Cell context-specific effects of the beta-tubulin glycylation domain on assembly and size of microtubular organelles.

Tubulin glycylation is a posttranslational modification found in cells with cilia or flagella. The ciliate Tetrahymena has glycylation on ciliary and cortical microtubules. We showed previously that mutating three glycylation sites on beta-tubulin produces immotile 9 + 0 axonemes and inhibits cytokinesis. Here, we use an inducible glycylation domain mutation and epitope tagging to evaluate the potential of glycylation-deficient tubulin for assembly and maintenance of microtubular systems. In axonemes, the major defects, including lack of the central pair, occurred during assembly, and newly made cilia were abnormally short. The glycylation domain also was required for maintenance of the length of already assembled cilia. In contrast to the aberrant assembly of cilia, several types of cortical organelles showed an abnormally high number of microtubules in the same mutant cells. Thus, the consequences of deficiency in tubulin glycylation are organelle type specific and lead to either insufficient assembly (cilia) or excessive assembly (basal bodies and cortical microtubules). We suggest that the diverse functions of the beta-tubulin glycylation domain are executed by spatially restricted microtubule-associated proteins.