Microtubule nucleation for spindle assembly: one molecule at a time

For a long time, it was unclear how the myriads of microtubules are generated in a spindle to orchestrate cell division. In recent years, many microtubule nucleation pathways that generate spindle microtubules have been identified and shed light on this question. Moreover, the γ-tubulin ring complex (γ-TuRC), once thought to be the cell’s sole nucleator, is now known to work synergistically with the protein XMAP215/ch-TOG. Recent high resolution cryogenic electron microscopy structures and in vitro single molecule studies of γ-TuRC provided clues towards its mode of action. It is an emerging concept that biomolecular condensation plays a role in microtubule nucleation in the spindle. The next challenge will be to determine how γ-TuRC is targeted to the right location in the spindle at the right time and subsequently turned on to generate the spindle framework. The cell orchestrates the dance of chromosome segregation with remarkable speed and fidelity. The mitotic spindle is built from scratch after interphase through microtubule (MT) nucleation, which is dependent on the γ-tubulin ring complex (γ-TuRC), the universal MT template. Although several MT nucleation pathways build the spindle framework, the question of when and how γ-TuRC is targeted to these nucleation sites in the spindle and subsequently activated remains an active area of investigation. Recent advances facilitated the discovery of new MT nucleation effectors and their mechanisms of action. In this review, we illuminate each spindle assembly pathway and subsequently consider how the pathways are merged to build a spindle. The cell orchestrates the dance of chromosome segregation with remarkable speed and fidelity. The mitotic spindle is built from scratch after interphase through microtubule (MT) nucleation, which is dependent on the γ-tubulin ring complex (γ-TuRC), the universal MT template. Although several MT nucleation pathways build the spindle framework, the question of when and how γ-TuRC is targeted to these nucleation sites in the spindle and subsequently activated remains an active area of investigation. Recent advances facilitated the discovery of new MT nucleation effectors and their mechanisms of action. In this review, we illuminate each spindle assembly pathway and subsequently consider how the pathways are merged to build a spindle. a eukaryotic organelle that acts as the basis for the formation of cilia and flagella. It consists of microtubules that are organized in a cylinder containing nine bundles of triplet microtubules. a complex containing the proteins Aurora B, INCENP, Borealin, and Survivin, which localize to the inner centromere. The CPC regulates attachment of microtubules to chromosomes during mitosis. the 2.2 MDa protein complex responsible for nucleating microtubules synergistically with XMAP215 in cells. In metazoans, the γ-TuRC is composed of five copies of γ-TuSC, additional GCP homologs, and other accessory proteins. a tetramer composed of two copies of γ-tubulin, one copy of γ-tubulin complex protein 2 (GCP2), and one copy of γ-tubulin complex protein 3 (GCP3). a complex of proteins assembled at the centromeres on chromosomes. The kinetochore forms the bridge between spindle microtubules and chromosomes, allowing the segregation of genetic material into daughter cells. parallel bundles of microtubules and associated proteins that connect the kinetochore with the spindle poles. a liquid-like, condensed protein region containing microtubule nucleation and centrosomal proteins that promotes spindle assembly in mammalian oocytes. the process during which biomolecules undergo a phase transition to form a concentrated and biochemically distinct compartment, often in the form of liquid-like droplets. cellular sites where microtubules are nucleated and then organized. Examples include the centrosome, the basal body, and the nuclear envelope, among others. New MTOCs are still being discovered. radial arrays of microtubules in which the microtubule minus ends are centrally anchored and the plus ends symmetrically extend outward. a dense, organized matrix of proteins that surrounds the centrioles within the centrosome. During metaphase, the PCM is expanded and organized into protein layers, including γ-tubulin, which is responsible for nucleating new microtubules. a signaling pathway that monitors the attachment of kinetochores to the spindle, thus acting as a safety feature for chromosome segregation. a protein downstream of the GTP-loaded protein Ran, which contains a nuclear localization sequence via which the importin α/β heterodimer binds and thereby inhibits the SAF. Ran GTP releases the SAF from importin α/β and thereby initiates spindle assembly. the major microtubule organizing center in yeast, analogous to the centrosome in higher organisms.