Spatial variation of microtubule depolymerization in large asters

© The Author(s), 2021. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Ishihara, K., Decker, F., Caldas, P., Pelletier, J. F., Loose, M., Brugués, J., & Mitchison, T. J. Spatial variation of microtubule depolymerization in large asters. Molecular Biology of the Cell, 32(9), (2021): 869-879, https://doi.org/10.1091/mbc.e20-11-0723.Microtubule plus-end depolymerization rate is a potentially important target of physiological regulation, but it has been challenging to measure, so its role in spatial organization is poorly understood. Here we apply a method for tracking plus ends based on time difference imaging to measure depolymerization rates in large interphase asters growing in Xenopus egg extract. We observed strong spatial regulation of depolymerization rates, which were higher in the aster interior compared with the periphery, and much less regulation of polymerization or catastrophe rates. We interpret these data in terms of a limiting component model, where aster growth results in lower levels of soluble tubulin and microtubule-associated proteins (MAPs) in the interior cytosol compared with that at the periphery. The steady-state polymer fraction of tubulin was ∼30%, so tubulin is not strongly depleted in the aster interior. We propose that the limiting component for microtubule assembly is a MAP that inhibits depolymerization, and that egg asters are tuned to low microtubule density.K.I. was supported by fellowships from the Honjo International Scholarship Foundation and Center of Systems Biology Dresden. F.D. was supported by the DIGGS-BB fellowship provided by the German Research Foundation (DFG). P.C. is supported by a Boehringer Ingelheim Fonds PhD fellowship. J.F.P. was supported by a fellowship from the Fannie and John Hertz Foundation. M.L.’s research is supported by European Research Council (ERC) Grant no. ERC-2015-StG-679239. J.B.’s research is supported by the Human Frontiers Science Program (CDA00074/2014). T.J.M.’s research is supported by National Institutes of Health Grant no. R35GM131753