Unit 6: Molecular motors and mechanobiology

I. Molecular Motors: Orientation

Molecular motors are proteins that convert chemical energy stored in ATP into directed mechanical work, driving transport, division, and force generation inside cells. They operate at the nanometre scale where thermal noise (Brownian motion) is comparable to the forces they produce, so they function as biased ratchets rather than classical engines.

• Energy source: Each motor hydrolyses ATP to ADP + Pᵢ, releasing ~20 kᵦT (~50 pN·nm) per cycle; conformational changes couple this to a mechanical "power stroke".
• Track dependence: Motors run along polar cytoskeletal filaments — kinesin and dynein on microtubules, myosin on actin — and read filament polarity for directionality.
• Processivity: The number of steps taken before detaching; processive motors (kinesin-1, myosin V) take many steps, non-processive motors (muscle myosin II) detach each cycle.
• Step size: Discrete displacements — kinesin ~8 nm, myosin V ~36 nm — set by the geometry of the track and lever arm.
• Force output: Single motors stall at ~5–7 pN, measured by optical trap experiments.
• Duty ratio: Fraction of the ATPase cycle a head spends bound to its track; high duty ratio enables single-motor transport, low duty ratio suits large teams.

II. Kinesin, Dynein and Myosin, and Intracellular Movement

Cytoskeletal motors move cargo, organelles, and chromosomes along filaments, giving cells spatial organisation that free diffusion cannot achieve over long distances.

A. Kinesin

Kinesin-1 is a plus-end-directed microtubule motor that carries vesicles and organelles toward the cell periphery.

• Structure: A homodimer with two N-terminal globular motor (head) domains, a coiled-coil stalk, and a C-terminal tail that binds cargo or adaptor proteins.
• Directionality: Moves toward the microtubule plus end (away from the centrosome), e.g. anterograde axonal transport toward the synapse.
• Hand-over-hand walking: The two heads alternate leading positions, advancing 8 nm per step — the tubulin dimer spacing — one ATP hydrolysed per step.
• Coordination (gating): Neck-linker docking in the ATP-bound leading head strains the trailing head, synchronising the two so they never release simultaneously; this keeps kinesin processive over hundreds of steps.

B. Dynein

Cytoplasmic dynein is the principal minus-end-directed microtubule motor, retrieving cargo toward the cell centre.

• Structure: A large complex built on two heavy chains, each with an AAA+ ATPase ring, a microtubule-binding stalk, and a linker that acts as the mechanical lever; assembled with intermediate and light chains.
• Directionality: Moves toward the microtubule minus end, driving retrograde transport and positioning the Golgi and nucleus.
• Regulation: Requires the dynactin complex and cargo adaptors (e.g. BicD2) to become processive; isolated dynein is weakly active.
• Mechanism: ATP binding in the AAA1 site drives a swing of the linker across the ring, producing the power stroke; step size is variable (8–32 nm), unlike kinesin's fixed step.

C. Myosin

Myosins are actin-based motors; different classes serve contraction versus transport.

1. Myosin II (non-processive, muscle): Assembles into bipolar thick filaments; low duty ratio means each head is bound only ~5% of the cycle, so hundreds act as a team to slide actin in the sarcomere.
2. Myosin V (processive, transport): Long lever arms give a 36 nm step matching the actin helical repeat, allowing a single dimer to walk cargo along actin toward the plus (barbed) end.

• Cross-bridge cycle: ATP binding detaches the head from actin; hydrolysis re-cocks the lever; Pᵢ release triggers the power stroke; ADP release resets — described by the Lymn–Taylor scheme.
• Directionality: Most myosins move toward the actin plus (barbed) end; myosin VI is the exception, moving toward the minus end.

D. Intracellular Movement

Motors organise the crowded cytoplasm by driving directed transport and mechanical events.

• Long-range transport: Microtubule motors carry cargo over micrometres (axons up to a metre) where diffusion would take years.
• Local delivery: Actin–myosin handoff moves cargo through the dense cortical actin near the membrane, completing the "highway-to-local-road" model.
• Tug-of-war: Opposing kinesin and dynein bind the same cargo; the net direction reflects the balance of engaged motors, enabling rapid reversals.
• Cell division: Kinesins and dynein position spindle poles and separate chromosomes; myosin II drives cytokinetic furrow constriction.
• Worked example — axonal transport rate: Fast axonal transport proceeds at ~1 µm/s; a 1 mm axon segment is traversed in 10³ s (~17 min), versus diffusion time t ≈ x²/2D ≈ (10⁻³)² / (2·10⁻¹¹) ≈ 5×10⁴ s (~14 h) for a small protein — demonstrating why motors are essential.

III. Microtubule Structure

Microtubules are the stiff, polar tracks that kinesin and dynein read, and they template the mitotic spindle.

A. Building Block and Assembly

The microtubule is a hollow tube polymerised from tubulin dimers.

• Subunit: A heterodimer of α-tubulin and β-tubulin, each ~55 kDa, bound head-to-tail into protofilaments.
• Protofilaments: Typically 13 align laterally to form a hollow cylinder of outer diameter ~25 nm and inner lumen ~15 nm.
• Polarity: The consistent α–β orientation gives a minus end (α exposed, near centrosome) and a plus end (β exposed, dynamic periphery) — the basis for motor directionality.
• Nucleation: γ-tubulin ring complexes at the centrosome/MTOC template the minus end.

B. Dynamic Instability

Microtubules stochastically switch between growth and shrinkage.

• GTP cap: β-tubulin binds GTP; a cap of GTP-tubulin stabilises the growing plus end.
• Catastrophe and rescue: GTP hydrolysis to GDP behind the tip destabilises the lattice; loss of the cap triggers catastrophe (rapid depolymerisation), while re-addition of GTP-tubulin gives rescue.
• Functional value: This search-and-capture behaviour lets microtubules probe cytoplasmic space and attach to kinetochores during mitosis.

C. Structural Properties and Regulation

Microtubule mechanics and stability set their cellular roles.

• Rigidity: Persistence length of millimetres makes microtubules the stiffest cytoskeletal filament, suited to compression-bearing and long-range tracks.
• Post-translational modifications: Detyrosination, acetylation, and polyglutamylation form a "tubulin code" read by motors and MAPs.
• Associated proteins: MAPs (tau, MAP2) stabilise lattices; +TIP proteins (EB1) track growing ends.
• Pharmacology: Taxanes stabilise and colchicine/vinblastine destabilise microtubules — exploited as anticancer and antimitotic agents.

IV. Mechanobiology and Its Importance in Human Health

Mechanobiology studies how cells sense, generate, and respond to mechanical forces, translating physical cues into biochemical signals.

A. Principles of Mechanotransduction

Cells convert mechanical stimuli into biochemical responses.

• Definition: Mechanotransduction is the conversion of physical force (tension, shear, compression) into intracellular signalling.
• Force-sensing structures: Integrin-based focal adhesions couple the extracellular matrix to the actomyosin cytoskeleton; mechanosensitive ion channels (Piezo1/2) open under membrane tension.
• Motor contribution: Myosin II generates cytoskeletal tension that cells use to probe substrate stiffness; the resulting force feeds back onto adhesion signalling (e.g. YAP/TAZ nuclear translocation).
• Matrix stiffness: Measured as elastic modulus (kPa); soft (~0.1–1 kPa) matrices resemble brain, stiff (~30+ kPa) matrices resemble bone.

B. Cellular Responses to Force

Mechanical environment directs core cell behaviours.

• Stem-cell fate: Substrate stiffness biases mesenchymal stem cell differentiation — soft matrices favour neurons, stiff matrices favour bone (Engler experiments).
• Cell migration: Cells undergo durotaxis, migrating toward stiffer substrates by traction-force sensing.
• Tissue morphogenesis: Coordinated actomyosin contraction folds and shapes epithelia during development.

C. Importance in Human Health

Disrupted mechanics underlies major diseases and guides therapy.

• Cancer: Tumour matrix stiffening promotes invasion and metastasis; altered mechanotransduction drives proliferation via YAP/TAZ.
• Cardiovascular disease: Endothelial cells sense blood shear stress; disturbed flow at arterial bends predisposes to atherosclerotic plaque.
• Fibrosis: Progressive tissue stiffening activates myofibroblasts, creating a feed-forward loop in lung, liver, and kidney fibrosis.
• Muscular and motor disorders: Mutations in motor proteins cause disease — kinesin/dynein defects impair axonal transport in neurodegeneration (e.g. hereditary spastic paraplegia, ALS), and myosin mutations cause hypertrophic cardiomyopathy.
• Hearing: Mechanosensitive channels in cochlear hair cells convert sound-induced deflection into electrical signals; their failure causes deafness.
• Bone remodelling: Osteocytes sense mechanical load and regulate bone deposition; unloading (microgravity, bed rest) causes bone loss, illustrating the therapeutic relevance of controlled mechanical stimulation.