Model for the role of tubulin isotypes in optimizing diverse microtubule functions. (a) Yeast microtubules polymerized from only the α-isotype Tub1 (red) optimize the Dynein-mediated spindle positioning mechanism at the expense of the Kar9 mechanism. (b) Those made from only Tub3 (blue) optimize the Kar9-mediated mechanism at the expense of the Dynein mechanism. (c) In WT cells, microtubules containing both Tub1 and Tub3 support both mechanisms sufficiently. Tub1 and Tub3 are likely to differentially support other microtubule-dependent functions as well. (d) Utilization of multiple α- and β-isotypes (e.g., in higher eukaryotes) would allow microtubules to be optimized for an increased range of molecular functions.