Differences in Intrinsic Tubulin Dynamic Properties Contribute to Spindle Length Control in Xenopus Species
William G. Hirst, Abin Biswas, Kishore K. Mahalingan, Simone Reber
Abstract
The function of cellular organelles relates not only to their molecular composition but also to their size. However, how the size of dynamic mesoscale structures is established and maintained remains poorly understood [1Mitchison T.J. Ishihara K. Nguyen P. Wühr M. Size scaling of microtubule assemblies in early Xenopus embryos.Cold Spring Harb. Perspect. Biol. 2015; 7: a019182Crossref PubMed Scopus (24) Google Scholar, 2Reber S. Goehring N.W. Intracellular scaling mechanisms.Cold Spring Harb. Perspect. Biol. 2015; 7: a019067PubMed Google Scholar, 3Heald R. Gibeaux R. Subcellular scaling: does size matter for cell division?.Curr. Opin. Cell Biol. 2018; 52: 88-95Crossref PubMed Scopus (12) Google Scholar]. Mitotic spindle length, for example, varies several-fold among cell types and among different organisms [4Crowder M.E. Strzelecka M. Wilbur J.D. Good M.C. von Dassow G. Heald R. A comparative analysis of spindle morphometrics across metazoans.Curr. Biol. 2015; 25: 1542-1550Abstract Full Text Full Text PDF PubMed Scopus (56) Google Scholar]. Although most studies on spindle size control focus on changes in proteins that regulate microtubule dynamics [5Loughlin R. Wilbur J.D. McNally F.J. Nédélec F.J. Heald R. Katanin contributes to interspecies spindle length scaling in Xenopus.Cell. 2011; 147: 1397-1407Abstract Full Text Full Text PDF PubMed Scopus (134) Google Scholar, 6Helmke K.J. Heald R. TPX2 levels modulate meiotic spindle size and architecture in Xenopus egg extracts.J. Cell Biol. 2014; 206: 385-393Crossref PubMed Scopus (46) Google Scholar, 7Milunovic-Jevtic A. Jevtic P. Levy D.L. Gatlin J.C. In vivo mitotic spindle scaling can be modulated by changing the levels of a single protein: the microtubule polymerase XMAP215.Mol. Biol. Cell. 2018; 29: 1311-1317Crossref PubMed Scopus (13) Google Scholar, 8Miller K.E. Session A.M. Heald R. Kif2a scales meiotic spindle size in Hymenochirus boettgeri.Curr. Biol. 2019; 29: 3720-3727.E5Abstract Full Text Full Text PDF PubMed Scopus (8) Google Scholar], the contribution of the spindle’s main building block, the αβ-tubulin heterodimer, has yet to be studied. Apart from microtubule-associated proteins and motors, two factors have been shown to contribute to the heterogeneity of microtubule dynamics: tubulin isoform composition [9Vemu A. Atherton J. Spector J.O. Moores C.A. Roll-Mecak A. Tubulin isoform composition tunes microtubule dynamics.Mol. Biol. Cell. 2017; 28: 3564-3572Crossref PubMed Google Scholar, 10Chaaban S. Jariwala S. Hsu C.T. Redemann S. Kollman J.M. Müller-Reichert T. Sept D. Bui K.H. Brouhard G.J. The structure and dynamics of C. elegans tubulin reveals the mechanistic basis of microtubule growth.Dev. Cell. 2018; 47: 191-204.e8Abstract Full Text Full Text PDF PubMed Scopus (26) Google Scholar] and post-translational modifications [11Xu Z. Schaedel L. Portran D. Aguilar A. Gaillard J. Marinkovich M.P. Théry M. Nachury M.V. Microtubules acquire resistance from mechanical breakage through intralumenal acetylation.Science. 2017; 356: 328-332Crossref PubMed Scopus (159) Google Scholar]. In the past, studying the contribution of tubulin and microtubules to spindle assembly has been limited by the fact that physiologically relevant tubulins were not available. Here, we show that tubulins purified from two closely related frogs, Xenopus laevis and Xenopus tropicalis, have surprisingly different microtubule dynamics in vitro. X. laevis microtubules combine very fast growth and infrequent catastrophes. In contrast, X. tropicalis microtubules grow slower and catastrophe more frequently. We show that spindle length and microtubule mass can be controlled by titrating the ratios of the tubulins from the two frog species. Furthermore, we combine our in vitro reconstitution assay and egg extract experiments with computational modeling to show that differences in intrinsic properties of different tubulins contribute to the control of microtubule mass and therefore set steady-state spindle length.