Motor Proteins

5 min read Updated Apr 18, 2026

Motor proteins convert chemical energy (ATP hydrolysis) into mechanical work. They literally walk along cellular tracks, carrying cargo or pulling on filaments to cause contraction. Three MCAT-relevant motor proteins, all walking different tracks.

Motor	Walks on	Direction	Typical cargo/job
Myosin	Actin filaments	Plus end (most myosins)	Muscle contraction; vesicle transport
Kinesin	Microtubules	Plus end (toward periphery, anterograde)	Axonal transport outward from cell body
Dynein	Microtubules	Minus end (toward center, retrograde)	Axonal transport inward; cilia and flagella beating

Myosin and Muscle Contraction

In skeletal muscle, myosin is organized into thick filaments. Actin forms thin filaments. Together they slide past one another to produce contraction. The unit of contraction is the sarcomere, bounded by two Z-discs.

The cross-bridge cycle is the four-step ATP-driven process that makes one myosin head “walk” along actin:

ATP binds the myosin head, releasing it from actin.
ATP hydrolysis cocks the myosin head into a high-energy position (like a spring being loaded).
Myosin binds actin and releases inorganic phosphate.
Power stroke: myosin swings back to its original shape, pulling actin toward the center of the sarcomere. ADP leaves.

Diagram of the four-step cross-bridge cycle: ATP binding releases myosin from actin, ATP hydrolysis cocks the head, myosin binds actin and releases phosphate, and the power stroke pulls actin while ADP leaves — The cross-bridge cycle. Each cycle consumes one ATP and produces one "step" of myosin along actin. Dozens of cycles in parallel produce muscle contraction. *Credit: OpenStax Anatomy and Physiology 2e, CC BY 4.0*

Rigor Mortis

After death, cells run out of ATP. Without ATP, myosin cannot release actin (step 1 of the cycle). Muscles seize up in a contracted state called rigor mortis. A few hours later, proteases begin degrading the myofibrils and the muscle relaxes again.

Sliding filament diagram showing relaxed and contracted sarcomere with actin filaments sliding past myosin filaments, bringing Z-discs closer together — Sliding filament model. During contraction, actin thin filaments slide past myosin thick filaments toward the sarcomere center, shortening the sarcomere. *Credit: OpenStax Anatomy and Physiology 2e, CC BY 4.0*

Kinesin

Kinesin is a two-headed motor that walks toward the plus end of microtubules. It has a heavy chain with motor domains, a stalk, and light chains that attach to cargo (vesicles, mitochondria, RNA granules). Each step hydrolyzes one ATP and covers about 8 nm.

The classic example is axonal transport: kinesin carries neurotransmitter vesicles and mitochondria down the axon, from the cell body to the synapse. Without kinesin, a long neuron could not supply its distant axon terminals with fresh proteins.

Dynein

Dynein is the opposite-direction partner. It walks toward the minus end of microtubules (back toward the cell body). It transports endocytosed material, recycles old organelles, and - in a completely different role - powers the beating of cilia and flagella.

In cilia and flagella, dynein molecules between adjacent microtubule pairs pull past one another, producing the wave-like bending that propels cells (sperm flagella, airway cilia). The axoneme structure - nine outer microtubule doublets plus two central singlets, “9+2” - is bound together by dynein arms and nexin links.

On which cytoskeletal filament does each motor protein walk: myosin, kinesin, dynein?

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Myosin walks on actin filaments. Kinesin and dynein both walk on microtubules. Kinesin moves toward the plus end (anterograde, outward in axons). Dynein moves toward the minus end (retrograde, inward).

Why does muscle go into rigor mortis after death?

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After death, ATP production stops. Myosin needs ATP to release from actin. Without ATP, the cross-bridges cannot detach and the muscle stays contracted. Rigor mortis resolves hours later when lysosomal proteases begin to digest the muscle fibers.

Which motor protein family is responsible for the beating of cilia and flagella?

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Dynein. Axonemal dyneins between adjacent microtubule doublets slide past one another while tethered by nexin links, producing the wave-like bending that moves cilia and flagella. The "9+2" arrangement of microtubules in cilia and flagella depends on dynein arms.