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Lophotrichous bacteria possess multiple flagella that arise from a single end. The majority of these flagella arise from the polar end. Flagella are usually long and few while cilia are short and numerous. Both comprise 9 fused pairs of protein microtubules with side arms of the motor molecule made of dynein that originates from a centriole. They form a ring around the inner central pair of microtubules that arises from a plate present near the cell surface. The arrangement of microtubules is referred to as a 2(9+2) arrangement. This complex of microtubules is surrounded via a sheath i.e. continuous with the cytoplasmic membrane.
Lophotrichous
Lophotrichous refers to an arrangement of flagella which have a presence of multiple flagella arising from the same point in the cell. Most of the time, these flagella arise at the polar end of the cell.
The bases of these flagella are mainly surrounded by a region of the cell membrane known as the polar organelle.
The mechanism of movement of lophotrichous flagella is quite similar to that of monotrichous flagella in that it is induced via different changes in the surrounding environment.
Since there are multiple flagella involved in the movement of lophotrichous flagella, all the flagella are required to move in the same direction and with the same speed to help in a smooth movement.
The flagella are usually controlled via different motors, but as all the motors are activated via similar stimuli, the thrust created by the flagella occurs together.
The change in direction is usually obtained by the subsequent separation of the flagella as a result of the stimuli present due to changes in the environment.
Examples of a lophotrichous arrangement of flagella such as Spirillium, Pseudomonas fluorescens, etc.
Mechanism of Flagella Formation
The process of flagella formation and assembly starts with the formation of the FliF ring complex in the basal body. This process takes place in the cytoplasmic membrane and proceeds both inwards and outwards. However, most of the studies related to the process of formation and assembly of the flagellum have been done on bacteria. As the flagellum consists of a complex membrane and structures made up of numerous proteins and their interactions.
The overall process of flagella formation and assembly has been clearly described below:
Formation of the basal body
The process first starts with the incorporation of FliF, which is an integral protein comprising MS-ring into the cytoplasm.
The MS-ring is important as it helps in determining the assembly of all the other structures of a flagellum.
The FliF proteins assemble into a single ring that forms two adjacent loops spanning the cytoplasmic membrane.
After the incorporation of FliF into the membrane, other proteins such as FliG, FliM, and FliN are also incorporated into the cytoplasmic face of the MS-ring.
These proteins are important for different functions of the flagella such as motility and the export of other flagellar component proteins.
The inward assembly of the basal body comprises the formation of a C-ring, which is generally formed in the cytoplasmic space. Within the C-ring, a flagellar export apparatus is formed to export flagellar axial proteins via the channel.
A flagellum most specifically type III protein export system binds and moves flagellar axial proteins towards the central channel of the flagellum.
The next step is the formation of the rod, which is a major component of the basal flagellar body. The rod is composed of five proteins that are attached to the FliF ring at the proximal portion and the hook at the distal end.
Formation of the hook
Proteins translocate through the rod into the hook, this results in the hook growing up to the length of 55 nm, which might differ in various types of cells.
Hook formation is mainly induced by a protein FlgD that is lacking in the completed flagella. As hook assembly begins, the rod cap protein gets replaced by hook capping proteins.
The FlgD is required for the polymerization of subunits into an α-helical structural arrangement.
The hook capping protein, FlgE, is exported from the cytoplasm via the central channel from base to tip.
Filament assembly
The assembly of filament occurs in the presence of the HAP2 pentamer complex that caps the distal end of the filaments as the filament monomers get assembled.
The cap is important to prevent the diffusion of subunits from the filament and induce a conformational change to enable polymerisation.
Conclusion
Flagella are usually the primary structures of locomotion in many bacteria so that bacteria can move towards the most favourable environment. The movement of bacteria arises in response to different stimuli that enable them to adapt to various environmental conditions. In eukaryotic cells such as sperm, flagella are important for motility and lastly fertilization.
Flagella play a crucial role in the colonization of tissue surfaces like a virulence factor to invade host tissues and develop within them. These are also vital for the non-pathogenic colonization of surfaces such as plant, soil, or animal surfaces.
