Premed · Premed · General Biology 1
Lecture 8: The Cytoskeleton and Cell Motility
General Biology I — Molecular & Cellular
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the three types of cytoskeletal filaments and their properties
- Explain how microfilaments contribute to cell shape, movement, and division
- Describe the structure and functions of microtubules, including their role in intracellular transport
- Distinguish between cilia and flagella and explain their mechanism of movement
- Compare the roles of motor proteins: myosin, kinesin, and dynein
Lecture Content
I. Overview of the Cytoskeleton
The cytoskeleton is a dynamic, interconnected network of protein filaments that extends throughout the cytoplasm of every eukaryotic cell. Far from being a static scaffold, it is continuously remodeled, enabling cells to change shape, move, divide, and transport cargo. The cytoskeleton provides structural support to maintain cell shape, drives cell motility (both whole-cell crawling and the beating of cilia and flagella), facilitates intracellular transport of organelles and vesicles along defined tracks, organizes cell division through the mitotic spindle and the contractile ring, and transmits mechanical signals from the extracellular matrix to the interior of the cell. Three main types of filaments compose the cytoskeleton, each with distinct structural properties and biological roles: microfilaments (actin filaments, 7 nm in diameter), intermediate filaments (8-12 nm), and microtubules (25 nm).
II. Microfilaments (Actin Filaments)
Microfilaments are the thinnest cytoskeletal elements, measuring just 7 nm in diameter. They are composed of the globular protein actin (G-actin), which polymerizes into two intertwined helical chains to form filamentous actin (F-actin). Like microtubules, actin filaments have a polar structure: the fast-growing plus end (also called the barbed end) and the slow-growing minus end (the pointed end). This polarity is critical for directional assembly and for the movement of motor proteins.
Actin filaments are highly dynamic, undergoing continuous treadmilling--a process in which monomers are added at the plus end and removed at the minus end simultaneously, so the filament appears to move through the cytoplasm even though individual subunits travel in only one direction. This dynamic behavior is regulated by ATP hydrolysis (G-actin binds ATP upon incorporation, which is then hydrolyzed to ADP) and by a suite of actin-binding proteins. Profilin promotes polymerization by loading G-actin with ATP. Cofilin accelerates depolymerization by severing older, ADP-bound filaments. The Arp2/3 complex nucleates new branches from existing filaments, creating the dense, dendritic networks that push the leading edge of a migrating cell forward. Formins nucleate unbranched filaments that grow in a linear fashion.
The functional repertoire of microfilaments is remarkably broad. Beneath the plasma membrane, a cortical actin network defines and maintains cell shape. In muscle cells, actin thin filaments interact with myosin thick filaments within the sarcomere to drive contraction. During amoeboid movement, rapid actin polymerization at the leading edge extends pseudopods in the direction of travel. In cytokinesis, a contractile ring of actin and myosin II assembles at the cell equator and constricts to pinch the dividing cell in two. Actin bundles form the structural core of microvilli in intestinal epithelial cells, vastly increasing the absorptive surface area. And during cell crawling, broad, sheet-like lamellipodia and thin, spike-like filopodia--both driven by actin polymerization--probe the environment and guide cell migration.
III. Intermediate Filaments
Intermediate filaments, at 8-12 nm in diameter, occupy a middle ground between the thin microfilaments and the thick microtubules. They are the most stable and least dynamic of the three filament types, providing cells with mechanical strength and resistance to tensile (stretching) forces. Unlike actin filaments and microtubules, intermediate filaments are not polar--they have no distinct plus or minus end--and they do not bind nucleotides (no ATP or GTP hydrolysis is involved in their assembly or disassembly).
The proteins that compose intermediate filaments vary by cell type, reflecting specialized mechanical demands. Keratins reinforce epithelial cells and form the structural basis of hair, nails, and skin. Vimentin provides support in fibroblasts and other mesenchymal cells. Desmin maintains the alignment of sarcomeres in muscle cells. Neurofilaments regulate axon caliber in neurons, directly influencing the speed of nerve impulse conduction. Lamins line the inner surface of the nuclear envelope, forming the nuclear lamina that supports nuclear shape and participates in organizing chromatin.
Intermediate filaments anchor organelles (the nucleus, via lamins), connect cells to one another at desmosomes (cell-cell junctions), and link cells to the extracellular matrix at hemidesmosomes. Their clinical importance is underscored by the consequences of mutations in intermediate filament genes: epidermolysis bullosa simplex results from keratin mutations that weaken the mechanical integrity of skin, causing blistering in response to minor friction, while progeria (Hutchinson-Gilford syndrome) arises from mutations in the lamin A gene and is characterized by dramatic premature aging.
IV. Microtubules
Microtubules are the largest cytoskeletal elements, with a 25 nm outer diameter and a 15 nm hollow core. They are assembled from alpha-tubulin and beta-tubulin heterodimers, which polymerize into linear protofilaments--thirteen of these arranged in a cylindrical wall form a single microtubule. Like actin filaments, microtubules are polar: the fast-growing plus end exposes beta-tubulin, while the slow-growing minus end exposes alpha-tubulin and is typically anchored at the centrosome, the cell's primary microtubule-organizing center.
A defining property of microtubules is dynamic instability--the rapid and stochastic switching between phases of growth and shrinkage. This behavior is driven by GTP hydrolysis: beta-tubulin binds GTP upon incorporation, and the hydrolysis of GTP to GDP after assembly destabilizes the polymer. As long as new GTP-tubulin subunits are added faster than GTP is hydrolyzed, a stabilizing GTP cap is maintained at the plus end, and the microtubule grows. If the cap is lost, the microtubule undergoes rapid depolymerization--a dramatic event called catastrophe. The reverse transition, from shrinkage back to growth, is termed rescue. Dynamic instability allows microtubules to explore the cytoplasm rapidly, searching for targets such as kinetochores during mitosis.
Microtubules serve as tracks for intracellular transport, resist compression to help define cell shape, form the mitotic spindle that segregates chromosomes during cell division, and provide the structural core of cilia and flagella. At the centrosome, two centrioles (each consisting of nine triplets of microtubules in a 9+0 arrangement) are surrounded by pericentriolar material containing gamma-tubulin ring complexes (gamma-TuRC) that nucleate new microtubules.
<image>A comparative diagram of the three cytoskeletal elements. Left column: Microfilaments — showing G-actin monomers polymerizing into a double-helical F-actin filament (7 nm), with plus and minus ends labeled. Middle column: Intermediate filaments — showing fibrous protein monomers forming dimers, tetramers, and ultimately a rope-like filament (8-12 nm), with no polarity indicated. Right column: Microtubules — showing alpha/beta tubulin heterodimers assembling into a hollow tube of 13 protofilaments (25 nm), with plus (beta) and minus (alpha) ends labeled. Below each, representative functions are illustrated: contractile ring for microfilaments, nuclear lamina for intermediate filaments, and mitotic spindle for microtubules.</image>
V. Motor Proteins
Motor proteins convert the chemical energy of ATP hydrolysis into mechanical movement along cytoskeletal tracks, enabling the directed transport of cargo through the cell.
Myosin moves along actin filaments. The myosin superfamily is large, but myosin II is the best known, driving muscle contraction and cytokinesis by moving toward the plus end of actin filaments. Myosin V transports vesicles along actin tracks, also toward the plus end. All myosins share a basic architecture: a head domain that binds actin and hydrolyzes ATP, a neck (lever arm) that amplifies conformational changes, and a tail that binds cargo.
Kinesin moves along microtubules toward the plus end, which in most cells points toward the periphery. Kinesin-1, the canonical member of this family, transports vesicles, organelles, and mRNA outward from the cell body--a process called anterograde transport. It walks in a hand-over-hand fashion, taking 8 nm steps that correspond to the spacing between tubulin dimers.
Dynein moves along microtubules toward the minus end, typically toward the centrosome and the center of the cell. Cytoplasmic dynein carries out retrograde transport, positions the Golgi apparatus near the nucleus, and moves chromosomes during mitosis. Axonemal dynein is a specialized form that generates the bending movements of cilia and flagella.
VI. Cilia and Flagella
Cilia and flagella are motile appendages that extend from the cell surface, sharing a common internal architecture: the axoneme, a "9+2" arrangement of microtubules in which nine outer doublets surround two central singlets. The outer doublets are connected by nexin links and bear dynein arms. When the dynein arms on one doublet "walk" along the adjacent doublet, they generate a sliding force. Because the nexin links prevent the doublets from sliding freely, this force is converted into bending--the basis of ciliary and flagellar movement. At the base of each cilium or flagellum, a basal body anchors the structure to the cell; the basal body is identical in structure to a centriole (9+0 triplets).
Cilia are short (2-20 micrometers), numerous, and beat in coordinated waves with a distinct power stroke and recovery stroke. They move fluid past the cell surface--respiratory cilia sweep mucus laden with trapped particles out of the airways, and cilia in the oviduct propel the egg toward the uterus. Flagella are long (up to 200 micrometers), usually one or two per cell, and move with an undulatory, whip-like motion, as exemplified by sperm propulsion.
Primary cilia are a distinct category: non-motile, possessing a 9+0 axoneme (lacking the central pair), they function as sensory antennae rather than propulsive devices. Primary cilia are involved in signaling pathways such as the Hedgehog pathway and mechanosensation in kidney tubules. Defects in primary cilia signaling are linked to polycystic kidney disease and a growing list of other conditions collectively termed ciliopathies.
<image>A two-panel figure. Panel A: Cross-section of a cilium/flagellum showing the 9+2 axoneme arrangement — nine outer microtubule doublets (A and B tubules) surrounding two central singlets, with outer dynein arms, inner dynein arms, nexin links between doublets, radial spokes connecting outer doublets to the central sheath, and the plasma membrane surrounding the entire structure. Panel B: Side view comparing cilia (short, numerous, rhythmic beating pattern shown with arrows) and flagella (long, single, undulatory wave pattern shown with arrows). A basal body is shown at the base of each, with its 9+0 triplet cross-section.</image>
VII. Cell Junctions (Brief Overview)
Animal cells are connected to one another and to the extracellular matrix by several types of junctions. Tight junctions seal adjacent cells together, creating a barrier that prevents leakage between cells--essential, for example, in the intestinal epithelium, where they prevent digestive enzymes and stomach acid from seeping between cells. Desmosomes are anchoring junctions that connect the intermediate filament networks of adjacent cells, providing resistance to mechanical stress in tissues like skin and heart muscle. Gap junctions contain channels called connexons that allow direct passage of ions and small molecules between neighboring cells, enabling rapid communication and coordination. In plants, plasmodesmata serve an analogous role, providing channels through the cell wall that connect the cytoplasm of adjacent cells.
