Flagella Definition

The hair-like structure protruding from some plant and animal cells are known as a flagellum or flagella. These are crucial for a variety of physiological processes carried out by the cell.

• “Flagellum” refers to the long, slender form of the structure, which resembles a whip and is derived from the Latin word for “whip”.
• Flagella are the distinguishing features of the Mastigophora group of protozoa, although they may also be found in a variety of microscopic and macroscopic creatures, including bacteria, fungus, algae, and mammals.
• The primary function of flagella is as an organelle for propulsion, in addition to aiding in food collection and circulation.
• Flagella present in many living things vary in terms of their structure, mechanism, motion, and even functions. Because it differs significantly from the flagellum found in bacteria, the flagellum found in archaea is known as an “archaellum.”
• Flagella and other hair-like protrusions termed cilia share a very similar structure, albeit they differ extensively in quantity, frequency, movement, and occasionally the functionality.
• These appendages have been studied in various animal groups for their functions in both mobility and feeling because they can detect changes in the pH and composition of the surrounding environment.
• Because of its size, the flagellum seen in Chlamydomonas reinhardtii is the one that is most frequently investigated.
• Despite the fact that most flagella are found at the polar extremities of cells, flagella vary in number and position while maintaining the same composition and activities.

Structure of Flagella

Prokaryotes and eukaryotes have distinct size, structures and flagella counts. The bacterial flagellum differs from the archaeal flagellum even among prokaryotes. Similarly, there are many distinct components and mechanisms involved in flagella development. However, the fundamental components of a flagellum includes several elements that are prevalent throughout all spheres of life.

The following components make up a flagellum’s fundamental structure:

1. Filament

• The filament, which makes up roughly 98% of all the flagellum’s structures, is the component that stands out the most.
• The filaments, which have an average length of 18 nm, stretch from the hook-like structure found within the cell’s cytoplasm. Various types of living things have different filament lengths, such as the bacterial flagella’s 20 nm filament and the archaea’s 10–14 nm filament.
• Outside of the cell membrane-specific flagellar staining techniques, filaments can be seen. The motor located in the cytoplasm regulates the motion of the filaments.
• Flagellins and hook proteins make up the self-assembling macromolecular structures known as filaments, and the quantity of flagellin and hook protein subunits may vary across various cell types.

2. Hook or anchoring structures.

• The flagellar hook, which joins the basal body or flagellar motor to the long filament, is a small, curved tubular structure.
• The hook’s primary purpose is to transfer motor torque to the helical filament so that it can travel in various directions to fulfil the needs. Additionally, it is crucial to the flagellum’s construction.
• Numerous hook protein subunits combine to create polymorphic supercoil structures that make up the hook.
• All types of cells have this structure close to the cell membrane, yet each cell may have a different form and precise composition.

3. Basal body or motor device

• The sole component of a flagellum that is found inside the cell membrane is the basal body. It is attached to the flagellum’s hook, which is subsequently attached to the lengthy filament.
• The rod-shaped basal body is made up of a network of microtubule rings. Diverse cell types have different basal body components.
• For certain purposes, the rods in the basal body work as a reversible motor to drive the filament in a different orientation.
• The transport of flagellar proteins from the cytoplasm to the hook and filament portions of the flagellum during construction depends on the basal body.

Flagella formation mechanism

• The development of the FliF ring complex in the basal body triggers the production and assembly of flagella. In the cytoplasmic membrane, the process takes place and moves both inward and outward. The majority of research on the development and assembly of the flagellum has been conducted using examples in the bacteria. 
• Following are some general descriptions of the creation and assembly of flagella:

1. Formation of the basal body

• The process starts with the cytoplasmic inclusion of FliF, an essential protein made up of an MS ring.
• The MS-ring is crucial because it controls how the other flagellum components are put together.
• The FliF proteins come together to create a single ring that divides the cytoplasmic membrane into two neighbouring loops.
• Other proteins, including FliG, FliM, and FliN are also integrated onto the cytoplasmic face of the MS-ring once FliF is absorbed into the membrane.
• These proteins are necessary for the flagella’s many activities, including motility and the export of other proteins that make up the flagellum.
• The C-ring, which develops in the cytoplasmic space, is necessary for the inward assembly of the basal body. To export flagellar axial proteins down the channel, a flagellar export machinery is created inside the C-ring.
• Flagellar axial proteins are bound and transported into the central channel of the flagellum via a type III protein export pathway that is unique to the flagellum.
• The creation of the rod, a crucial part of the basal flagellar body, comes next. The FliF ring at the proximal end of the rod and the hook at the distal end are connected by five proteins.

2. Formation of the hook

• Proteins are moved from the rod into the hook, causing it to lengthen to 55 nm, albeit this length may vary across various types of cells.
• A protein called FlgD, which is absent in fully developed flagella, stimulates the production of hooks. The rod cap protein is changed by hook capping proteins when hook assembly gets going.
• The polymerization of subunits into a helical structural arrangement requires the FlgD.
• From base to tip, the central channel transports the FlgE hook capping protein from the cytoplasm.

3. Filament assembly

• The HAP2 pentamer complex, which seals the distal end of the filaments when the filament monomers are put together, is present throughout filament assembly.
• The cap is necessary to cause a conformational shift that enables polymerization and prevents the diffusion of subunits from the filament.

Types of Flagella

1. Bacterial flagella

• The helically coiled flagella of bacteria are much longer than those of archaea and eukaryotes.
• Compared to eukaryotic flagella, they are thinner.
• About 20 nanometers would be the diameter.
• Variable bacterial species that are predominantly involved in movement have different numbers of flagella. The flagella may occasionally serve as sensory organs that can pick up on environmental changes.
• Because the filaments travel counterclockwise, they frequently possess a left-handed helix and are lengthier than those of archaea, which results in swimming motility. The length of the flagella may also differ.

2. Archaeal flagella

• The flagellum in archaea is a special motility device that differs in composition but is put together similarly to the bacterial flagellum.
• Nearly all of the major domain groups, including the halophiles, methanogens, and thermophiles, contain flagella.
• Archaeal flagella differ from bacterial flagella in diameter.
• The right-handed helix-shaped arrangement of the proteins in the flagella causes the clockwise spinning of the flagella. As the cell is pushed by the clockwise rotation, the speed of the archaeal flagella increases.
• In contrast to bacteria, archaeal flagella have hooks that vary in length between species.
• Numerous studies have also shown that the switching mechanisms and sensory regulation of archaeal flagella vary from those of bacterial flagella.

3. Flagella of eukaryotes

• Many eukaryotic cells, including some animal cells like sperm and many algae, have flagella.
• The primary functions of eukaryotic flagella in eukaryotic animals are cell movement, cell nutrition, and reproduction. These also serve, in some algae, as sensory antennae.
• The architecture, composition, mechanism, and assembly of eukaryotic flagella are distinct from those of bacterial flagella. While bacterial flagella only comprise roughly 30 proteins, eukaryotic flagella are made up of hundreds of distinct proteins.
• Similar to how bacterial flagella are driven by a rotational motor located in the basal body, eukaryotic flagella are dependent on limited dynein-dependent microtubule sliding for their movement.
• The microtubule-based centriole that makes up eukaryotic flagella targets the proteins from which the axoneme extends. In eukaryotic cells, the centriole is frequently referred to as the basal body of the flagella.

Bacterial flagella arrangement

1. Monotrichous

• A monotrichous configuration of flagella is one in which every cell contains a single flagellum. If the flagellum is found at the polar end, the distribution is referred to as monotrichous.
• Different chemoreceptors that cause cell motility control the basic process by which monotrichous flagella move.
• The bacterial flagella motor is powered by a transmembrane electrochemical gradient of ions that is produced when different sensory receptors detect changes in the surrounding environment.
• The filament and hook receive the push produced by the motor, which causes the flagella to rotate anticlockwise.
• The flagella rotate in a counterclockwise direction which causes the cell to advance or “run”.
• The counterclockwise movement of the flagella is what causes the orientation of the cell to shift in a monotrichous arrangement. This motion causes the germs to recoil and allows them to reorient.
• Bacteria like Vibrio cholerae, Campylobacter spp., and E. coli are some examples of a monotrichous arrangement of flagella.

2. Lophotrichous

• A lophotrichous configuration of flagella is one in which many flagella are present in the same area of the cell. These flagella are generally observed close to the polar end of the cell.
• The polar organelle, a section of the cell membrane, frequently encircles the bases of these flagella.
• Similar to monotrichous flagella, lophotrichous flagella move by a process that is triggered by a variety of environmental conditions.
• To achieve smooth movement, all of the flagella that are engaged in lophotrichous flagella’s movement must travel in the same direction and at the same pace.
• Although various motors operate the flagella, the propulsion produced by the flagella happens simultaneously because all of the motors are activated by the same stimulus.
• The direction shift is caused by the flagella’s consequent separation in response to external stimuli brought on by environmental changes.
• The flagella of some organisms, such as Spirillium and Pseudomonas fluorescens, are arranged in a lophotrichous manner.

3. Amphitrichous

• An amphitrichous distribution of flagella is the presence of one or more flagellums at each of the polar ends of the cell.
• The polar organelle, a part of the cell membrane, is where the flagella are located when they have several flagella at the ends.
• The operation of the amphitrichous arrangement of flagella is less understood than that of other flagellal layouts.
• According to several researchers, the two flagella function differently because they can each only travel in one direction, either monotrichously or lophotrichously.
• According to different research, the two flagella function concurrently but rotate in separate directions.
• However, the movement mechanism is comparable to other systems.
• Rhodospirillum rubrum, Magnetospirillum, and Campylobacter jejuni are some examples of bacteria possessing amphitrichous flagella.

4. Peritrichous

• While flagella are scattered all over the cellular body and oriented in diverse directions, they are grouped in a peritrichous pattern.
• The flagella in the peritrichous arrangement bundle together to propel the cell toward the stimulus during its “run” motion.
• In the case of repellant, a phosphorylation cascade is set up that modifies the CheY regulator’s phosphorylation state.
• The motor switch proteins are then immediately contacted by the active regulator, causing the flagella to rotate in the clockwise direction.
• The contact alters the movement’s speed and direction by tearing apart bundles and separating the flagella.
• They can rearrange themselves until a subsequent stimulus is arbitrarily detected due to the Brownian motion of the cell.
• Salmonella, Bacillus subtilis, Klebsiella, and Escherichia coli are among organisms with peritrichous flagella arrangements.

Functions of Flagella

• Many bacteria move by using their flagella as their main locomotional components in order to get to the most hospitable environment. Bacteria move in reaction to numerous stimuli, allowing them to adapt to various environmental circumstances. Flagella are necessary for motility and, finally, fertilisation in eukaryotic cells like sperm.
• Flagella are crucial for the colonisation of tissue surfaces because they function as a virulence factor for the invasion of host tissues and the development of bacteria inside them.
• These are crucial for the non-pathogenic colonisation of surfaces such as those of plants, soil, or animals.
• By disrupting the bacteria’s nutrient-poor, waste-rich shell, the flagella of some bacteria participate in the exchange of nutrients and waste.
• The sodium-driven flagella of alkaliphilic bacteria allow sodium to enter the cytoplasm again, keeping the pH of the cytoplasm at a neutral level.

Examples of Flagella

1. Flagella in Helicobacter pylori

• Helicobacter pylori is a flagellated bacteria that moves through tissue surfaces using its flagella.
• The bacteria have between 4 and 8 unipolar flagella, which are crucial virulence factors for the many illnesses the bacteria causes.
• pylori flagella may travel through liquid and semisolid media with a swimming or swarming motility.
• The flagella affect several physiological processes such as immune evasion, cell-like inflammation, and colonisation in H. pylori.

2. Flagellum in human sperm cells

• A sperm cell’s flagellum is necessary for human in vivo fertilisation and motility.
• Inability to move the flagella and push the cell might prevent fertilisation from occurring during human sexual reproduction.
• Microtubules grouped in a 9+2 pattern, together with components including the dynein regulatory complex, radial spokes, and dynein arms, make up the core of the flagella.
• Additionally, because it directs the sperm in a certain direction during fertilisation, it is crucial for sperm penetration into the egg.

References

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• Nikhil A. Thomas, Sonia L. Bardy, Ken F. Jarrell, The archaeal flagellum: a different kind of prokaryotic motility structure, FEMS Microbiology Reviews, Volume 25, Issue 2, April 2001, Pages 147–174, https://doi.org/10.1111/j.1574-6976.2001.tb00575.x
• Vonderviszt F, Namba K. Structure, Function and Assembly of Flagellar Axial Proteins. In: Madame Curie Bioscience Database [Internet]. Austin (TX): Landes Bioscience; 2000-2013. Available from: https://www.ncbi.nlm.nih.gov/books/NBK6250/
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• Jonathan Moran, Paul G. McKean, Michael L. Ginger, Eukaryotic Flagella: Variations in Form, Function, and Composition during Evolution, BioScience, Volume 64, Issue 12, December 2014, Pages 1103–1114, https://doi.org/10.1093/biosci/biu175
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