Myosin is a motor protein that interacts with actin filaments to generate mechanical force for various types of cellular movement. It plays a central role in muscle contraction, cell motility, cytokinesis, and intracellular transport.
Structure:
Myosin proteins share a common basic structure:
- Head domain:
- Binds to actin
- Has ATPase activity—hydrolyzes ATP to provide energy for movement
- Neck (lever arm) domain:
- Acts as a hinge that amplifies movement from the head
- Tail domain:
- Varies in structure and function
- Responsible for binding cargo, other myosins, or cell structures
Classes of Myosin:
| Myosin Type | Function | Example |
|---|---|---|
| Myosin II | Muscle contraction, cytokinesis | Found in skeletal, cardiac, and smooth muscle |
| Myosin I | Endocytosis, membrane trafficking | Non-muscle cells |
| Myosin V | Organelle and vesicle transport | Transports cargo along actin filaments |
| Myosin VI | Moves toward the minus end of actin (unusual) | Endocytosis, stereocilia function in ear |
How Myosin Works (Muscle Example):
- ATP binds to myosin → myosin releases actin.
- ATP is hydrolyzed → myosin head becomes “cocked”.
- Myosin binds new position on actin.
- Power stroke: release of phosphate triggers the head to pivot, pulling actin filament.
- ADP is released → cycle repeats.
This cycle is central to the sliding filament model of muscle contraction.
Biological Functions:
| Function | Description |
|---|---|
| Muscle contraction | Myosin II slides actin filaments past each other to shorten muscle fibers |
| Cell motility | Myosin helps drive cell crawling, wound healing, and morphogenesis |
| Cytokinesis | Myosin forms the contractile ring that divides cells during mitosis |
| Intracellular transport | Carries vesicles, organelles, and RNA along actin tracks |
Clinical Relevance:
- Cardiomyopathies: Mutations in myosin heavy chain genes can lead to heart muscle diseases.
- Hearing loss: Mutations in myosin VI and VIIA affect stereocilia in the inner ear.
- Cancer: Abnormal myosin activity can affect cell migration and metastasis.
Summary Table:
| Feature | Description |
|---|---|
| Protein Type | Motor protein |
| Interacts With | Actin filaments |
| Energy Source | ATP hydrolysis |
| Main Functions | Muscle contraction, transport, cell division |
| Direction of Movement | Usually toward actin’s plus (+) end (except myosin VI) |
| Examples | Myosin I, II, V, VI |