Branching Microtubule Nucleation in Xenopus Egg Extracts Mediated by Augmin and TPX2

• Published in: Cell Vol. 152; no. 4; pp. 768 - 777
• Main Authors: Petry, Sabine, Groen, Aaron C., Ishihara, Keisuke, Mitchison, Timothy J., Vale, Ronald D.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 14-02-2013
• Subjects: Animals; branching; Cell Cycle Proteins - metabolism; centrosomes; chromatin; eggs; guanosine triphosphate; Meiosis; Microscopy - methods; Microtubule-Associated Proteins - metabolism; microtubules; Microtubules - metabolism; Nuclear Proteins - metabolism; Ovum - chemistry; Ovum - metabolism; Phosphoproteins - metabolism; tubulin; Xenopus; Xenopus laevis - metabolism; Xenopus Proteins - metabolism
• ISSN: 0092-8674; 1097-4172
• DOI: 10.1016/j.cell.2012.12.044

The microtubules that comprise mitotic spindles in animal cells are nucleated at centrosomes and by spindle assembly factors that are activated in the vicinity of chromatin. Indirect evidence has suggested that microtubules also might be nucleated from pre-existing microtubules throughout the spindle, but this process has not been observed directly. Here, we demonstrate microtubule nucleation from the sides of existing microtubules in meiotic Xenopus egg extracts. Daughter microtubules grow at a low branch angle and with the same polarity as mother filaments. Branching microtubule nucleation requires γ-tubulin and augmin and is stimulated by factors previously implicated in chromatin-stimulated nucleation, guanosine triphosphate(GTP)-bound Ran and its effector, TPX2. Because of the rapid amplification of microtubule numbers and the preservation of microtubule polarity, microtubule-dependent microtubule nucleation is well suited for spindle assembly and maintenance.
[Display omitted]
► A Xenopus egg extract assay reveals MT nucleation from the sides of existing MTs ► Newly nucleated daughter MTs grow with the same polarity as the mother MT ► Branching MT nucleation is stimulated by RanGTP and its effector TPX2 ► Augmin, which increases spindle MT density, is required for branching MT nucleation
Visualizing MTs in meiotic Xenopus egg extracts reveals branching nucleation from the sides of existing MTs. Branching requires γ-tubulin and augmin and is stimulated by the Ran effector TPX2, linking a Ran-GTP signal from chromosomes to spindle dynamics.