nucleator

A nucleator is a molecule or multi-protein complex that initiates the formation of a nucleus for polymerization or crystallization. In cellular biology, nucleators seed the assembly of cytoskeletal filaments by stabilizing an initial rate-limiting oligomer, thereby lowering the energy barrier to filament growth. Nucleation is the first step of polymer formation; without a nucleator, spontaneous nucleation can be inefficient or misregulated.

Biological examples include nucleators of microtubules, actin, and other polymers. The gamma-tubulin ring complex (gamma-TuRC) serves as a primary microtubule nucleator at microtubule organizing centers, providing a template for the addition of alpha-/beta-tubulin dimers. In actin networks, the Arp2/3 complex nucleates new branches off existing filaments, while formins promote the growth of unbranched actin filaments. Other actin nucleators such as Spire, Cobl, and certain WH2-domain proteins contribute to nucleation or elongation in specific cellular contexts. Nucleators are often regulated by signaling pathways, interactions with profilin, capping proteins, and post-translational modifications, enabling spatially controlled assembly.

In materials science and crystallography, nucleators or seeding agents initiate crystal formation and can determine crystal size, morphology, and rate of growth. Nucleation can be homogeneous (within the bulk) or heterogeneous (on surfaces or templates).