In most eukaryotic cells, tubulin is subjected to posttranslational 
glutamylation, a conserved modification of unclear function. The glutamyl side 
chains form as branches of the primary sequence glutamic acids in two 
biochemically distinct steps: initiation and elongation. The length of the 
glutamyl side chain is spatially controlled and microtubule type specific. Here, 
we probe the significance of the glutamyl side chain length regulation in vivo 
by overexpressing a potent side chain elongase enzyme, Ttll6Ap, in Tetrahymena. 
Overexpression of Ttll6Ap caused hyperelongation of glutamyl side chains on the 
tubulin of axonemal, cortical, and cytoplasmic microtubules. Strikingly, in the 
same cell, hyperelongation of glutamyl side chains stabilized cytoplasmic 
microtubules and destabilized axonemal microtubules. Our observations suggest 
that the cellular outcomes of glutamylation are mediated by spatially restricted 
tubulin interactors of diverse nature.