We analyzed the role of tubulin polyglycylation in Tetrahymena thermophila using in vivo mutagenesis and immunochemical analysis with modification-specific antibodies. Three and five polyglycylation sites were identified at glutamic acids near the COOH termini of α- and β-tubulin, respectively. Mutants lacking all polyglycylation sites on α-tubulin have normal phenotype, whereas similar sites on β-tubulin are essential. A viable mutant with three mutated sites in β-tubulin showed reduced tubulin glycylation, slow growth and motility, and defects in cytokinesis. Cells in which all five polyglycylation sites on β-tubulin were mutated were viable if they were cotransformed with an α-tubulin gene whose COOH terminus was replaced by the wild-type COOH terminus of β-tubulin. In this double mutant, β-tubulin lacked detectable polyglycylation, while the α-β tubulin chimera was hyperglycylated compared with α-tubulin in wild-type cells. Thus, the essential function of polyglycylation of the COOH terminus of β-tubulin can be transferred to α-tubulin, indicating it is the total amount of polyglycylation on both α- and β-tubulin that is essential for survival.

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Tubulin undergoes glutamylation, a conserved posttranslational modification of poorly understood function. We show here that in the ciliate Tetrahymena, most of the microtubule arrays contain glutamylated tubulin. However, the length of the polyglutamyl side chain is spatially regulated, with the longest side chains present on ciliary and basal body microtubules. We focused our efforts on the function of glutamylation on the ␣-tubulin subunit. By site-directed mutagenesis, we show that all six glutamates of the C-terminal tail domain of ␣-tubulin that provide potential sites for glutamylation are not essential but are needed for normal rates of cell multiplication and cilium-based functions (phagocytosis and cell motility). By comparative phylogeny and biochemical assays, we identify two conserved tubulin tyrosine ligase (TTL) domain proteins, Ttll1p and Ttll9p, as ␣-tubulin-preferring glutamyl ligase enzymes. In an in vitro microtubule glutamylation assay, Ttll1p showed a chain-initiating activity while Ttll9p had primarily a chain-elongating activity. GFP-Ttll1p localized mainly to basal bodies, while GFP-Ttll9p localized to cilia. Disruption of the TTLL1 and TTLL9 genes decreased the rates of cell multiplication and phagocytosis. Cells lacking both genes had fewer cortical microtubules and showed defects in the maturation of basal bodies. We conclude that glutamylation on ␣-tubulin is not essential but is required for efficiency of assembly and function of a subset of microtubule-based organelles. Furthermore, the spatial restriction of modifying enzymes appears to be a major mechanism that drives differential glutamylation at the subcellular level.The principal components of microtubules, heterodimers of ␣-and ␤-tubulin, are known to undergo several types of conserved posttranslational modifications (PTMs), including acetylation, detyrosination, phosphorylation, palmitoylation, glutamylation, and glycylation (61). Some of these PTMs strongly influence interactions between microtubules and microtubuleassociated proteins, including motors and plus-end tracking proteins (27,32,34,43,49). Specific PTMs are enriched on microtubules in restricted subcellular areas, suggesting that these mechanisms act as marks that locally adapt the microtubule polymer for specific functions (reviewed in reference 58). How the spatially restricted modified microtubules are generated within the cell is not well understood.Glutamylated microtubules are generated by the sequential addition of multiple glutamates to the ␥-carboxyl group of specific glutamic acids of the primary sequence of the C-terminal tail (CTT) domain of ␣-and ␤-tubulin (14). This reversible PTM creates glutamyl side chains of variable length and is enriched on microtubules in cilia, centrioles/basal bodies, the mitotic spindle, microtubules of nerve projections, and pellicular arrays in protists (2,5,6,31,35,45,63). Recently, we have identified tubulin glutamylases as proteins with a tubulin tyrosine ligase homology (28). The identification of forward enzymes for...