Unit 6: Molecular motors and mechanobiology - Subjective Questions

Molecular motors are specialized protein machines that convert chemical energy, typically derived from the hydrolysis of ATP (adenosine triphosphate), into mechanical work and directed movement.

Key roles in cellular function:

• Intracellular transport: They carry cargo such as vesicles, organelles, and proteins along cytoskeletal tracks.
• Cell division: Motors help separate chromosomes during mitosis by acting on the mitotic spindle.
• Muscle contraction: Myosin motors interact with actin to generate contractile force.
• Cell motility: They drive movement of cilia, flagella, and the cell body itself.

Common features:

• A motor (head) domain that binds the track and hydrolyzes ATP.
• A tail domain that binds specific cargo.
• Directionality, moving toward a defined end of their cytoskeletal filament.

The efficiency of energy conversion in molecular motors can approach , making them far more efficient than most man-made engines.

Kinesin is a motor protein that transports cargo toward the plus end of microtubules (anterograde transport).

Structure:

• Two heavy chains and two light chains forming a dimer.
• Two globular motor (head) domains at the N-terminus that bind microtubules and contain ATP-binding sites.
• A coiled-coil stalk connecting the heads to the tail.
• A tail domain that binds cargo via light chains.

Mechanism of movement (hand-over-hand):

1. One head binds to the microtubule while the other is detached.
2. ATP binding to the bound head causes a conformational change in the neck linker.
3. This swings the trailing head forward by about , so it becomes the new leading head.
4. ATP hydrolysis and release reset the cycle.

Each step advances kinesin by approximately (one tubulin dimer length), and one ATP is hydrolyzed per step. This processive motion allows a single kinesin to take many steps without detaching.

Dynein is a large minus-end-directed microtubule motor responsible for retrograde transport and ciliary/flagellar beating.

Structure:

• Composed of heavy chains, intermediate chains, and light chains.
• Each heavy chain contains a large motor domain built from a ring of six AAA+ ATPase modules.
• A stalk with a microtubule-binding domain projects from the ring.
• A stem/tail links to cargo and accessory proteins.

Types:

• Cytoplasmic dynein: transports cargo, positions organelles, moves toward the minus end.
• Axonemal dynein: drives the bending of cilia and flagella.

Function:

• Retrograde transport of vesicles and organelles toward the cell center.
• Positioning of the Golgi apparatus and other organelles.
• Requires the accessory complex dynactin for cargo binding and processivity.
• Powers movement using ATP hydrolysis at the AAA+ domains, producing conformational changes that generate a power stroke.

Myosin is an actin-based motor protein central to muscle contraction and various cell movements.

Structure of Myosin II:

• Two heavy chains and four light chains.
• Two globular head (motor) domains that bind actin and ATP.
• A neck (lever arm) region stabilized by light chains.
• A long coiled-coil tail allowing assembly into thick filaments.

Sliding Filament Mechanism:

1. ATP binding to myosin head causes it to detach from actin.
2. ATP hydrolysis () cocks the head into a high-energy state.
3. The head binds a new site on the actin filament forming a cross-bridge.
4. release triggers the power stroke, sliding the actin filament past the myosin.
5. ADP release and new ATP binding restart the cycle.

As thin (actin) filaments slide over thick (myosin) filaments, the sarcomere shortens, but the filament lengths remain constant. This produces muscle contraction and force generation.

Comparison of Kinesin and Dynein:

Summary:

• Both are microtubule-based motors powered by ATP hydrolysis.
• They work in opposite directions, allowing bidirectional transport within cells.
• Kinesin has a simpler, more processive mechanism, while dynein is structurally complex and depends on regulatory partners.

Intracellular movement is the regulated transport of vesicles, organelles, and macromolecules within a cell, achieved by molecular motors moving along cytoskeletal tracks.

Key components:

• Microtubules: highways for kinesin (outward) and dynein (inward) transport.
• Actin filaments: tracks for myosin motors, important for short-range transport near the cell periphery.

Organization of transport:

• Anterograde transport: kinesin moves cargo from the cell center toward the periphery (e.g., toward the axon tip in neurons).
• Retrograde transport: dynein moves cargo from the periphery back toward the nucleus.
• Cargo hand-off: vesicles can switch between microtubule and actin tracks for precise delivery.

Regulation:

• Adaptor and scaffolding proteins ensure correct cargo–motor binding.
• Motors can be switched on/off to control direction and stopping points.

Importance:

• Essential in neurons, where materials must travel long distances along axons.
• Disruption leads to neurodegenerative diseases and impaired cellular organization.

Microtubules are hollow, cylindrical polymers of the cytoskeleton that provide structural support and serve as tracks for motor proteins.

Subunit composition:

• Built from tubulin dimers, each consisting of one -tubulin and one -tubulin subunit.
• Dimers stack head-to-tail to form protofilaments.
• Typically 13 protofilaments align side by side to form the hollow tube.

Dimensions:

• Outer diameter ~, inner lumen ~.

Polarity:

• Plus end: fast-growing, exposes -tubulin.
• Minus end: slow-growing, often anchored at the centrosome (MTOC).

Assembly (Dynamic Instability):

• Requires GTP bound to tubulin.
• Microtubules alternate between growth (polymerization) and rapid shrinkage (catastrophe).
• A GTP cap stabilizes the growing end; loss of the cap triggers depolymerization.

This dynamic behavior enables microtubules to rapidly reorganize during processes like mitosis and cell shape changes.

Dynamic instability refers to the coexistence of growing and shrinking microtubules within a cell, where individual microtubules switch stochastically between polymerization and depolymerization.

Mechanism:

• Tubulin dimers bind GTP before adding to the microtubule plus end.
• A GTP cap at the growing end stabilizes the structure.
• After incorporation, GTP is hydrolyzed to GDP, weakening lateral bonds.
• If the GTP cap is lost, the microtubule undergoes catastrophe (rapid shrinkage).
• Regaining a GTP cap causes rescue (return to growth).

Key transitions:

• Catastrophe: switch from growth to shrinkage.
• Rescue: switch from shrinkage to growth.

Biological significance:

• Enables microtubules to search and capture chromosomes during mitosis.
• Facilitates rapid remodeling of the cytoskeleton.
• Allows cells to explore intracellular space efficiently.
• Target of anticancer drugs (e.g., taxol) that suppress dynamics to block cell division.

Mechanobiology is an interdisciplinary field that studies how physical forces and changes in cell/tissue mechanics influence biological processes, including development, physiology, and disease.

Core ideas:

• Cells sense mechanical cues such as stiffness, stretch, shear stress, and compression.
• These signals are converted into biochemical responses through mechanotransduction.

Key mechanisms:

• Mechanosensors: ion channels, integrins, and cytoskeletal elements.
• Signal conversion: mechanical force alters protein conformation, opening channels or triggering signaling cascades.

Importance in human health:

• Cardiovascular system: blood flow shear stress regulates endothelial cell function; abnormalities lead to atherosclerosis.
• Bone remodeling: mechanical loading strengthens bone (Wolff's law); disuse causes osteoporosis.
• Cancer: tumor tissue stiffening promotes progression and metastasis.
• Development: mechanical forces guide tissue morphogenesis.

Understanding mechanobiology enables improved tissue engineering, regenerative medicine, and design of mechanically informed therapies.

Mechanotransduction is the process by which cells convert mechanical stimuli into biochemical signals, allowing them to respond to their physical environment.

Steps of mechanotransduction:

1. Sensing: A mechanical force (stretch, pressure, shear) is applied to the cell.
2. Transmission: Force is relayed through membrane proteins and the cytoskeleton.
3. Conversion: Force alters protein conformation, opening channels or exposing binding sites.
4. Response: Downstream signaling changes gene expression, growth, or movement.

Major mechanosensors:

• Integrins: transmembrane receptors linking the extracellular matrix (ECM) to the cytoskeleton; sense ECM stiffness.
• Mechanosensitive ion channels (e.g., Piezo1/Piezo2): open in response to membrane tension, allowing ion flux.
• Cytoskeleton (actin, microtubules): transmits and distributes forces internally.
• Focal adhesions: protein complexes that anchor cells and sense substrate mechanics.
• Cadherins: sense cell–cell forces.

Significance: Proper mechanotransduction is essential for tissue homeostasis, and its dysregulation contributes to cancer, fibrosis, and cardiovascular disease.

Comparison of Microtubules and Actin Filaments:

Summary:

• Both are dynamic cytoskeletal polymers with defined polarity.
• Microtubules serve as long-distance transport highways.
• Actin filaments support short-range transport and force generation near the cell surface.
• Together they coordinate cell shape, movement, and internal organization.

Molecular motors couple the chemical energy of ATP hydrolysis to mechanical steps, producing directed motion through cyclic conformational changes.

General ATP cycle:

Kinesin ATPase cycle (per head):

1. ATP binding: The leading head binds ATP tightly to the microtubule; the neck linker docks forward.
2. Neck linker docking: This conformational change throws the trailing head forward toward the next binding site (~ swing).
3. ATP hydrolysis: ATP is hydrolyzed to in the now-trailing head.
4. release: Weakens microtubule binding of the trailing head, allowing detachment.
5. Reattachment: The free head binds the next tubulin site, completing an advance.

Directionality:

• Achieved because conformational changes are biased in one direction by the asymmetric structure of the neck linker and the polarity of the microtubule.
• The alternating catalytic cycle of the two heads ensures processive, unidirectional stepping.

Thus, chemical energy is rectified into mechanical work with efficiencies approaching .

Molecular motors are essential for the assembly and function of the mitotic spindle, ensuring accurate chromosome segregation during cell division.

Key roles:

• Spindle assembly: Motors organize microtubules into a bipolar spindle structure.
• Chromosome capture and alignment: Motors at kinetochores help attach chromosomes to spindle microtubules and align them at the metaphase plate.
• Chromosome separation (anaphase):
  • Anaphase A: Kinetochore microtubules shorten, pulling chromosomes toward the poles (dynein-assisted).
  • Anaphase B: Motors push spindle poles apart, elongating the spindle.

Specific motors involved:

• Kinesin-5 (Eg5): cross-links and slides antiparallel microtubules to separate poles.
• Kinesin-14: minus-end-directed, focuses spindle poles.
• Cytoplasmic dynein: positions the spindle and anchors microtubules at the cortex.

Significance:

• Errors in motor function cause chromosome missegregation and aneuploidy, contributing to cancer.
• Motor proteins like Eg5 are targets for anticancer drugs.

A processive motor is one that can take many consecutive steps along its cytoskeletal track without fully detaching, transporting cargo over long distances.

Processive motors:

• At least one head remains attached to the track at all times.
• Move in a coordinated hand-over-hand fashion.
• Examples: Kinesin-1, Myosin V, cytoplasmic dynein.

Non-processive motors:

• Detach after each ATPase cycle; a single motor takes only one step before releasing.
• Must work in large teams to produce continuous movement.
• Examples: Myosin II (muscle myosin).

Comparison:

Duty ratio — the fraction of the ATPase cycle a head remains attached — is high for processive motors and low for non-processive ones.

Mechanobiology provides critical insights into how physical forces and altered tissue mechanics drive cancer development and spread.

Key mechanical changes in tumors:

• Increased matrix stiffness: Tumor tissues are often stiffer than normal tissue due to excess collagen deposition and cross-linking.
• Elevated interstitial pressure: Affects drug delivery and cell behavior.

Mechanisms linking mechanics to cancer:

• Stiffness sensing: Cancer cells sense increased ECM stiffness via integrins and focal adhesions, activating pro-growth signaling (e.g., YAP/TAZ pathway).
• Enhanced proliferation: Stiff environments promote uncontrolled cell division.
• Invasion and metastasis: Altered cytoskeletal mechanics enable cells to migrate, squeeze through tissues, and enter blood vessels.
• EMT (epithelial–mesenchymal transition): Mechanical cues can trigger this transition, increasing invasiveness.

Clinical importance:

• Tissue stiffness is used diagnostically (e.g., palpation, elastography).
• Targeting mechanotransduction pathways offers new therapeutic strategies.
• Understanding mechanics improves drug delivery to dense tumors.

The axoneme is the microtubule-based core structure of cilia and flagella that enables their beating movement.

Structure (9+2 arrangement):

• Nine outer doublet microtubules arranged in a ring.
• Two central single microtubules in the middle.
• Radial spokes connect the outer doublets to the central pair.
• Nexin links join adjacent doublets, limiting sliding.

Role of axonemal dynein:

• Dynein arms project from each outer doublet toward the adjacent one.
• Dynein motors use ATP hydrolysis to walk along the neighboring doublet, generating a sliding force.
• Because nexin links restrain free sliding, the sliding is converted into bending.

Movement mechanism:

• Coordinated activation of dynein on one side, then the other, produces the characteristic whip-like beating.

Biological significance:

• Drives sperm motility, movement of mucus in airways, and fluid flow in the brain.
• Defects in axonemal dynein cause primary ciliary dyskinesia, leading to respiratory problems and infertility.

Neurons critically depend on molecular motors because of their extreme length; materials synthesized in the cell body must travel great distances along axons.

Axonal transport:

• Anterograde transport (kinesin): carries synaptic vesicle precursors, mitochondria, and membrane proteins from the cell body to the axon terminal.
• Retrograde transport (dynein): returns signaling endosomes, damaged organelles, and recycled materials to the cell body.

Importance:

• Maintains synaptic function by delivering neurotransmitter machinery.
• Distributes mitochondria to regions of high energy demand.
• Transmits survival and stress signals from axon terminals to the nucleus.

Consequences of motor defects:

• Impaired transport leads to accumulation of cargo and axonal swelling.
• Associated diseases include:
  • Amyotrophic lateral sclerosis (ALS)
  • Alzheimer's disease (tau disrupts transport)
  • Charcot–Marie–Tooth disease (kinesin/dynein mutations)
  • Hereditary spastic paraplegia

Thus, molecular motors are vital for neuronal health, and their dysfunction is a hallmark of many neurodegenerative disorders.

Molecular motors operate against loads, and their velocity decreases as the opposing force increases, characterized by a force–velocity relationship.

Force–velocity relationship:

• At zero load, a motor moves at its maximum velocity .
• As opposing force increases, velocity decreases.
• A common approximate relationship:

where is the stall force.

Stall force ():

• The maximum load a motor can sustain before its net velocity becomes zero.
• At this point, the mechanical work opposing the motor balances the energy from ATP hydrolysis.
• For a single kinesin, –.

Energetic basis:

• Work done per step: , where (kinesin step).
• The energy available per ATP is about .
• Maximum force: (theoretical upper bound).

Significance: The force–velocity relationship characterizes motor efficiency and is measured using optical tweezers.

Studying molecular motors requires techniques capable of measuring nanometer displacements and piconewton forces.

Single-molecule and biophysical techniques:

• Optical tweezers (laser traps):

  • Use a focused laser beam to trap a microscopic bead attached to a motor.
  • Measure step size (~ for kinesin) and stall force (~–).

• Magnetic tweezers:

  • Apply controlled forces using magnetic beads; useful for torque and force measurements.

• Atomic Force Microscopy (AFM):

  • Probes surface topology and measures forces via a cantilever tip.

• Total Internal Reflection Fluorescence (TIRF) microscopy:

  • Visualizes single fluorescently labeled motors moving along filaments.

• Fluorescence Resonance Energy Transfer (FRET):

  • Detects conformational changes within motor proteins.

Mechanobiology tools:

• Traction force microscopy: measures forces cells exert on their substrate.
• Micropipette aspiration and substrate stiffness assays: study cellular mechanical responses.

Significance: These techniques reveal how motors convert chemical energy into mechanical work and how cells sense forces, bridging molecular mechanisms with physiological function.

Mechanobiology explains how mechanical forces regulate tissue maintenance in both bone and the cardiovascular system.

Bone remodeling:

• Bone continuously adapts to mechanical loading, described by Wolff's Law — bone strengthens in response to stress and weakens without it.
• Osteocytes act as mechanosensors, detecting fluid flow through canaliculi during loading.
• Mechanical signals regulate the balance between:
  • Osteoblasts (bone formation)
  • Osteoclasts (bone resorption)
• Consequences:
  • Weight-bearing exercise increases bone density.
  • Disuse (bed rest, microgravity) causes bone loss/osteoporosis.

Cardiovascular function:

• Blood flow generates shear stress on endothelial cells lining vessels.
• Endothelial cells sense shear via mechanosensitive channels (Piezo1) and the glycocalyx.
• Laminar (smooth) flow: promotes healthy, anti-inflammatory endothelial behavior.
• Disturbed/turbulent flow: promotes inflammation and plaque formation, leading to atherosclerosis.
• The heart also responds to mechanical load; chronic overload causes cardiac hypertrophy.

Significance: Understanding these mechanical cues informs treatments for osteoporosis, prevention of cardiovascular disease, and design of implants and prosthetics.