Motor Proteins and Directed Movement Along Tracks
Myosins, kinesins and dyneins use ATP-dependent cycles to generate movement on polar filaments. Direction depends on the particular motor and the track's orientation. Opposite-direction motors support transport both ways along a track.
What a learner can do afterwards
- Explains how ATP binding and hydrolysis support the muscle myosin cross-bridge cycle without assigning that cycle to every motor.
- Predicts which way a cargo travels given the motor and the track polarity.
- Explains why a cell needs motors moving in both directions along the same track.
1 · Read
Motors use ATP-dependent changes in shape and attachment to generate movement. Their cycles differ. In muscle myosin, ATP binding releases the head from actin; hydrolysis resets it. The head attaches again, phosphate release accompanies the power stroke, and ADP leaves. One ATP is used per cross-bridge cycle. One ATP per exact step is not a rule for every motor, and a motor can pause or take a backward step.
Myosins use actin; kinesins and dyneins use microtubules. Conventional kinesin-1 usually moves toward plus ends, while cytoplasmic dynein moves toward minus ends. Some other kinesins move toward minus ends or change filament length instead of hauling cargo. Most myosins move toward actin plus ends, but myosin VI moves toward the minus end. Name the particular motor when predicting direction.
Plus does not always mean outward, or minus inward. Read the track orientation. On a microtubule with its minus end at the center and plus end at the edge, kinesin-1 carries cargo outward and dynein carries it inward. Reverse the track and those spatial directions reverse. Cells regulate opposite-direction motors so cargo can travel both ways. A trip does not always require a handoff.
In skeletal muscle, calcium binds troponin. Tropomyosin moves away from myosin-binding sites on actin, permitting cross-bridge cycling while ATP is available. Heads contribute at different times, rather than pulling in unison. After a power stroke, a bound head needs new ATP to detach. Without it, repeated cycling cannot continue.
Microtubules often support long-distance transport, including transport along axons. Actin and myosin often support local delivery and tension near the membrane. These are common arrangements, not exclusive rules about distance or location. Available tracks and motors determine how cargo is delivered. Motor transport also differs from force generated by filament assembly, such as actin pushing a leading edge.
Motor cycles differ. Predict cargo direction from the named motor and track orientation. Common transport roles are not exclusive rules.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Referenced videos are by Khan Academy.
More to read
An end direction is not a destination
To predict a delivery, first identify the motor, then locate the track's ends. Conventional kinesin-1 usually moves toward microtubule plus ends. Cytoplasmic dynein moves toward minus ends. Neither statement alone tells you whether cargo moves toward the cell center or its edge.
Family names can also hide differences. Some kinesins move toward minus ends or regulate microtubule length. Most myosins move toward actin plus ends, while myosin VI moves toward the minus end. A named motor and a stated track orientation make the prediction more precise.
Separate the stages of a muscle motor cycle
Follow a muscle myosin head after its power stroke. ATP binding allows release from actin. Hydrolysis then prepares the head with ADP and phosphate bound. The head can attach again, phosphate release accompanies the next power stroke, and ADP is released afterward.
Calcium controls access to the actin sites in skeletal muscle. It binds troponin, allowing tropomyosin to shift away. This regulation permits cycling while ATP is available; calcium does not replace ATP. Other motor proteins have different cycles, so this sequence should not be used as a universal motor mechanism.
Cells use more than one way to generate movement
Microtubules often provide routes for long-distance cargo transport, including transport along axons. Actin and myosin often support local deliveries and tension near the membrane. These patterns describe common arrangements, not restrictions on where either system can work.
Transport along a track is also different from changing the track itself. Motors generate movement through ATP-dependent interactions with filaments. Actin assembly can generate force by adding subunits near a membrane. Distinguishing these processes helps connect cargo transport to the earlier lesson about a protruding cell edge.
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