Microtubule nucleation

Microtubule nucleation requires the action of a third type of tubulin, γ-tubulin, which is distinct from the α and β subunits which compose the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a circular structure known as the γ-tubulin ring complex (γ-TuRC). This complex acts as a scaffold for α/β tubulin dimers to begin polymerization. It also acts as a cap of the (−) end while microtubule growth continues towards the (+) direction.

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The γ-TuRC is typically found as the core functional unit in a microtubule organizing center (MTOC), such as the centrosome in animal cells or the spindle pole bodies in fungi and algae. However, the cells of higher plants for example lack distinct MTOCs and microtubules are nucleated at γ-TuRCs distributed around the nuclear envelope. Microtubules can also nucleate in the cytoplasm, forming so-called astral microtubules radiating star-like from a diffusible nucleation center.

Several proteins are involved in the formation of the γ-TuRC and the temporal and spatial control of microtubule nucleation. These include for example coiled-coil proteins with structural functions and regulatory proteins, such as components of the Ran cycle.

In some filamentous fungi gamma tubulin localizes at the tip.