The cytoskeleton is an interconnected system of protein filaments and associated proteins that gives a cell mechanical organization and supports movement, intracellular transport, and division. In eukaryotes, its principal filament systems are actin filaments, microtubules, and intermediate filaments, although their distribution varies among organisms and cell types. Rather than being a permanent internal scaffold, the cytoskeleton continually changes its organization through filament assembly, disassembly, and interactions with regulatory and motor proteins. Accessory proteins connect its filaments to one another, to organelles, and to the cell membrane. (ncbi.nlm.nih.gov)
Principal filament systems
Actin filaments, also called microfilaments, are approximately 7 nanometers in diameter. Their actin subunits form a two-stranded helical structure with distinct ends, conventionally called the plus, or barbed, end and the minus, or pointed, end. This structural polarity affects both filament growth and the direction of motor movement. Actin can form parallel bundles, contractile arrays, or branched networks. A dense actin-rich cortex immediately beneath the membrane contributes to cell shape and surface mechanics; actin bundles also support projections such as microvilli. (ncbi.nlm.nih.gov)
Microtubules are hollow tubes approximately 25 nanometers in diameter, usually composed of 13 longitudinal protofilaments. Their building blocks are paired alpha- and beta-tubulin subunits. Like actin filaments, microtubules have structural polarity. They are relatively stiff and provide tracks for transport as well as components of the machinery that separates chromosomes. In many animal cells, microtubules radiate from a centrosome, but other arrangements occur, and not all microtubules originate there. Their organization depends on nucleation sites, stabilizing proteins, and interactions with other cellular structures. (ncbi.nlm.nih.gov)
Intermediate filaments are approximately 10 nanometers in diameter and assemble from elongated proteins into flexible, rope-like structures. Unlike actin filaments and microtubules, they lack overall structural polarity. Their proteins differ among cell types: keratins occur in epithelial cells, desmin in muscle, and neurofilament proteins in neurons. Lamins form a supporting meshwork associated with the inner nuclear membrane of the cell nucleus. Intermediate filaments are particularly important for resistance to mechanical stress, although their assembly and organization are also regulated. (ncbi.nlm.nih.gov)
Assembly and dynamic organization
Cytoskeletal filaments are polymers, but their subunits are joined through noncovalent interactions rather than covalent bonds along the filament. This permits reversible assembly. Formation normally begins with nucleation—the establishment of a small assembly capable of further growth—followed by addition of subunits. Cells regulate these processes through proteins that nucleate, cap, sever, bundle, branch, or stabilize filaments. Consequently, the same basic subunits can produce structures with very different shapes and functions. (ncbi.nlm.nih.gov)
Actin assembly is coupled to the binding and hydrolysis of adenosine triphosphate (ATP). Under suitable conditions, subunits enter one filament end and leave the other, producing treadmilling while overall filament length remains approximately constant. Microtubules use guanosine triphosphate (GTP) and commonly show dynamic instability: individual ends switch between growth and rapid shortening. Intermediate filaments do not use this same nucleotide-dependent assembly mechanism; changes such as phosphorylation regulate their reorganization, including disassembly of the nuclear lamina during certain forms of mitosis. (ncbi.nlm.nih.gov)
Motors, transport, and movement
Motor proteins convert chemical energy from ATP into mechanical movement along cytoskeletal tracks. Myosins interact with actin and participate in muscle contraction, intracellular transport, and cellular contractility. Kinesins and dyneins operate on microtubules. Many kinesins move toward plus ends, whereas cytoplasmic dynein moves toward minus ends. Motor-associated adaptor proteins help connect these proteins to particular cargoes, including membrane-bound vesicles and organelles. (ncbi.nlm.nih.gov)
This transport is especially important in a neuron, whose long processes require delivery and retrieval of cellular components over considerable distances. Microtubule motors carry vesicles and mitochondria between the cell body and distant regions. Cytoskeletal filaments also organize the core of a eukaryotic cilium or flagellum. In motile forms, dynein-driven sliding between neighboring microtubules is converted into bending. Cell crawling uses a different combination: actin assembly pushes the surface outward, adhesion provides traction, and contractile activity helps move the cell body. (ncbi.nlm.nih.gov)
Division and mechanical integration
During cell division, microtubules form the mitotic spindle, which organizes and separates chromosomes. Its operation depends on coordinated microtubule growth, shortening, attachment, and motor activity. During cytokinesis in many animal cells, an actin–myosin contractile ring constricts the surface between the daughter cells. These temporary structures illustrate how cytoskeletal components are reorganized for different stages of cellular activity. (ncbi.nlm.nih.gov)
The filament systems function together rather than as independent frameworks. Cross-linking proteins couple different networks, while membrane-associated attachment complexes connect them to neighboring cells or external surfaces. These connections distribute forces and coordinate changes in shape. Cytoskeletal architecture therefore reflects both the intrinsic properties of individual filaments and their arrangement into larger, mechanically integrated networks. (ncbi.nlm.nih.gov)
Cytoskeletal systems in bacteria
Cytoskeletal proteins also occur in bacteria, although bacterial systems are not simply miniature versions of the three eukaryotic networks. FtsZ, a tubulin homologue, assembles at future division sites; its ring-like localization in Escherichia coli was demonstrated in 1991. MreB is an actin homologue involved in maintaining cell shape in many rod-shaped bacteria. Structural studies established its relationship to actin, while subsequent work showed that MreB can form antiparallel double filaments. These discoveries established that filament-based cellular organization extends beyond eukaryotes. (pubmed.ncbi.nlm.nih.gov)