Chapter 9. Cytoskeletal Motor Proteins

Energy for Molecular Motor Proteins Generated By

Hydrolysis of ATP provides energy for movement along cytoskeletal filaments.

Three Categories of Molecular Motors

Kinesins, dyneins, and myosins.

Types of Cargo Moved by Motor Proteins

Vesicles, organelles, mRNA, proteins, chromosomes, and cytoskeletal filaments.

Structure of Kinesin

Composed of two heavy chains with motor domains (heads), a stalk (coiled-coil dimerization domain), and a cargo-binding tail.

Cytoskeletal Element Used by Kinesin

Microtubules serve as the track for kinesin movement.

Direction of Kinesin Movement

Moves toward the plus (+) end of microtubules, away from the centrosome.

Specificity for Cargo Achieved By

Interaction between the cargo-binding domain of kinesin and adaptor proteins that recognize specific cargo.

Highly Processive Meaning

A motor protein moves along a filament for long distances without detaching.

Functions of Dynein

Retrograde transport (toward the minus end), organelle positioning, mitotic spindle function, and cilia/flagella movement.

Structure of Dynein

Large multi-subunit protein with heavy chains (motor domains), intermediate chains, and light chains for cargo binding.

Direction of Dynein Movement

Moves toward the minus (-) end of microtubules, toward the centrosome.

Reason Dynein Needs Dynactin

Dynactin links dynein to cargo and regulates motor activity.

Mechanism of Cilia and Flagella Locomotion

Dynein arms generate sliding forces between microtubule doublets, bending the axoneme in a coordinated, wave-like motion.

Myosin Definition & Two Groups

Myosins are actin-based motor proteins.

Two groups: Conventional (Type II) for muscle contraction and Unconventional for intracellular transport.

Conventional Myosins Generate Force In

Muscle cells (skeletal, cardiac, smooth) and nonmuscle cells (cytokinesis, cell migration, stress fibers).

Structure of Myosin II

Two heavy chains with head, neck, and tail domains; each heavy chain is associated with two light chains.

Muscle Fiber

A single, multinucleated muscle cell composed of numerous myofibrils.

Myofibril

A cylindrical bundle of contractile filaments (actin and myosin) within a muscle fiber, responsible for contraction.