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Microtubule structure. (A) Tubulin heterodimers are composed of α and β subunits that polymerize head-to-tail to form protofilaments (B). Thirteen protofilaments form lateral contacts to create the hollow cylindrical structure of the microtubule (C and D) with β-tubulin exposed at the microtubule plus end (+) and α-tubulin exposed at the microtubule minus end (−).
