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Cell Biology Unit 5: The Cytoskeleton and Motility

This unit covers the three cytoskeletal filament systems, the motor proteins that move along them, and how they power muscle contraction, ciliary beating and cell crawling.

Covers: Actin filaments and their assembly, Microtubules and dynamic instability, Intermediate filaments, Motor proteins: myosin, kinesin and dynein, Muscle contraction, Cilia and flagella, Cell crawling and its regulation

Study guide

The cytoskeleton has three types of filaments. Actin filaments (microfilaments, about 7 nm) are two-stranded helices of globular actin. Microtubules (about 25 nm) are hollow tubes of 13 protofilaments made of αβ-tubulin dimers. Intermediate filaments (about 10 nm) are rope-like polymers of fibrous proteins such as keratins, vimentin, neurofilaments and nuclear lamins. Actin and microtubules are polar, with fast-growing plus ends and slower minus ends, and serve as tracks for motors. Intermediate filaments are nonpolar, have no motors, and mainly resist mechanical stress.

Actin and tubulin bind nucleotides (ATP and GTP) and hydrolyze them after joining a filament. The critical concentration is the free subunit level at which addition and loss balance; above it, ends grow, and below it, they shrink. Because the plus end has a lower critical concentration, an intermediate subunit concentration causes treadmilling. Microtubules show dynamic instability: a growing end keeps a cap of GTP-tubulin, but if hydrolysis catches up and the cap is lost, the end switches to rapid shrinking (catastrophe) until rescued. Microtubules are nucleated by γ-tubulin at the centrosome; actin is nucleated by Arp2/3 (branched networks) and formins (unbranched bundles).

Motor proteins convert ATP hydrolysis into movement. Myosins walk on actin, most toward the plus end. Kinesins move most cargo toward microtubule plus ends, while cytoplasmic dynein moves toward minus ends. In axons, where microtubule plus ends point outward, kinesin carries cargo toward synapses and dynein brings it back. Each motor has a cycle of binding, a conformational power stroke, and release. Drugs reveal roles: colchicine and nocodazole depolymerize microtubules, taxol stabilizes them, cytochalasin and latrunculin block actin polymerization, and phalloidin stabilizes actin filaments.

Skeletal muscle sarcomeres contain thin actin filaments anchored at Z discs and thick myosin II filaments in the middle. Contraction follows the sliding filament model: filaments slide past each other, so the sarcomere, I band and H zone shorten while the A band does not. In the cross-bridge cycle, ATP binding releases myosin from actin, hydrolysis cocks the head, binding to actin and Pi release cause the power stroke, and ADP leaves. Contraction is switched on when Ca²⁺ released from the sarcoplasmic reticulum binds troponin and shifts tropomyosin off the binding sites. Without ATP, heads stay bound, causing rigor.

Eukaryotic cilia and flagella have a 9 + 2 axoneme of microtubule doublets. Axonemal dynein arms slide neighboring doublets, and nexin links turn that sliding into bending. Defects cause primary ciliary dyskinesia, with lung infections and infertility. Crawling cells extend lamellipodia (Arp2/3 networks) and filopodia (formin bundles), attach the front through integrins at focal adhesions, and use myosin II to pull the rear forward. The small GTPases Cdc42, Rac and Rho trigger filopodia, lamellipodia and stress fibers respectively, linking signals from outside the cell to cytoskeletal change.

Key ideas

• Actin and microtubules are polar tracks for motors; intermediate filaments are nonpolar and provide strength.
• Filament ends grow above and shrink below their critical concentration.
• GTP hydrolysis in tubulin underlies dynamic instability.
• Kinesin moves toward plus ends, dynein toward minus ends, and myosin along actin.
• Muscle contracts by filaments sliding, controlled by Ca²⁺ binding troponin.
• Cilia bend when axonemal dynein slides constrained microtubule doublets.
• Cell crawling cycles through protrusion, adhesion and contraction, controlled by Rho-family GTPases.

Formulas and rules

Common mistakes

• Saying ATP binding causes the power stroke; ATP binding releases myosin, and Pi release drives the stroke.
• Thinking filaments shorten during muscle contraction; they slide.
• Mixing up kinesin and dynein directions.
• Assuming intermediate filaments have motors or polarity.
• Confusing taxol (stabilizes microtubules) with colchicine (prevents polymerization).

Before the test, make sure you can

• Compare the structure, polarity and function of actin, microtubules and intermediate filaments.
• Explain critical concentration, treadmilling and dynamic instability.
• Identify the direction and track of myosin, kinesin and dynein.
• Predict the effects of colchicine, taxol, cytochalasin, latrunculin and phalloidin.
• Describe the cross-bridge cycle and the role of Ca²⁺, troponin and tropomyosin.
• Explain how axonemal dynein bends cilia and what happens when it fails.
• Describe the steps of cell crawling and the roles of Rho, Rac and Cdc42.
• Calculate filament subunit numbers, motor ATP use and transport times.

Words to know

Actin filament

A 7 nm, two-stranded polymer of actin with plus (barbed) and minus (pointed) ends.

Microtubule

A 25 nm hollow tube of 13 protofilaments made of αβ-tubulin dimers.

Intermediate filament

A 10 nm nonpolar rope-like filament, such as keratin or lamin, that gives tensile strength.

Critical concentration

The free subunit concentration at which a filament end neither grows nor shrinks.

Treadmilling

Net addition at the plus end and loss at the minus end so subunits flow through a filament of constant length.

Dynamic instability

Random switching of microtubule ends between growth and rapid shrinkage due to loss of the GTP cap.

Centrosome

The main microtubule-organizing center, containing centrioles and γ-tubulin ring complexes.

Arp2/3 complex

A complex that nucleates new actin filaments as branches on existing filaments.

Formin

A protein that nucleates and elongates unbranched actin filaments.

Kinesin

A microtubule motor that usually moves toward the plus end.

Dynein

A large microtubule motor that moves toward the minus end; also bends cilia.

Myosin II

The actin motor in muscle thick filaments and in contractile rings and stress fibers.

Sarcomere

The contractile unit of striated muscle, between two Z discs.

Troponin–tropomyosin

Regulatory proteins on thin filaments that block myosin binding until Ca²⁺ binds troponin.

Axoneme

The 9 + 2 microtubule core of cilia and flagella.

Lamellipodium

A broad, sheet-like protrusion at the leading edge of a crawling cell, filled with branched actin.

Focal adhesion

A site where integrins connect actin stress fibers to the extracellular matrix.

Sample practice questions

30 more questions like these, from level 1 to 5, are in the practice set.