Scientists deconstruct process of bacterial division

Normally, bacteria divide by forming a ring that pinches the cell in two. The ring is called a “Z ring” after the protein FtsZ, which forms a ring-shaped scaffold and then squeezes it smaller. In bacteria, the Z ring also contains a dozen other proteins, all believed to be essential for division.

The Z ring normally pulls in on the cell membrane by binding to another protein, FtsA, which has one end attached to the inner cell membrane and the other end connected to FtsZ. When the Z ring constricts, it completely pulls in the membrane and nips the bacterium in two.

But cell biology research scientist Masaki Osawa, Ph.D., cut FtsA out of the system by making an FtsZ that could bind directly to the membrane, and called it "membrane targeted FtsZ" or FtsZ-mts.

First, Osawa demonstrated that the new protein, FtsZ-mts, assembled Z rings in bacteria.

Then he constructed a greatly simplified cell-division machine in microscopic oil droplets, called liposomes, that demonstrated the important role of FtsZ in the division process. He was able to assemble Z rings in this completely artificial system, the liposome, a tiny hollow sphere of fat that mimics natural cell membranes.

To do this, Osawa mixed the liposomes with FtsZ and GTP, a molecule that provides energy. On a microscope slide the liposomes fused and stretched into tubes that mimicked the shape of E. coli and other rod-shaped bacteria.

“It was a happy coincidence that the size and shape of the liposomes was similar to that of rod-shaped bacteria,” says co-author Harold Erickson, professor of cell biology. “These tubular liposomes are a new micro-structure, and their formation is still a mystery.”

During the experiment, fluorescently labeled FtsZ-mts was initially on the outside of the liposomes, but some of the tubular liposomes ended up with FtsZ on the inside. “We don’t know how this happens, but it is a key to the discovery,” Osawa said.

Inside the liposome the FtsZ formed multiple closed rings that aligned perpendicular to the length of the tube, just as Z rings form in bacteria. They also slid back and forth, and where they collided, they stayed together and formed brighter Z rings. And as the Z rings grew in brightness, they visibly pulled the wall of the liposome inward.

“The Z rings are clearly generating force and causing the constriction,” Osawa said. A movie the team made shows several constrictions in the wall occurring at the sites of the bright Z rings. When the GTP in the liposome is used up, the tube eases out of its constrictions into its original shape.

“We believe our simple system may recreate the mechanism that the earliest bacteria used to divide. They probably had FtsZ alone,” Erickson said. “Osawa’s experiments show that FtsZ, a membrane tether, and the inside surface of a tubular membrane are all that’s needed to assemble the Z ring and generate a constriction force.”

The artificial Z rings were not sufficient to pinch the liposomes in half, “probably because their walls are much thicker than the membrane of a bacterium,” Osawa noted. “We are now working to make thinner liposomes, so that we can achieve complete division.”

Erickson said that FtsZ is the bacterial ancestor of tubulin, the protein that makes the microtubules in animal cells and is the target of a number of anti-cancer drugs like taxol. Although FtsZ is not sensitive to taxol, anything learned about the bacterial ancestor will help us understand microtubules, which help animal cells to keep their shape and control their movements, he explained.

Source: Duke University