25.2 : Adaptability of Cytoskeletal Filaments

The cytoskeleton is a complex dynamic structure performing varied functions based on cellular requirements. The adaptability of the individual filaments in the cytoskeleton determines their ability to perform various functions within the cell. It can undergo rapid reorganization during processes like cell division or remain stable for several hours as in the interphase. The adaptability of these filaments depends on stringent regulatory mechanisms. The microfilament and microtubules of the three cytoskeletal filaments are dynamic, while the intermediate filaments are considered static.

Microfilaments are present throughout the cell body and can reorganize into different structures. About 150 different proteins in the cell have been identified that can associate with actin monomers or filaments to regulate their assembly, disassembly, stability, and network structure. The microfilaments provide mechanical support to the plasma membrane, determine cell shape, and help in cell movement by forming lamellipodia and filopodia. A crosslinker protein regulates the formation and stabilization of parallel tight bundles or antiparallel loose bundles. Here, the kinetics of this interaction is also responsible for the architecture of the network. For example, a higher crosslinker protein dissociation rate from the actin filaments leads to alignment into uniform bundles, whereas low dissociation rates lead to a randomly arranged network.

The microtubules are dynamic cytoskeletal filaments; their role in cell division is well-established. During cell division, the centrioles form the spindle fibers comprising microtubule arrays to pull the sister chromatids to the opposite poles. Microtubules are abundant in cilia and flagella, where, with the help of axonemal dyneins, microtubules form locomotory and sensory appendages such as cilia and flagella. In plant cells, these cytoskeletal filaments determine the direction of cell wall formation.

Intermediate filaments are known to provide mechanical support to the cell components. These filaments are abundantly found in the nuclear envelope, where they main the structural integrity of the membrane. In migrating cells along with actin filaments, the keratin fibers, a type of intermediate filament, are also present. The different types of intermediate filaments adapt to perform different functions within the cell.