Tubulin Synthesis, Structure, and Function: What Are the Relationships?

In most eukaryotes, families of tubulin genes give rise to multiple isoforms of tubulin, which may be modified post-translationally. The synthesis of isotubulins is spatially and temporally regulated, leading to the presence of different tubulins within an organism. The cellular localization of tubulin is also nonrandom with discrete isoforms residing in specific regions of some cells. Much work, dependent upon interrelated molecular and immunological technologies, has gone into determining why cells produce multiple isotubulins. One proposal would have us believe that isotubulins are functionally discrete and that the isotubulin composition of a microtubule determines its function. A second idea is that tubulin multigene families arose by gene duplication and subsequent genetic drift. The duplicated genes survive in modified form, as they provide the cell with the ability to effectively regulate tubulin synthesis at the transcriptional level while yielding multifunctional tubulins. Analysis of mutant tubulin genes is revealing functionally important regions of tubulin, prompting a return to the consideration of those properties of the molecule that are fundamental to microtubule formation rather than function per se. A model for autoregulation of tubulin synthesis has been developed, but little is yet known of other molecular signals or mechanisms involved in regulation of tubulin production. Further study of flagella formation promises to change this situation. Complementing the genetic studies is the examination of tubulin posttranslational modifications including detyrosination/tyrosination, acetylation/deacetylation, and phosphorylation. Enzymatic mechanisms mediating tubulin posttranslational changes are partially elucidated, as are the influences of the modifications on microtubule properties. Potential functions of the posttranslational changes are the modulation of microtubule-associated protein binding to microtubules, the provision of a mechanism to indicate microtubule age, or as mediators of cellular morphogenesis, a role which could be dependent upon the first two functions. Despite the progress that has been made, the physiological significance of having several isotubulins within a single cell and the molecular details of microtubule function remain obscure.