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Edited by Ronald D. Vale, Howard Hughes Medical Institute and University of California, San Francisco, CA, and approved November 13, 2014 (received for review October 6, 2014)
Significance
How the cell cytoplasm is spatially organized is of fundamental interest. In ordinary animal cells the cytoplasm is organized by a radial array of microtubules, called an aster. Aster microtubules are nucleated by the centrosome and elongate to the periphery. We investigated how asters grow in an extremely large cell, the frog egg, using microscopy of an extract system. Asters were initially nucleated at centrosomes, but then additional microtubules nucleated far from the centrosome, apparently stimulated by preexisting microtubules. The resulting growth process allows asters to scale to the size of huge egg cells while maintaining a high density of microtubules at the periphery. Microtubule-stimulated microtubule nucleation might be a general principle for organizing large cells.
Abstract
A major challenge in cell biology is to understand how nanometer-sized molecules can organize micrometer-sized cells in space and time. One solution in many animal cells is a radial array of microtubules called an aster, which is nucleated by a central organizing center and spans the entire cytoplasm. Frog (here Xenopus laevis) embryos are more than 1 mm in diameter and divide with a defined geometry every 30 min. Like smaller cells, they are organized by asters, which grow, interact, and move to precisely position the cleavage planes. It has been unclear whether asters grow to fill the enormous egg by the same mechanism used in smaller somatic cells, or whether special mechanisms are required. We addressed this question by imaging growing asters in a cell-free system derived from eggs, where asters grew to hundreds of microns in diameter. By tracking marks on the lattice, we found that microtubules could slide outward, but this was not essential for rapid aster growth. Polymer treadmilling did not occur. By measuring the number and positions of microtubule ends over time, we found that most microtubules were nucleated away from the centrosome and that interphase egg cytoplasm supported spontaneous nucleation after a time lag. We propose that aster growth is initiated by centrosomes but that asters grow by propagating a wave of microtubule nucleation stimulated by the presence of preexisting microtubules.
Footnotes
- ↵1To whom correspondence may be addressed. Email: kishihar{at}fas.harvard.edu or timothy_mitchison{at}hms.harvard.edu.
This contribution is part of the special series of Inaugural Articles by members of the National Academy of Sciences elected in 2014.
Author contributions: K.I. and T.J.M. designed research; K.I., P.A.N., A.C.G., and C.M.F. performed research; K.I., P.A.N., A.C.G., and C.M.F. contributed new reagents/analytic tools; K.I. analyzed data; and K.I. and T.J.M. wrote the paper.
The authors declare no conflict of interest.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1418796111/-/DCSupplemental.
References
Growth of large microtubule asters in frog eggs
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