Microtubules are dynamic filaments required in countless cellular processes, so their nucleation and growth has always been a point of interest for many biologists. Microtubule nucleation from centrosomes is the best understood mechanism for microtubule nucleation, yet centrosomes are not always necessary for microtubule growth within a mitotic spindle. A recent paper shows direct evidence for microtubule nucleation from existing microtubules in meiotic frog extracts, through the use of TIRF microscopy. Petry and colleagues show that these new daughter microtubules nucleate from existing microtubules at a low “branch” angle. In addition, daughter microtubules have the same polarity as mother microtubules, which can help maintain mitotic spindle integrity. Branching microtubule nucleation requires γ-tubuin and augmin, a protein that increases microtubule density. RanGTP, which is required for chromatin-mediated microtubule nucleation, and its effector protein TPX2 both stimulate branching microtubule nucleation. In the images above, microtubules branch from existing microtubules after the addition of both RanGTP and TPX2, resulting in fan-like microtubule structures. Lower panel is an enlarged view of the area marked with the asterisk. Long arrows point to daughter microtubules nucleating at a clear branched angle, while short arrows point to daughter microtubules growing along the length of mother microtubules.
BONUS!! Here's a mesmerizing movie of branching microtubules after the addition of RanGTP and TPX2, similar to the above image.
Petry, S., Groen, A., Ishihara, K., Mitchison, T., & Vale, R. (2013). Branching Microtubule Nucleation in Xenopus Egg Extracts Mediated by Augmin and TPX2 Cell, 152 (4), 768-777 DOI: 10.1016/j.cell.2012.12.044
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