In cell anatomy, Microtubule nucleation is the event that initiates de novo formation of microtubules. These filaments of the cytoskeleton typically form through polymerization of α- and β-tubulin dimers, the basic building blocks of the microtubule, which initially interact to nucleate a seed from which the filament elongates.

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. The kinetics barriers of such a process however mean that the rate at which microtubules spontaneously nucleate is relatively low.

In vivo, cells get around this kinetic barrier by using various proteins to aid microtubule nucleation. The primary pathway by which microtubule nucleation is assisted requires the action of a third type of tubulin, γ-tubulin, which is distinct from the α and β subunits that compose the microtubules themselves. The γ-tubulin combines with several other associated proteins to form a conical structure known as the γ-tubulin ring complex (γ-TuRC). 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 also acts as a cap of the (−) end while the microtubule continues growth from its (+) end. This cap provides both stability and protection to the microtubule (-) end from enzymes that could lead to its depolymerization, while also inhibiting (-) end growth.