Molecular Motors Study Notes

Molecular Motors: Proteins that move or generate force using actin, microtubules, or DNA as tracks.
- Functions:
- Carry objects around the cell.
- Help position organelles.
Energy Source:
- Utilize chemical energy of ATP to generate motion.

Groups of Motor Proteins in Cytoplasm: Three main types: Myosins, Kinesins, and Dyneins.
- Myosins:
- Move along actin filaments.
- Kinesins and Dyneins:
- Move along microtubules.
Core Structure:
- Motor proteins (myosins, kinesins, and dyneins) share a similar core structure.
- Size Comparison:
- Dyneins: Largest (~1000 kDa).
- Kinesins: Smallest (~380 kDa).
- Myosins: Approximately 520 kDa.

Functionality: Myosin is a superfamily of motors with diverse functions.
- Motor Domains: Share similar motor domains which indicate a common evolutionary origin.
- Movement Direction: Most myosin types move towards the plus end of actin filaments except for Myosin VI.
Species Examples:
- Found across various organisms including Arabidopsis, maize, human, mouse, rat, C. elegans, Drosophila, Saccharomyces, Plasmodium, Toxoplasma.

Myosin II:
- Functions:
- Contractile activity in muscle and nonmuscle cells.
- Cytokinesis (pinching apart a dividing cell into two daughter cells).
- Forward translocation of the cell body during migration.
Myosin I:
- Contains a second actin-binding or membrane-binding site in their tails.
- Functions in intracellular organization and protrusion of actin-rich structures at the cell surface.
Myosin V:
- Function: Vesicle and organelle transport.
Myosin VI:
- Role in the inner ear; mutations linked to deafness.

Structure:
- N-terminus: Coiled-coil structure of two alpha helices.
- C-terminus: Composed of two heavy chains and four light chains, with specific domains:
- Globular Head Domain: Binds ATP and actin.
- Long Tails: Form coiled coils and provide structural stability.
- Hinge/Neck Region: Functions as a lever arm.
Dimensions:
- Myosin heads: 150 nm
- Bare zone: 100 nm
- Total length of myosin molecule: 500 nm

Head Domain:
- Binds actin.
- Hydrolyzes ATP.
- Couples ATP hydrolysis to actin binding and motion generation.
Neck Domain:
- Binds light chains, stiffening the neck and regulating function.
- Acts as a lever arm, enhancing movement efficiency.
Tail Domain:
- Protein-protein interaction domain for binding with other myosins, cargo, or attachment to membranes.

Measuring Myosin II Motion:
- Experimental setup involves coating surfaces with myosin and introducing fluorescently labeled actin.
Myosin V's Role as Organelle Transporter:
- Myosin V walks along actin filaments carrying vesicles.
- C-terminal tail attaches to cargo, while the N-terminal head interacts with actin filaments.
Cytoplasmic Streaming:
- Myosin molecules are responsible for moving organelles in plant cells through cytoplasmic streaming.

Class II Myosins:
- Step size: 10-14 nm.
- Functions include contraction, endocytosis, and membrane association.
Class V Myosins:
- Step size: 5-10 nm.
- Function mainly in organelle transport and vesicle movement.

Dyneins and Kinesins:
- Dyneins: Move towards the minus (-) end of microtubules.
- Kinesins: Move towards the plus (+) end of microtubules.
Kinesin Structure:
- Similar structural features to myosins, walks along microtubules, facilitated by light chains binding to cargo.
Cytoplasmic Dynein:
- Heavy molecular weight (approximately 1000 kDa).
- Interacts with microtubules through a stalk and utilizes the dynactin complex for cargo attachment.
Competition:
- Kinesins and dyneins exhibit a competitive mechanism, often described as a 'tug-of-war' for directional movement.

Rotary Motors:
- Examples include flagellar motors providing locomotion in bacteria.
- F1FO ATPase utilizes a proton gradient to generate ATP, representing a different mode of molecular motion.