Myosin – The Actin Motor Protein

• Myosins are a family of proteins that utilize ATP hydrolysis to move along actin filaments.

• Myosin II was the first identified and isolated from skeletal muscle; previously believed to be the only type.

• Today, multiple classes of myosin are known, each with diverse functions:

  • Movement of organelles

  • Muscle contraction

  • Cell migration

• Myosins categorize as mechanochemical enzymes or motor proteins.

Myosin Structure

• Composition:

  • Heavy chains (1-2) and several light chains.

• Regions:

  • Head: Actin binding and nucleotide binding sites (ATP).

  • Neck: Contains α-helical regions, associates with light chains that regulate head activity.

  • Tail: Binding sites that define myosin specificity.

3D Structure of Myosin Head Domain

• Consists of:

  • Head and Neck Domain: Includes actin-binding site.

  • Nucleotide-binding site.

  • Light chains: Essential and regulatory types.

Classes of Myosins

• Class I: 10-14 nm step size, functions in membrane association, endocytosis.

• Class II: 5-10 nm step size, primarily involved in contraction.

• Class V: 36 nm step size, movement of organelles.

Mechanism of Myosin Walking along Actin Filaments

1. In the absence of ATP, the myosin head is firmly attached to the actin (rigor mortis).

2. Binding ATP: Myosin head releases from actin.

3. Hydrolysis of ATP: Converts ATP to ADP + Pi, inducing a rotation and storing energy in a "cocked state".

4. Binding to Actin: Myosin in cocked state attaches to actin.

5. Power Stroke: Release of Pi couples with elastic energy release, moving the actin filament.

6. Myosin remains bound as ADP is released, awaiting ATP rebinding.

Skeletal Muscle Sarcomere Structure

• Skeletal Muscles: Composed of muscle fibers with bundles of multinucleated cells.

• Each fiber contains myofibrils made up of sarcomeres:

  • Z Disk: Located at each end of a sarcomere, shared with adjacent sarcomeres.

  • I Bands: Actin thin filaments anchored to Z disk, not overlapped with myosin.

  • A Band: Thick filaments interdigitate with thin filaments.

Sliding-Filament Model of Muscle Contraction

• Relaxed Sarcomere: Low Ca2+ concentration.

• Contraction: Increased Ca2+ stimulates ATP-dependent contraction, shortening the sarcomere length by pulling actin filaments toward the center.

Ca2+-Dependent Thin-Filament Regulation

• Tropomyosin (TM): Coiled-coil dimer covering actin, blocking myosin binding in low Ca2+

• Troponin: Complex that regulates TM position via Ca2+ binding.

  • Low Ca2+: TM prevents cross-bridge cycling.

  • High Ca2+: TM exposes myosin-binding sites on actin, allowing contraction.

• Mutations in TN or TM can lead to cardiomyopathies.

Microtubule Motor Proteins

Kinesins

• 14 distinct families; humans have 45.

• Move processively using ATP, mainly transporting cargo to + end.

Dyneins

• Move towards the minus end of microtubules.

• Composed of heavy chains (1-3), can be cytoplasmic or axonemal.

• Vital for organelle movement and cilia/flagella beating.

Dynein Structure and Mechanism

• Contains multiple subunits, ATP hydrolysis facilitates movement.

• Dynactin Complex: Assists cytoplasmic dynein in cargo transport; binds to dynein and cargo, regulated by protein interactions.

Cargo Movement by Kinesins and Dyneins

• Kinesin transports cargo to the + end; dynein to the - end.

• Various cargoes include organelles like lysosomes, mitochondria, Golgi, etc.

Molecular Motors Summary

• Types of motors: Myosin V, Kinesin 1, Dynein.

• Functions supported by cargo-binding tail domains and motor head domains.