Flagellum

A flagellum (plural: flagella) is a thin, hair-like structure that sticks out from certain cells, helping them move. It is found on some plant and animal sperm cells, fungal spores called zoospores, and many microorganisms. Protists that have flagella are often called flagellates. Although flagella exist in all three domains of life—Bacteria, Archaea, and Eukaryota—their structure, protein makeup, and way of moving differ, yet they all serve the same purpose: motility. The Latin word *flagellum* means "whip," referring to the lashing motion these appendages make during swimming.

In prokaryotes (bacteria and archaea), flagella work like rotors, spinning to propel the organism. The archaeal version is called an archaellum to distinguish it from the bacterial one, even though they look similar. Eukaryotic flagella are quite different: they are large membrane extensions supported by the cytoskeleton, and they wave back and forth instead of rotating. Eukaryotic cilia are nearly identical to flagella but much shorter. Both surface-attached cilia and flagella can help cells swim or move fluids across surfaces.

Depending on the species and cell type, a cell may have zero, one, or many flagella. For instance, the gram-negative bacterium *Helicobacter pylori* uses its flagella to swim through the stomach and reach the mucous lining, where it can colonize the epithelium and potentially cause gastritis, ulcers, and increase the risk of stomach cancer. Flagella can also have other jobs besides movement. *Salmonella typhimurium*, for example, senses how wet its surroundings are using its flagella.

There are three main types of flagella: bacterial, archaeal, and eukaryotic. Eukaryotic flagella contain dynein and microtubules, which allow a bending motion. Bacteria and archaea lack these components and instead rely on a rotary mechanism. Bacterial flagella are helical filaments with a rotary motor at their base that can spin clockwise or counterclockwise, enabling two kinds of bacterial movement. Archaeal flagella (archaella) look similar to bacterial ones and also have a rotary motor, but they differ in many details and are not considered homologous. Eukaryotic flagella—found in animal, plant, and protist cells—are complex projections that lash back and forth. They share the same structure as motile cilia but differ in length, waveform, and function. Primary cilia are immotile and have a 9+0 axoneme, unlike the 9+2 axoneme seen in flagella and motile cilia.

Building and operating a bacterial flagellum requires over 50 proteins working together. The filament is made of flagellin protein subunits and forms a hollow tube about 20 nanometers thick. It is helical, with a sharp bend just outside the outer membrane called the "hook," which points the helix away from the cell. A shaft connects the hook to the basal body, passing through protein rings in the cell membrane that act as bearings. Gram-positive bacteria have two basal body rings (one in the peptidoglycan layer, one in the plasma membrane), while gram-negative bacteria have four: the L ring (associated with lipopolysaccharides), the P ring (peptidoglycan), the M ring (plasma membrane), and the S ring (attached to the cytoplasm). The filament ends with a capping protein. In most studied bacteria—such as *Escherichia coli*, *Salmonella typhimurium*, *Caulobacter crescentus*, and *Vibrio alginolyticus*—the filament consists of 11 protofilaments running parallel to its axis, each a series of tandem protein chains. *Campylobacter jejuni*, however, has only seven protofilaments.

The basal body shares features with certain secretory pores, including a hollow, rod-like plug that extends through the plasma membrane. These similarities between bacterial flagella and type-three secretion systems (TTSS) support the idea that flagella evolved from such secretion systems. The atomic structures of both bacterial flagella and the TTSS injectisome have been mapped in detail, especially using cryo-electron microscopy. The best-understood parts are the scaffolding rings of the inner and outer membranes, along with the rod/needle (injectisome) or rod/hook (flagellum) sections.

The bacterial flagellum is powered by a rotary engine called the Mot complex, made of protein and anchored at the inner cell membrane. This engine runs on proton-motive force—the flow of protons across the membrane driven by a concentration gradient from the cell's metabolism. (Some *Vibrio* species have two kinds of flagella, lateral and polar, and a few use sodium ions instead of protons.) As the rotor moves protons across the membrane, it turns. Alone, the rotor can spin at 6,000 to 100,000 rpm, but with the filament attached, it usually reaches only 200 to 1,000 rpm. The motor switch can change the direction of rotation almost instantly, thanks to a small shift in the position of a protein called FliG in the rotor. The torque-generating unit, the stator, is a proton-powered rotary motor that drives the whole system.