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Figure 5
Possible modes for the cooperative binding of doublecortin to microtubules and for microtubule bundling. Schematic microtubules are shown as grey spheres polymerized from α-tubulin and β-tubulin (shades of grey). The exposure of β-subunits at the (+)-end and of α-subunits at the (–)-end of the microtubule determines its polarity. N-DCX (red spheres) binds to the vertex of four αβ-tubulin dimers. Only every other N-DCX binding site around the circumference of the microtubule is occupied in this scheme. Distances between these N-DCX molecules are given, along with the dimension of the staggered C-DCX tetramer (left) and the maximum dimension of the linker region between N-DCX and C-DCX (centre). The linker between N-DCX and C-CDCX is drawn as a black line (not to scale). The microtubule is less densely decorated with doublecortin in vivo (estimated at a physiological doublecortin:αβ-tubulin ratio of ∼1:70; Taylor et al., 2000BB45). A few possible orientations of the staggered doublecortin tetramer are shown. Those limited to a single microtubule (left) would explain the cooperative binding of doublecortin, and those cross-linking two microtubules would explain microtubule bundling by doublecortin. In the absence of tertiary interface formation, only a domain-swapped doublecortin dimer may bind and cross-link microtubules, but with lower cooperativity. A few schematic monomers with the open C-DCX conformation, prior to dimer formation, are also indicated. The C-terminal Pro/Ser-rich part of doublecortin is not included in the figure. Note that although the staggered tetramer has C2 symmetry, this has to be broken by linker flexibility in order to bundle microtubules with the same polarity. On the lower right, the disease-causing mutations of one protomer of C-DCX are mapped in the context of the staggered tetramer.

Journal logoSTRUCTURAL
ISSN: 2059-7983
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