For bacteria, navigating a microscopic environment can be especially difficult given their relative size and variety of living conditions. In order to ensure long-term survival, bacteria utilize “flagella,” long hair-like structures made of proteins, to drive movement throughout their environment. To achieve this, flagella rotate much like a propeller. However, until recently, the molecular structures that allow flagella to rotate freely were unknown. A group of researchers at Zhejiang University in China recently published a complete structure of a flagellum, giving us new insights into the structure and related function of these massive motor proteins.

To accomplish this, the group used a technique called cryo-electron microscopy (cryo-EM). For a cryo-EM experiment, proteins of interest are sprayed on a small grid, then frozen at incredibly low temperatures. Next, a beam of electrons is shot at the grid, which produces small images of the protein. Recent advancements in cryo-EM imaging allows for advanced software to combine a series of images to produce these groundbreaking structures. Before cryo-EM, molecular details of proteins could not be determined for structures of this size.

The key mystery of the flagellar motor lies in the propeller-like movement of the structure. Given the size and shape, scientists wonder how the molecular details allow for circular motions that give rise to movement. This study solidified and made clear how the flagellar motor acts differently. A central rod-like protein runs down the center of the complex. This rod connects two major parts of the motor: a ring of proteins at the base of the structure, and a “hook” of flexible proteins at the top. Much like man-made motors, the ring at the base of the structure rotates. The rod transfers this rotation to the hook at the top. This allows the hook to spin, propelling the cell through its environment much like a propeller in a boat engine. While this function is not dissimilar to what was originally thought, the molecular details illustrate an interesting finding—the machines that people invent often work off the same principles as the machines that drive small biological life. Researchers hope to use these findings to draw evolutionary comparisons to bacterial ancestors and better understand how these structures developed over the history of life on earth.

This study was a collaborative effort co-authored by Jiaxing Tan, Xing Zhang, Xiaofei Wang, and Caihuang Xu from the Department of Biophysics and Department of Pathology at Zhejiang University and coordinated by the corresponding author Yongqun Zhu from the Department of Biophysics and Department of Pathology at Zhejiang University in Hangzhou, Zhejiang, China.

Managing Correspondent: Koby Ljunggren

Original Article: Structural basis of assembly and torque transmission of the bacterial flagellar motor. Cell.

Image Credit: Pixabay

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