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Carmichael’s Concise Review


that a mechanism must exist to prevent collisions. Because TIRF microscopy does not provide suffi cient resolution to demonstrate a relationship between trains and microtubules, the authors took the correlative light and EM (CLEM) approach. In order to provide suffi cient spatiotemporal resolution to analyze IFT trains, they developed a new method to provide millisecond resolution in correlative TIRF and three-dimensional (3D) EM. T e experimental cell was rapidly fi xed with glutaraldehyde during time-lapse TIRF imaging of IFT trains. T e position of each fi xed IFT train was cross-referenced to its direction of movement just before fi xation. T e same cell was then prepared for EM tomography. T ey obtained suffi cient spatial resolution to observe the structural details of each train and its interaction with microtubules. In this manner they identifi ed anterograde and retrograde trains unambig- uously, as well as trains that remained stationary ( Figure 1 ). Using these and other techniques Stepanek and Pigino showed


that each microtubule doublet is used as a bidirectional double-track railway with anterograde IFT trains moving toward the ciliary tip along B-microtubules and retrograde trains moving toward the cell body along A-microtubules. Because A- and B-microtubules are thought to share the same tubulin arrangement, they suggested that IFT trains recognize specifi c post-translational modifi cations on A- versus B-microtubules. T us, the microtubule doublet geometry provides direction-specifi c rails to coordinate bidirectional transport of ciliary and fl agellar components. At the very least, this study highlights the critical role played by microtubule doublets in the assembly of cilia and fl agella.


References [1] L Stepanek and G Pigino , Science 352 ( 6281 ) ( 2016 ) 721 – 24 . [2] T e authors gratefully acknowledge Dr. Gaia Pigino for reviewing this article.


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