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Flagellar microtubule doublet assembly in vitro reveals a regulatory role of tubulin C-terminal tails

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Science  18 Jan 2019:
Vol. 363, Issue 6424, pp. 285-288
DOI: 10.1126/science.aav2567

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Assembly of the ciliary microtubule doublet

The cilium is a conserved organelle that is crucial for motility as well as for sensing the extracellular environment. Its core structure is characterized by nine microtubule doublets (MTDs). The mechanisms of MTD assembly are unclear. Schmidt-Cernohorska et al. developed an assay to reconstitute MTD assembly in vitro. Tubulin carboxyl-terminal tails played a critical inhibitory role in MTD formation. Molecular dynamics revealed that carboxyl-terminal tails of the A11 microtubule protofilament regulated MTD initiation. Furthermore, live-cell imaging showed an unexpected bidirectional isotropic elongation of the MTD.

Science, this issue p. 285

Abstract

Microtubule doublets (MTDs), consisting of an incomplete B-microtubule at the surface of a complete A-microtubule, provide a structural scaffold mediating intraflagellar transport and ciliary beating. Despite the fundamental role of MTDs, the molecular mechanism governing their formation is unknown. We used a cell-free assay to demonstrate a crucial inhibitory role of the carboxyl-terminal (C-terminal) tail of tubulin in MTD assembly. Removal of the C-terminal tail of an assembled A-microtubule allowed for the nucleation of a B-microtubule on its surface. C-terminal tails of only one A-microtubule protofilament inhibited this side-to-surface tubulin interaction, which would be overcome in vivo with binding protein partners. The dynamics of B-microtubule nucleation and its distinctive isotropic elongation was elucidated by using live imaging. Thus, inherent interaction properties of tubulin provide a structural basis driving flagellar MTD assembly.

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