Dynamic Instability

Core Concept

Dynamic instability is the property by which an individual microtubule alternates between slow growth and rapid shrinkage, with abrupt, stochastic switches between the two states. The behavior is driven by a race between tubulin addition at the plus end and the lagging hydrolysis of GTP on β-tubulin. As long as a GTP cap (a tip of unhydrolyzed GTP-tubulin) is present, the microtubule grows; if the cap is lost, the GDP-tubulin lattice becomes unstable and the end depolymerizes catastrophically. A switch back to growth is called rescue. Discovered by Mitchison and Kirschner in 1984, dynamic instability lets the cell rapidly rebuild its microtubule array to explore space, as in the "search-and-capture" of chromosomes during mitosis.

Key Components

• GTP cap: the terminal layer(s) of GTP-bound β-tubulin at the plus end that stabilizes the otherwise GDP-rich lattice.
• GDP-tubulin core: the bulk of the polymer; GDP-tubulin has a curved conformation and tends to peel into protofilament curls when exposed.
• Catastrophe: the switch from growth to rapid depolymerization (cap loss).
• Rescue: the switch from shrinking back to growth (cap regain).
• Catastrophe factors: proteins such as kinesin-13/MCAK that actively promote cap loss and depolymerization.
• +TIPs: plus-end tracking proteins (e.g., EB1, CLIP-170) that bind growing plus ends and regulate dynamics and attachment.
• Stabilizers: MAPs and drugs (paclitaxel) that suppress catastrophe; destabilizers (colchicine, nocodazole) that promote it.

Mechanism / How It Works

1. GTP-tubulin adds to the plus end faster than the GTPase lag, so a growing end carries a GTP cap.
2. GTP hydrolysis (on β-tubulin, seconds after incorporation) converts the cap's base into GDP-tubulin, which is curved and strained.
3. While the cap persists, it acts like a "helmet" holding the straight lattice together, and the microtubule grows.
4. If growth slows (low free-tubulin, or a catastrophe factor arrives) and the cap is consumed faster than it is renewed, the GDP lattice is exposed.
5. Exposed GDP-protofilaments splay outward into curls and peel away — catastrophe — releasing dimers rapidly (up to tens of µm/min).
6. Occasionally, a shrinking end reacquires GTP-tubulin (or encounters a GTP-rich island) and switches back to growth — rescue.
7. At steady state in a population, some microtubules grow while others shrink; individual microtubules switch stochastically between phases, a "non-equilibrium steady state" that persists only while GTP is available.

Energy and Directionality

Dynamic instability consumes GTP, not ATP: each tubulin dimer hydrolyzes one GTP to GDP + Pᵢ. The energy of hydrolysis is stored as lattice strain and released during catastrophe, making depolymerization much faster than growth. The process is directional: net addition and hydrolysis both occur at the plus end (where β-tubulin's E-site is), so the minus end is largely inert. It is also irreversible in the thermodynamic sense — the polymer turns over continuously and would collapse to a simple equilibrium if GTP were exhausted.

Experimental Evidence / Technique

• Mitchison & Kirschner (1984): using purified tubulin and video-enhanced differential-interference-contrast (DIC) microscopy, they directly filmed individual microtubules switching between growth and rapid shrinkage, coining "dynamic instability."
• Non-hydrolyzable GTP analogs (GMPCPP): microtubules polymerized with GMPCPP do not catastrophize, proving hydrolysis (not GTP binding) is required for instability.
• EB1-GFP live imaging: fluorescent +TIP "comets" mark growing plus ends, allowing quantification of growth rate, catastrophe frequency, and rescue frequency in living cells.
• Tubulin dilution/flow assays: rapidly lowering free tubulin triggers synchronous catastrophe, demonstrating the cap's dependence on ongoing addition.
• Taxol treatment: stabilizes GDP lattices and abolishes catastrophe, confirming the central role of lattice nucleotide state.

How it works

1. GTP-tubulin adds to the plus end faster than the GTPase lag, so a growing end carries a GTP cap.
2. GTP hydrolysis (on β-tubulin, seconds after incorporation) converts the cap's base into GDP-tubulin, which is curved and strained.
3. While the cap persists, it acts like a "helmet" holding the straight lattice together, and the microtubule grows.
4. If growth slows (low free-tubulin, or a catastrophe factor arrives) and the cap is consumed faster than it is renewed, the GDP lattice is exposed.
5. Exposed GDP-protofilaments splay outward into curls and peel away — catastrophe — releasing dimers rapidly (up to tens of µm/min).
6. Occasionally, a shrinking end reacquires GTP-tubulin (or encounters a GTP-rich island) and switches back to growth — rescue.
7. At steady state in a population, some microtubules grow while others shrink; individual microtubules switch stochastically between phases, a "non-equilibrium steady state" that persists only while GTP is available.

Common confusions

• "Dynamic instability means the whole cell's microtubules shrink at once." — No. It is a property of individual microtubules; in a population, different microtubules are in different phases at any instant.
• "Catastrophe is the same as shrinking." — Catastrophe is the switch (growth → rapid shrink); the rapid depolymerization itself is the consequence.
• "GTP hydrolysis happens as the dimer binds." — No; hydrolysis is delayed after incorporation, which is exactly what permits the cap to exist.
• "Dynamic instability and treadmilling are the same." — Different. Treadmilling is steady net flux with balanced ends; dynamic instability is large stochastic excursions at one end.
• "Rescue requires new GTP-tubulin to arrive from nowhere." — Rescue occurs when a shrinking end regains a GTP cap, often at a GTP-rich region or where +TIPs/stabilizers bind.

Quick review

• Microtubules switch stochastically between growth, catastrophe (rapid shrink), and rescue.
• The GTP cap is the molecular switch; lagging hydrolysis creates it and its loss triggers catastrophe.
• Powered by GTP hydrolysis; directional (plus end); a non-equilibrium steady state.
• Demonstrated by Mitchison & Kirschner (1984) with video-DIC microscopy.
• Enables mitotic search-and-capture and is the target of taxane/vinca chemotherapy.
• Regulated by +TIPs (EB1), MAPs, kinesin-13, and drugs.