Multiple forms of tubulin: Different gene products and covalent modifications

Tubulin, the subunit protein of microtubules, is an α/β heterodimer. In many organisms, both α and β exist in numerous isotypic forms encoded by different genes. In addition, both α and β undergo a variety of posttranslational covalent modifications, including acetylation, phosphorylation, detyrosylation, polyglutamylation, and polyglycylation. In this review the distribution and possible functional significance of the various forms of tubulin are discussed. In analyzing the differences among tubulin isotypes encoded by different genes, some appear to have no functional significance, some increase the overall adaptability of the organism to environmental challenges, and some appear to perform specific functions including formation of particular organelles and interactions with specific proteins. Purified isotypes also display different properties in vitro. Although the significance of all the covalent modifications of tubulin is not fully understood, some of them may influence the stability of modified microtubules in vivo as well as interactions with certain proteins and may help to determine the functional role of microtubules in the cell. The review also discusses isotypes of γ-tubulin and puts various forms of tubulin in an evolutionary context.