Bacteria make tracks to the pole

Steady-state helices of the actin homolog MreB inside bacteria: Dynamics without motors

“…In B. subtilis, if the three MreB isoforms (MreB, Mbl, and MreBH) bundle together into a triplex structure, the total number of monomers should be the sum from each homologue, which we estimate is 23000 − 40000. We see that with current experimental pitch and abundance estimates, E. coli has n ∈ [3,16] protofilaments in each polarized MreB helical bundle. Under the assumption that they are bundled independently of the other isoforms in B. subtilis we estimate that Mbl has n ∈ [9,21], while MreB has n ∈ [3,7], with pitches from [7].…”

“…The ultrastructure of the MreB helix -the precise arrangement, orientation and length distribution of the individual protofilaments that make up the helical bundle -remains a mystery. Several hypotheses have been put forward [16] and Fig. 1 illustrates five basic possibilities.…”

“…A second possibility is that these proteins simply bind to the helix and advect with the continuous treadmilling, eventually ending up at one of the polar tips. A third possibility is that they associate with leading tips of MreB protofilaments [16], perhaps via intermediary proteins analogous to formin for actin filaments. Here we quantitatively analyze these possibilities, which are depicted in Fig.…”

“…Proteins that could bind either directly, or via putative tip-binding proteins (analogous to formin in eukaryotes [16,45]) to the barbed end of free protofilaments could be translocated in the opposite direction to the treadmilling advection. In an unbuckled (F B = 0) filament, lateral protofilaments treadmill at the same rate of the net backwards advection of the bundle, accomplishing no net movement relative to the cell's axis.…”

“…The MreB helix appears to be composed of a bundle of individual "protofilaments" [15][16][17]. Quantitative immunoblotting has been used to measure the molecular abundance of various MreBs.…”