A role for mechanosensitive channels in chloroplast and bacterial fission

Abstract

The division site in both chloroplasts and bacteria is established by the medial placement of the FtsZ ring, a process that is in part regulated by the evolutionarily conserved components of the Min system. We recently showed that mechanosensitive ion channels influence FtsZ ring assembly in both Arabidopsis thaliana chloroplasts and in Escherichia coli; in chloroplasts they do so through the same genetic pathway as the Min system. Here we describe the effect of heterologous expression of the Arabidopsis MS channel homolog MSL2 on FtsZ ring placement in E. coli. We also discuss possible molecular mechanisms by which MS channels might influence chloroplast or bacterial division.

Figure 1. Heterologous expression of MSL2 in E. coli causes filamentation and disrupts normal Z-ring placement. (A and B). Light micrographs of Frag-1 cells harboring the indicated constructs after 1 h of induction in 0.1 mM IPTG. Cells were immobilized on agarose pads as previously described. (C–N). Immunofluorescence micrographs of FtsZ (pseudocolored green) and 4′,6-diamidino-2-phenylindole (DAPI) staining of nucleoids (pseudocolored red) in Frag-1 cells harboring an empty vector (C–H) or pCTC-MSL2 (I–N). Immunofluorescence and DAPI staining were performed as described previously. All cells were treated with 0.1 mM IPTG for 20 min; + signs indicate cells that were further treated with 10 mM cephalexin for 1 h prior to fixing. White and purple arrowheads indicate single and double or polar FtsZ rings, respectively. Size bar = 10 μm.

Figure 2. Mechanisms by which MS channels may influence chloroplast or E. coli cell division. The possible effects of mechanosensitive channel (MSC) activity (represented in blue, using the predicted topology of Arabidopsis MSL2) on the membrane association and/or function of components of the division machinery (MinD is used as an example and is represented in orange). In Model 1, MSCs directly interact with MinD, stabilizing its association with the membrane and allowing it to function in the inhibition of FtsZ ring assembly. In the absence of the MSC, MinD is unable to properly associate with the membrane and FtsZ assembly is no longer appropriately inhibited. In Model 2, MSCs are required to maintain chloroplast ion homeostasis or to relieve membrane tension in the inner plastid envelope for normal MinD function. Top, in the absence of the MSC, Ca²⁺ or Mg²⁺ ions accumulate in the plastid stroma, enhancing the ATPase activity of MinD and leading to its continual membrane disassociation. Bottom, increased membrane tension alters the biophysical properties of the membrane, preventing MinD from inserting its amphipathic helix into the lipid bilayer.