Actin Filaments: Polarity, Treadmilling and Cell Shape
Actin filaments have different plus and minus ends. During steady-state treadmilling, plus-end addition balances minus-end loss. Actin assembly near the membrane helps a crawling cell extend its leading edge.
What a learner can do afterwards
- Explains treadmilling in terms of different critical concentrations at the two ends.
- Links filament growth at the front of a cell to the direction it moves.
- Predicts what a drug that blocks subunit addition does to a crawling cell.
1 · Read
Actin filaments have plus and minus ends. Under comparable conditions, subunits add faster at the plus end. An end's critical concentration is the free-actin concentration where addition balances loss. In a basic ATP-actin model, this threshold is lower at the plus end. Between the thresholds, the plus end grows and the minus end shrinks. Below both thresholds, both ends shrink overall; above both, both grow.
During treadmilling, subunits enter mainly at the plus end and leave mainly at the minus end. At steady state, equal addition and loss rates preserve filament length. A concentration between the thresholds gives opposite net changes, but does not by itself prove equal rates. Subunits are added and removed at the ends; they do not walk along the filament.
In a crawling-cell model, actin assembly at the leading edge pushes the membrane outward. Adhesion to a surface and contraction help the rest of the cell follow. If the cell protrudes only at its left edge and these other steps work, it advances left. Many filaments rebuild the network; this is not one filament flowing through the cell.
Actin commonly joins a filament with ATP bound. After joining, ATP is hydrolyzed to ADP and phosphate. Phosphate is released later. Older ADP-rich regions are generally less stable than regions containing ATP or ADP with phosphate still bound. These nucleotide changes help support turnover; end polarity and free-actin concentration also affect it.
If a drug blocks all actin-subunit addition, existing filaments may still lose subunits. Assembly-driven protrusion cannot continue, so actin-based crawling is impaired. The cell may stop advancing or round up. Blocking addition does not instantly freeze every filament or every process in the cell.
Steady-state treadmilling balances plus-end addition with minus-end loss. Actin assembly extends a leading edge; adhesion and contraction help the cell follow.
Lesson, questions and flashcards written for this dataset.
Readings and flashcards written for this dataset. Referenced videos are by Khan Academy.
More to read
A network that changes shape
A cell can change its shape by rearranging its cytoskeleton. Actin may form bundles or a mesh beneath the membrane. Microtubules are hollow structures made from tubulin. Intermediate filaments help the cell withstand pulling forces.
These structures do different jobs. Actin assembly can extend an edge, while myosin acting on actin can produce tension. Microtubules also provide tracks for transport. Calling the cytoskeleton a skeleton should not suggest a fixed frame: its organization can change as the cell moves or rearranges its contents.
Length and turnover are different measurements
Measuring filament length tells you the net result of addition and loss. It does not identify which subunits remain. During steady-state treadmilling, plus-end additions balance minus-end losses, so length can stay constant while the filament's membership changes.
Do not picture subunits walking along a track inside the filament. They join and leave at its ends. A cell also regulates many filaments by capping, branching or severing them. Turnover of an entire actin network is therefore more varied than the basic single-filament model.
Turnover is different from sliding
Actin turnover changes which subunits belong to a filament. Muscle sliding changes the position of actin relative to myosin. A muscle can shorten through sliding without requiring either filament to shorten.
In skeletal muscle, calcium binds troponin and moves tropomyosin away from myosin-binding sites. ATP binding allows a myosin head to release actin, and hydrolysis prepares it for another cycle. Many heads contribute at different times. This ATP-dependent sliding mechanism should not be confused with ATP-actin joining and leaving a filament.
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