Noncentrosomal Microtubule Assembly and Its Regulation

Microtubule nucleation is a critical initiating step in microtubule assembly during the cell cycle. This fundamental process is primarily mediated by γ-tubulin-containing complexes, namely, the γ-tubulin small complex (γ-TuSC) and its higher-order assembly, the γ-tubulin ring complex (γ-TuRC). These complexes are recruited to specialized cellular structures called microtubule-organizing centers (MTOCs) through spatially regulated targeting factors. The centrosome represents the predominant MTOC in animal cells, with its functional counterpart in yeasts being the spindle pole body (SPB). Centrosome-derived microtubule networks form radial arrays that not only orchestrate spindle assembly but also govern intracellular organelle positioning. Intriguingly, plant cells and differentiated animal cells have developed noncentrosomal MTOCs (ncMTOCs) that exhibit cell type-specific localization patterns. These alternative nucleation sites, including the Golgi apparatus, the nuclear envelope, preexisting microtubules, and the cell cortex, play crucial roles in a wide range of cellular activities, including the regulation of cellular morphology, polarity establishment, and cell division. This review systematically examines the regulatory mechanisms underlying noncentrosomal microtubule nucleation from diverse ncMTOCs, with particular emphasis on their context-dependent functional specializations.