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#dynamic instability

3 public questions tagged with this topic.

Cytoskeleton

Which phase in microtubule dynamics involves sudden rapid depolymerization?

Dynamic instability coined by Mitchison and Kirschner describes microtubule switching without intermediate steady state enabling rapid reorganization. Four parameters define behavior: growth rate shrinkage rate catastrophe frequency rescue frequency. Growth corresponds to net addition of GTP tubulin maintaining cap of a few layers. Catastrophe is stochastic abrupt switch from growth to rapid shrinkage where depolymerization proceeds at about 20 microns per minute protofilaments curl outward releasing dimers. Rescue reverse switches shrinkage to growth via capture of GTP islands remaining in lattice or reduced activity of depolymerase. Treadmilling is separate describing net flux when plus growth balanced by minus loss maintaining constant length with polymer turnover. Nucleation is initial templated formation by gamma TuRC. Sudden depolymerization crucial for spindle reorganization kinetochore error correction and cell migration because destroying misoriented microtubules allows quick regrowth toward new targets making network adaptable to signals and cell shape changes during development. Additional regulatory inputs including phosphorylation, small GTPases, and cargo adaptors fine tune filament assembly stability and motor activity matching cellular demands during division, migration, and mechanical stress responses efficiently.

Ref: Mitchison & Kirschner, Nature 1984 – Catastrophe phase sudden rapid depolymerization in dynamic instability.

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Cytoskeleton

What is the role of GTP hydrolysis in microtubule assembly?

Microtubules do not require GTP hydrolysis to polymerize hydrolysis follows incorporation and modulates stability rather than providing assembly energy which is entropically driven. After alpha beta dimer carrying GTP on beta adds to plus end longitudinal contacts induce GTPase activity converting beta bound GTP to GDP plus inorganic phosphate retained initially as GDP Pi lattice. GDP tubulin prefers bent conformation by about 12 degrees but constrained straight within wall creating mechanical strain stored as elastic energy. As long as terminal layers retain GTP cap maintains stabilizing influence over underlying strained lattice. When addition slows or stochastic fluctuations expose GDP layers at tip strain released protofilaments splay outward lateral bonds break rapid depolymerization ensues known as catastrophe releasing stored energy. Thus hydrolysis acts as timer converting stable polymer into unstable intermediate powering dynamic instability required for exploratory search and generating polymerization force harnessed at kinetochores and cortical force generation for spindle positioning.

Ref: Desai & Mitchison, Annu Rev Cell Dev Biol 1997 – GTP hydrolysis causing depolymerization and catastrophe timer.

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Cytoskeleton

Which component of microtubules is responsible for its dynamic instability?

Microtubule dynamic instability reflects differential nucleotide state of tubulin subunits within polymer lattice storing strain energy. Each heterodimer contains alpha tubulin with nonexchangeable GTP permanently trapped at N site maintaining structural integrity and longitudinal interface and beta tubulin with exchangeable E site that binds GTP in soluble pool. Upon incorporation at growing plus end beta GTP forms GTP cap of one to three dimer layers that prefers straight protofilament conformation and strong lateral bonds mediated by M loop. Soon after incorporation beta GTP hydrolyzed to GDP via catalytic glutamate contributed by adjacent alpha tubulin converting lattice to GDP bound state that favors curved conformation with stored elastic strain. Alpha GTP never hydrolyzes beta does not bind ATP. Therefore GTP bound beta tubulin confers stability its hydrolysis introduces vulnerability. Loss of cap exposes GDP lattice protofilaments peel outward causing catastrophe. This mechanism distinguishes microtubules from nucleotide independent intermediate filaments and explains polymerization force generation.

Ref: Alberts et al., Molecular Biology of the Cell 7th ed., Chapter 14: GTP beta tubulin role in dynamic instability cap model.

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