One of the simplest and highly conserved bacterial shapes is a rod. In order to grow, rod-shaped bacteria restrict their growth so that they maintain a constant width as they elongate along their length. However, it is not understood how bacteria form into a rod shape, much less how they maintain rod shape as they grow. One key protein, MreB, plays an important role in this process. Bacteria that lose MreB become spherical. MreB polymerizes into short, curved membrane-bound filaments, interacting with the enzymes that build the cell wall. These MreB filament/enzyme complexes, as they insert new material into the growing cell wall, move in circle around the cell width, and perpendicularly to the cell length. It is thought that this circumferential motion causes the insertion of “hoops” of new material around the rod width, an organized arrangement that helps the rod shape resist the high pressures within the cell.
Saman Hussain and Carl Wivagg sought to understand how this organized movement arises: how do these short, independent filaments know how to move around the rod width, instead of moving in any other direction? To study this, they examined MreB motion in cells of different shapes. When cells were rods of any width, they could see that MreB moved circumferentially. But when cells became round, they were surprised to see that MreB motion became random, with filaments moving in all directions. If they confined these round cells into rod shapes in microfluidic molds, the motion of the filaments became oriented again, indicating MreB filaments can sense the local cell shape to move around the cell.
To test if the MreB filaments were moving along tracks of old material in the cell wall, they removed the cell wall. In round wall-less cells, filaments were oriented in all directions, but when they confined the wall-less cells into rods, MreB filaments aligned around the rod width. To test if purified MreB filaments alone were able to orient around tubes, they examined (in collaboration with Piotr Szwedziak in the Löwe lab at Cambridge University) the structure of pure MreB assembled inside liposomes. MreB filaments deformed the liposomes into rod-like tubes, and oriented along the circumference, just like in cells. Putting all of this data together, Carl and Saman realized that these inwardly curved MreB filament orient so they point along the most inwardly curved surface thus maximizing membrane interaction. In round cells, there is no difference in the two curvatures causing MreB filaments to have no preference in their orientation, resulting in a random distribution of orientations.
In collaboration with Felix Wong in the Amir lab at Harvard, they showed by mathematical modelling that circumferential membrane binding is the most energetically favorable conformation for highly curved filaments, a behavior predicted to be robust no matter how wide the cell, or pressurized it is. Thus, by sensing the existing cell shape and orienting new cell wall synthesis to reinforce that shape, curved MreB filaments create a local feedback loop allowing rod shape to be robustly maintained.
This local feedback of sensing then reinforcing local shape may also help bacteria to form into rod shapes in the first place. When round cells were forced to reform into rods, they found that rods form not from the sphere thinning down to make a rod, but instead they suddenly emerge from small local outward bulged on the sphere surface. These bulges create small rod like regions where MreB filaments could orient within, allowing them to quickly elongate as growing rods. Therefore, the curvature-orienting properties of MreB not only help cells maintain rod shape, they also help cells form into rods in the first place.