles. We show that the amphipathic helix of EcMreB is both necessary and sufficient to confer membrane-binding activity, and demonstrate that this membrane-binding activity of EcMreB is essential for the function of MreB in cell shape determination
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