[1] Microtubule nucleation occurs spontaneously in vitro, with solutions of purified tubulin giving rise to full-length polymers.
The tubulin dimers that make up the polymers have an intrinsic capacity to self-aggregate and assemble into cylindrical tubes, provided there is an adequate supply of GTP.
This complex, with its 13-fold symmetry, acts as a scaffold or template for α/β tubulin dimers during the nucleation process—speeding up the assembly of the ring of 13 protofilaments that make up the growing microtubule.
The γ-TuRC is typically found as the core functional unit in a microtubule organizing center (MTOC), such as the centrosome in some animal cells or the spindle pole bodies in fungi and algae.
The γ-TuRCs in the centrosome nucleate an array of microtubules in interphase, which extend their (+)-ends radially outwards into the cytoplasm towards the periphery of the cell.
[4] These non-centrosomal microtubule arrays can take on various geometries—such as those leading to the long, slender shape of myotubes, the fine protrusions of an axon, or the strongly polarized domains of an epithelial cell.
Through use of TIRF microscopy, researchers have visually observed the nucleation of branching microtubules in Drosophila cells during the formation of the mitotic spindle.
Recent studies have provided evidence towards the concept that the promotion of microtubule nucleation is possible with a combination of α- and β-tubulin dimers and the aforementioned MAP TPX2, even in the absence of γ-TuRC.