SUMMARY

Diversity of tubulin isotypes is illustrated by consideration of the β-tubulin isotypes of higher plants and the eukaryotic microbe, Physarum polycephalum, and by the α-tubulin isotypes of the protozoan, Trypanosoma brucei. The carrot plant expresses six, well-defined β-tubulin isotypes that possess characteristic two-dimensional gel coordinates. These six β-tubulin isotypes are differentially expressed during development of the flowering plant. In a similar manner, Physarum expresses three separate β-tubulin isotypes during its life cycle; of the two β1 isotypes, one is expressed solely in the myxamoeba whilst the other is expressed both in the myxamoeba and in the Plasmodium. A further β-tubulin isotype, β2, is expressed only in the Plasmodium. In carrot and in Physarum the generation of β-tubulin diversity appears, in the main, to be generated by the differential expression of a β-tubulin multi-gene family. However, tubulin isotypes can also be generated by post-translational modifications and T. brucei utilizes two different modifications within one cell. First, the primary translation product, the α1-tubulin isotype, can be acetylated to produce the α3 isotype. Second, both the α1 and α3 isotypes appear to exist in both tyrosinated and detyrosinated forms. The generation of these α-tubulin isotypes within the same cell and their presence in particular cellular domains, modulated throughout the cell cycle, reveals a complex relationship between α-tubulin isotypes produced by post-translational modifications and the dynamics of microtubule construction.

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