Protofilament curved structure

Fig. 2 The conformational switch of GDP-tubulin. a The straight conformation of tubulin when embedded in the MT (two tubulin segments of two adjacent protofilaments are drawn), b The curved structure of non-microtubular GDP-tubulin, as visualized here in a T2SLD complex. The first a subunit (at the bottom) is in the same orientation as the one at the bottom left of a. c Model of a 13 protofilaments MT [8] top view (left) and lateral view (right). One protofilament has been extended by a modeled tubulin assembly having the same curvature as in T2SLD (green). Figures 2, 3, 6 and 7 were generated with MOLSCRIPT [9], RASTER 3D [10] and PYMOL [11]...

Jimenez et al. (2002) proposed a molecular model for the insulin protofilament based on these data and on electron cryomicroscopy (cryo-EM) reconstructions of insulin fibrils. The fibrils show a number of twisted morphologies that seem to be alternative packings of similar protofilaments. The protofilaments have cross sections of 30x40 A. The authors suggest a complete conversion to / -structure and model the amyloid monomer as having four jS-strands (Fig. 3B). Each insulin chain contributes two of these jS-strands, and the chains align in a parallel stack, constrained by the interchain disulfide bonds. One pair of stacked /i-stran ds is curved... 

By contrast with the straight arrangement of tubulin subunits in protofilaments both in Zn-induced sheets and in MT, the T2R complex is curved. Interestingly, the curvature of T2R is very similar to that of the depolymerization products of MT, suggesting that the structure of tubulin in T2R reflects the structures of GDP/tubu-lin in oligomers and in curved tubulin protofilaments. Therefore, comparison of the structures of curved and straight tubulins can help one to understand the mechanism of the MT assembly-disassembly process. 

Fig. 3 Structural changes of tubulin subunits upon MT disassembly, a Structure of a 3 subunit of the T2R complex (pdb id 1SA0 [15]). The monomer is sub-divided in an N-terminal domain (blue) with bound GDP (ball-and-stick drawing, grey), the central helix H7 yellow), an intermediate domain green), and the C-terminal helices red), b Comparison of the 3 subunit conformation in the T2R complex (same color code as in a) and in a straight protofilament (nucleotide binding domain and C-terminal helix hairpin in cyan, H7 helix in salmon, intermediate domain in pink, pdb id 1JFF [70]) after superposition of the secondary structural elements of their N-terminal domain, c Schematic representation recapitulating the movements between straight and curved tubulin monomers (domains are color-coded as in a the nucleotide is depicted as a red sphere)...

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