6.2 Flagellar Motor Atlas of Bacterial and Archaeal Cell Structure Home

Motor structure

This is an average of the flagellar motors from more than one thousand Bdellovibrio bacteriovorus cells. Working upward from the base, the major parts of the rotor are the C-ring (for Cytoplasmic), the MS-ring (for Membrane and Supramembrane), the rod, the hook, and finally the filament. The hook and filament are at different angles in different images so they wash out in the average. The rotor is surrounded by stationary parts: the stator ring (which is dynamic, with various conformations that wash out when averaged, so we cannot resolve the stators as they cross the inner membrane and connect to the C-ring) and a series of bushings that allow rotation within the cell wall (the Periplasmic or P-ring) and outer membrane (Lipopolysaccharide or L-ring). Additional cytoplasmic components form the export apparatus, which is involved in assembly (discussed on the next page).

Diversity

As you can see in these averages of flagellar motors from different species [52] [53] [54] [10] [55] [56] [57] [58] [59], bacteria have evolved structural adaptations of their motors to better suit their environments. For instance, if your cell is a pathogen colonizing an animal’s intestinal tract (like Campylobacter jejuni, second from the right in the middle row), it will be swimming in more viscous conditions and may therefore have evolved a wider stator ring to generate more torque, along with reinforced anchors in the cell wall and outer membrane to withstand that added torque. The motors are arranged here according to the species’ evolutionary relatedness.

Flagellar Motor

The motor that spins the flagellum is a complicated molecular machine made of many copies of dozens of different proteins, spanning the cell envelope with components in the cytoplasm, the periplasm (in diderms), and outside the cell, as you can see in this Bdellovibrio bacteriovorus. To get a closer look, we can average the individual motors from many cells (⇩). Broadly, the motor consists of stationary “stators” that drive rotation of the “rotor” to spin the filament. The torque for spinning the flagellum comes from small movements in the stators that kick the rotor in a circle. The energy for these movements comes from the ion potential across the cell membrane that we discussed in Chapter 2; the stators provide a conduit for protons (in most species) or sodium ions (in some marine species) to diffuse down their chemical gradient into the cytoplasm, powering a conformational change in the stators in the process. The energy demands of the machine are high: a single rotation requires about 1,000 protons to flow through the stators, and the motor may spin at more than 100 rotations per second. The fact that cells pay this energetic cost indicates a strong evolutionary selection for motility, or in other words, the powerful advantage your cell can gain by learning to swim.

While the basic plan of the motor is the same in different species, there are structural differences that reflect the different environments those species encounter (⇩).

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