The extended tubulin superfamily

Although most eukaryotic cells can express multiple isotypes of alpha beta -tubulin, the significance of this diversity has not always been apparent. Recent data indicate that particular alpha beta -tubulin isotypes, both gen ome encoded and those derived by post-translational modification, can direc tly influence microtubule structure and function-thus validating ideas orig inally proposed in the multi-tubulin hypothesis over 25 years ago. It has also become increasingly evident over the past year that some (but i ntriguingly not all) eukaryotes encode several other tubulin proteins, and to date five further members of the tubulin superfamily, gamma, delta, epsi lon, zeta and eta, have been identified. Although the role of gamma -tubuli n in the nucleation of microtubule assembly is now well established, far le ss is known about the functions of delta-, epsilon-, zeta- and eta -tubulin . Recent work has expanded our knowledge of the functions and localisation of these newer members of the tubulin superfamily, and the emerging data su ggesting a restricted evolutionary distribution of these 'new' tubulin prot eins, conforms to established knowledge of microtubule cell biology. On the basis of current evidence, we predict that delta-, epsilon-, zeta- and eta -tubulin all have functions associated with the centriole or basal body of eukaryotic cells and organisms.