Fig. 7. Microtubule adaptations in tubulin.

Structural superimposition of two OdinTubulin subunits (green and gray) onto two laterally related microtubule subunits: (A) β-tubulins (orange and yellow) or (B) α-tubulins (pink and yellow). The microtubule inserts 280 to 282 (blue), relative to OdinTubulin, create a protrusion used for interprotofilament contacts, centered around Tyr²⁸³ and His²⁸³ in β- and α-tubulins, respectively. The second α-tubulin inserts 365 to 372 (red) interact with the nucleotide sensor motif, indicated by an asterisk. (C and D) Cartoons depicting how the straight-to-curved protofilament transition is used in the two tubule systems. (C) OdinTubulin polymerizes as straight protofilaments, which curve on GTP hydrolysis. The curved protofilaments assemble into tubules. End on view. (D) α/β-Tubulin heterodimers assemble as tubules of typically 13 protofilaments (5 are shown). GTP hydrolysis in the central region of the tubule causes strain; however, the microtubule architecture prevents curving. GTP hydrolysis at the microtubule + ends leads to curving and dissociation, via the loss of curved stretches of protofilaments or monomers. GTP hydrolysis occurs in alternate subunits, reducing the strain and cooperativity within microtubule protofilaments, relative to OdinTubulin, leading to longer more stable protofilaments in the straight form. Side view.