Myosins and Molecular Motors Overview

Myosins and Molecular Motors

• Myosins are biological machines driving movement in living organisms.

Myosin Classes

• Eighteen myosin classes are identified in the human genome.
• Myosin I-XVIII;
• Skeletal, cardiac, and smooth muscle myosins fall under Class II.

Myosin I

• A low molecular weight motor protein associated with cell membranes.
• Contains a tail rich in basic amino acids that can bind to anionic phospholipids.
• Contains a glycine, proline, alanine (GPA) domain facilitating actin binding.
• Involved in membrane-membrane interactions and functions in the brush borders of small intestines.
• Possibly involved in transporting Golgi-derived vesicles.

Myosin V

• Involved in organelle transport (synaptic vesicles, melanosomes, vacuoles, mRNA).
• Found in all brain neurons; required for melanosome transport.
• Mutations linked to neurological disorders and depigmentation, especially hair.

Elejalde Syndrome

• Characterized by silvery hair and CNS dysfunction.
• Large melanin granules unevenly distributed in hair shaft.
• Abnormal melanocytes and melanosomes might be present in fibroblasts.
• Suggests defective mechanisms for melanin movement and distribution.

Myosin VI

• Expressed in most cells and tissues.
• Extended neck region with one calmodulin (IQ motif) binding region per head.
• A dimeric molecule with a coiled tail and two globular ends (one per monomer).
• Defects in the gene cause hearing problems due to hair movement defects in the inner ear.

Kinesins

• Microtubule-based molecular motors.
• Do not form filaments like some myosins.
• Generate motion by conformational changes upon ATP binding and hydrolysis.
• Similar mechanism to myosin actin-based motors.
• Structure includes two heavy chains and two light chains.
• Includes microtubule and ATP binding sites in the head and cargo-binding sites in the tail.

Kinesin 1

• Na/K-ATPase containing vesicle transport via kinesin 1 on microtubules (KLC2, KHC).

Dynein

• Axonemal dynein affecting flagella and cilia movement.
• Cytoplasmic dynein affecting cytoplasmic structure distribution and organization.
• Structure includes a spherical globular head, stalk, and light chains attached to a cargo. Movement on microtubule.

Molecular Motor Comparison

• Comparison diagrams illustrating the modes of action for dynein, myosin, and kinesin heavy chain movement along their respective tracks (microtubules, actin filaments).

Additional Information

• T. Devlin's Textbook of Biochemistry (pp. 979-981) provides further details.