25.1: Introduction to the Cytoskeleton
Overview of the Cytoskeleton
The cytoskeleton is a network of protein filaments present within the cell, having three distinct filaments ̶ microfilaments, microtubules, and intermediate filaments. Each has characteristic features that distinguish them, including the dynamics of their assembly and disassembly, mechanical properties, polarity, and the type of molecular motors associated with them. Earlier, they were thought to be present only in eukaryotic cells; however, their homologs were eventually found in prokaryotic cells. Studies on bacterial homologs of cytoskeletal proteins hypothesize that the cytoskeleton originated in bacteria and archaea.
Despite using the word 'skeleton,' the cytoskeleton is not a fixed structure. It is a dynamic and adaptive structure that participates in various cellular functions. These functions can broadly be categorized as i) Spatial organization of cellular content, ii) Connecting the cell to its external environment both physically and biochemically iii) Generation of coordinated forces that help in cell movement and change in cell shape. Although these filaments are organized into networks that resist deformation, they can undergo rapid reorganization in response to external signals or forces.
Microfilaments or filamentous actin (F-actin) are right-handed spiral filaments of globular actin (G-actin) monomers. These are polar filaments owing to the rate of polymerization at either end. These filaments steadily elongate to produce a strong sustained force required to carry out motility and cell shape changes. Microtubules are hollow cylindrical structures having thirteen protofilaments made up of alpha-beta-tubulin heterodimers. Microtubules are known to have the most complex assembly and disassembly dynamics. Unlike microfilaments, the microtubules rapidly switch between polymerization and depolymerization. The microtubule dynamics are regulated by a structure known as Microtubule Organizing Centres (MTOCs). The third component, intermediate filaments, are long fibrous proteins composed of multiple subunits formed through multistep processes. These filaments are generally static structures, their dynamics regulated through post-translational modifications like phosphorylation and dephosphorylation.
