Diversity among tubulin subunits: Toward what functional end?

The cytoplasm of most eukaryotic cells is continuously remodeled by hundreds of growing or shrinking microtubules. These polymers not only provide structural architecture to the cytoplasm, but also contribute to several forms of intracellular motility including chromosome segregation during meiosis and mitosis and the distribution and transport of various organelles such as the Golgi and endoplasmic reticulum. In addition, microtubules have specific structural and motile roles in specialized cells including flagellar-mediated locomotion, the specification and maintenance of the asymmetric morphology of neurons, and the transport of organelles and vesicles into the axonal processes of such cells. One of the intriguing questions is what specifies which microtubules are used for each of these roles? An obvious place to look for such information is within the structure of the basic subunit, the tubulin heterodimer of one a and one p tubulin polypeptide. It is now clear that (with the exception of the simplest eukaryotes) tubulin exists in all cells as a mixture of similar, but not identical, polypeptides. As might be expected, within an individual cell this diversity is generated at two levels: first is the selective transcriptional activation of one or more genes encoding either subunit. Second, there are a variety of post-translational modifications of the primary gene products. We shall focus this review on the extent to which this tubulin diversity contributes to overall microtubule function.