10.2 Molecular motors: myosin, kinesin, and dynein

Molecular motors are tiny protein machines that power cell movement and transport. Myosin, kinesin, and dynein convert chemical energy from ATP into mechanical work, enabling muscle contraction, cell division, and cargo transport along cytoskeletal tracks.

These motors have unique structures tailored to their functions. Myosin works with actin filaments, while kinesin and dynein move along microtubules. Their cyclic interactions with cytoskeletal tracks, coupled with ATP hydrolysis, generate force and directed movement within cells.

Molecular motors and cytoskeletal components

Types of molecular motors and their associated cytoskeletal components

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• Myosin, kinesin, and dynein are the three main types of molecular motors
  • Myosin associates with actin filaments
  • Kinesin and dynein associate with microtubules
• Molecular motors convert chemical energy from ATP hydrolysis into mechanical work
  • Enables movement along cytoskeletal tracks and cargo transport within the cell
• Specific motor-cytoskeleton associations allow for directed movement and force generation in various cellular processes (muscle contraction, vesicle transport, cell division)

Role of molecular motors in cellular processes

• Molecular motors are enzymes that convert chemical energy into mechanical work
• Enable movement along cytoskeletal tracks and transport cargo within the cell
• Specific associations between motors and cytoskeletal components allow for directed movement and force generation
  • Examples: muscle contraction, vesicle transport, cell division

Structure and function of myosin, kinesin, and dynein

Myosin structure and function

• Large, multi-subunit protein with a globular head domain and a long tail domain
  • Globular head domain binds to actin and hydrolyzes ATP
  • Long tail domain involved in cargo binding and thick filament formation
• Responsible for muscle contraction and various other cellular movements
  • Examples: cytokinesis, cell migration, vesicle transport

Kinesin structure and function

• Dimeric protein with two globular head domains, a neck linker, and a tail domain
  • Globular head domains bind to microtubules and hydrolyze ATP
  • Neck linker undergoes conformational changes
  • Tail domain binds cargo
• Primarily transports cargo towards the plus end of microtubules in anterograde transport
  • Examples: transport of organelles, vesicles, and protein complexes

Dynein structure and function

• Large, multi-subunit protein complex with two heavy chains and several intermediate, light intermediate, and light chains
  • Heavy chains contain globular head domains that bind to microtubules and hydrolyze ATP
  • Intermediate, light intermediate, and light chains involved in cargo binding and regulation
• Mainly transports cargo towards the minus end of microtubules in retrograde transport
  • Examples: transport of organelles, vesicles, and protein complexes

Mechanisms of force generation and movement

Cyclic interaction between motor domains and cytoskeletal tracks

• Molecular motors generate force and movement through cyclic interactions between motor domains and cytoskeletal tracks
  • Coupled with ATP hydrolysis
• Myosin power stroke
  • Triggered by the release of inorganic phosphate from the myosin head
  • Causes a conformational change that pulls the actin filament, resulting in muscle contraction
• Kinesin and dynein hand-over-hand mechanism
  • Two motor domains alternate in binding to the microtubule and undergoing conformational changes
  • Results in a stepping motion along the microtubule

Conformational changes and coordination of motor domains

• Kinesin neck linker undergoes a conformational change upon ATP binding
  • Propels the unbound head forward to the next binding site on the microtubule
  • Leads to processive movement
• Coordination of the two heads in kinesin and dynein is essential for efficient and directional movement
  • Regulation of their affinity for the microtubule track

ATP hydrolysis in molecular motor activity

ATP hydrolysis as the primary energy source

• ATP hydrolysis provides the necessary chemical energy to drive conformational changes and force generation in molecular motors
• Myosin
  • ATP binding causes dissociation from actin
  • ATP hydrolysis and subsequent release of inorganic phosphate and ADP lead to the power stroke and force generation
• Kinesin and dynein
  • ATP binding and hydrolysis in the globular head domains cause conformational changes in the neck linker or linker domain
  • Drives the stepping motion along the microtubule

Coupling of ATP hydrolysis with mechanical cycles

• Coupling of ATP hydrolysis with the mechanical cycles of molecular motors ensures efficient energy conversion into directed movement and force production
• Rate of ATP hydrolysis and efficiency of energy transduction can regulate the speed and processivity of molecular motors
  • Implications for their specific cellular functions (fast vs. slow muscle contraction, long-distance vs. short-distance transport)