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#microtubule polymerization

2 public questions tagged with this topic.

Cytoskeleton

What is the function of XMAP215?

Microtubule growth in cells exceeds intrinsic tubulin addition rate due to polymerase action of Dis1 ch-TOG XMAP215 family enhancing incorporation efficiency. XMAP215 contains five Tumor Overexpressed Gene TOG domains each with two HEAT repeats capable of binding free alpha beta tubulin dimers with micromolar affinity. Mechanism involves processive tracking: TOG array recruits soluble dimers tethers to plus end via interaction with EB1 and lattice then delivers dimers to growing tip in orientation favoring incorporation increasing growth rate several fold. Function does not require ATP hydrolysis; instead GTP tubulin incorporation provides energy for assembly. Overexpression accelerates growth depletion shortens microtubules and delays mitosis formation of small spindles. It antagonizes catastrophe factors like XKCM1 kinesin-13 balancing dynamic instability. Unlike microtubule minus capping or severing action XMAP215 binds free dimers and enhances polymerization making it central regulator of rapid spindle assembly and cell morphology maintenance and growth cone dynamics in neurons 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: Al-Bassam & Chang, Trends Cell Biol 2011 – XMAP215 polymerase binds tubulin dimers enhances polymerization rate.

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Cytoskeleton

In microtubule polymerization, which end exhibits faster growth?

Polarity of microtubules dictates directionality of transport and regulation by plus end tracking proteins +TIP network. Lattice assembled from head to tail alpha beta dimers creates structurally distinct ends with different chemistry. Minus end shows alpha tubulin outward typically embedded in gamma TuRC within MTOC exchange slow often capped by patronins CAMSAP family. Plus end exposes beta tubulin with GTP binding pocket capable of rapid subunit addition with on rate about 7 per micromolar per second versus minus about half. In vitro growth rates show plus about two to three fold faster elongation also more dynamic undergoing frequent catastrophe and rescue events regulated by EB1 XMAP215. In cells EB family and XMAP215 selectively accumulate at plus ends accelerating growth while minus dynamics dampened and stabilized. This asymmetry enables plus ends to explore cell periphery searching kinetochores and cortical capture sites while minus ends provide stable anchor for force generation during spindle formation and maintain organized radial arrays supporting polarized transport highways.

Ref: Alberts et al., Molecular Biology of the Cell, Chapter 14 – Plus end exhibits faster growth due to polarity.

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