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The extended tubulin superfamily

Paul G. McKean, Sue Vaughan and Keith Gull*

School of Biological Sciences, University of Manchester, 2.205 Stopford Building, Oxford Road, Manchester M13 9PT, UK
* Author for correspondence (e-mail: k.gull{at}man.ac.uk )



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  		Fig. 1. (A) Electron micrograph of a section through a mammalian centriole showing the typical nine triplet organisation of microtubules (A, B and C tubules).

(B) Electron micrograph of a longitudinal section through a trypanosome cell, showing the basal body subtending the flagellum. Basal bodies at the base of flagella (and cilia) are structurally similar to centrioles and display a ninefold symmetry. Also visible beneath the basal body is the kinetoplast, an organelle containing the mitochondrial genome of trypanosomes, which is linked to the basal body by a series of filaments (reviewed by Gull, 1999Go). Scale bars are indicated on each figure.

Abbreviations: f, flagellum; bb, basal body; k, kinetoplast; m, mitochondrion.

 


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  		Fig. 2. Current model of microtubule structure at atomic resolution. The crystal structure of tubulin, represented as a ribbon model, is docked onto a low-resolution 3-D image of tubulin protofilaments. The figure shows a section through a microtubule; on the left is a view from inside the microtubule and on the right from one side of a protofilament. Lateral interactions between protofilaments occur through the M-loops on one side contacting the helix H3 in the adjacent protofilament. At the top of each monomer is a guanine-nucleotide-binding site; GTP (bound to {alpha}-tubulin) and GDP (bound to ß-tubulin) is shown occupying sites at the interface between tubulin subunits. The nucleotide is contacted by loop T7 of the next subunit in the protofilament. Also shown (as a space-filling molecule) in a pocket on the inside surface of the ß-tubulin subunit is the microtubule-binding drug taxol. At the tubulin C-terminus, the last residues visible by electron microscopy ({alpha}440 and ß437) are indicated by black circles; the conformations of the last 8-10 residues are unknown. Figure reproduced from Amos, 2000Go, with permission from Elsevier Science.

 


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  		Fig. 3. Sequence alignment of the six members of the tubulin superfamily in T. brucei. Dark blue boxes indicate sequence identity, and light blue boxes indicate sequence similarity, a minimum of three residues being shown in each box. This manual alignment indicates the presence of a number of insertions and deletions in the {delta}-, {epsilon}- and {zeta}-tubulin sequence in comparison with {alpha}ß-tubulin. Secondary structural elements of pig {alpha}-tubulin are indicated (PDB number 1TUB) to suggest where these insertions and deletions may be located. T1-T7 indicate loops that contain amino acid residues directly involved in nucleotide contacts (Nogales et al., 1998bGo).

 


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  		Fig. 4. Schematic representation of the alignment shown in Fig. 3, indicating the location of insertions and deletions in the T. brucei {delta}-, {epsilon}- and {zeta}-tubulin sequence. Red boxes indicate insertions and green chevrons indicate deletions of >5 amino acids; the actual length of each insertion/deletion is indicated. The proposed positions of these insertions/deletions with reference to the secondary structural elements of pig {alpha}-tubulin shown in Fig. 3 are denoted; for instance, S9-H10 indicates an insertion present on the loop between strand 9 and helix 10. The exact length of each tubulin sequence is shown at the right-hand side of the figure. Also indicated are the limits of the tubulin regions identified as the nucleotide-binding domain, the intermediate domain and the C-terminal domain.

 




© The Company of Biologists Ltd 2001