Now They Can Be Shown

We couldn’t run these images in last issue’s electron cryotomography article (“Cellular CAT Scans”) because of an embargo by Nature. At right is the first-ever high-resolution, 3-D view of a working molecular motor. As coauthor Grant Jensen, assistant professor of biology, remarked in E&S, “If he [grad student Gavin Murphy, the Nature paper’s lead author] thawed the cells out, they’d swim away.” The cells in question are a bacterium known as Treponema primitia, and the motor spins at about 300 revolutions per second to drive a flagellum that propels the cell.

A marvel of complexity, the motor is assembled from molecules of about 25 different proteins. Other researchers had isolated and purified some of them to determine their structures, or had pulled the motor out of the cell to examine it, losing pieces in the process. Seen here for the first time is the ring-shaped torque generator, called the stator (yellow), which is embedded in the inner cell membrane. Nested inside the stator is the moving part, the rotor (dark blue), which is attached to a rod (red) that turns the flagellum. A cross section through the stator (below right) shows that each of its 16 symmetric units grabs the rotor in two places, like a person’s two hands gripping the rail of a playground merry-go-round in order to spin it. Also revealed are the stator’s connections to the P collar (light blue), which is basically a bushing, and the C ring (green), which acts like a transmission to select clockwise or counterclockwise rotation. The paper, whose third author is Associate Professor of Environmental Microbiology Jared Leadbetter, ran in Nature’s August 31 issue. —DS