The cytoskeletal core of cilia and flagella, called the axoneme, is composed of microtubules (MTs), the dynamics of which can be dramatically influenced by post-translational modifications (PTMs) to tubulin C-terminal tails. In mammals, the presence of many tubulin genes, protein isoforms and potential combinations of PTMs make understanding the roles each might have in ciliary function extremely challenging. The biflagellate unicellular algae Chlamydomonas reinhardtii, however, possesses only four tubulin genes, which encode two pairs of identical α- and β-tubulins. In this study (Kubo et al., 2023), Toshiyuki Oda and colleagues take advantage of the simplicity of Chlamydomonas tubulin expression to explore the ciliary roles of tubulin PTMs by mutating key residues to block either glutamylation or glycylation on α- or β-tubulin tails, respectively. Loss of glutamylation of α-tubulin causes severe loss of cell motility despite having no clear effect on axoneme structure, suggesting that it influences the physical properties of ciliary MTs. In contrast, blocking glycylation of β-tubulin produces truncated, nonmotile flagella lacking central axonemal MTs, resembling other Chlamydomonas mutants for the MT-severing ATPase katanin. These findings thus illuminate distinct but crucial functions for both PTMs in ciliary motility, which might also contribute to understanding pathogenic mechanisms in human ciliopathies.
