Post-translational modifications (PTMs) added to microtubules (MTs) may act as a “Tubulin Code” that guides the activities of kinesins, dyneins, and other MT-binding proteins. Ciliary MTs are highly decorated with PTMs, but the functions of PTMs in cilia are largely unknown. We previously showed that loss of CCPP-1, a predicted MT deglutamylase, caused defective localization of the ciliary receptor PKD-2 and the kinesin-3 motor KLP-6, and abnormally fast movement of the kinesin 2 OSM-3, in male-specific B-type neuronal cilia. In amphid channel cilia, ccpp-1 mutants displayed a progressive Dyf phenotype and deterioration of MT structure. Loss of a CCPP-1 homolog in mice also affects ciliated cells, causing degeneration of cerebellar Purkinje neurons, olfactory mitral cells, and retinal photoreceptors, and also causes sperm immotility. Therefore, MT glutamylation may play a conserved role in cilia and flagella.
Here we show that select Tubulin Tyrosine Ligase-Like (TTLL) MT glutamylases oppose the activity of CCPP-1. Mutations in ttll-4, ttll-5, or ttll-11 suppressed the ccpp-1 progressive Dyf phenotype, but did not suppress PKD-2 ciliary localization defects. The ttll-11 mutation suppressed ccpp-1 effects on kinesin-3 KLP-6 localization and kinesin-2 OSM-3 velocity.
Ciliary MT structure typically contains doublets composed of A- and B- tubules. Ultrastructural analysis revealed loss of ciliary B-tubules in ccpp-1, while ttll-11 mutants displayed abnormally long ciliary doublet regions. MT glutamylation reduced B-tubule stability in both neuronal types, despite differences in structural details. We hypothesize that glutamylation targets MTs for degradation by MT-severing enzymes. To elucidate the pathways by which glutamylation controls ciliary MT stability, we performed a screen for suppressors of the ccpp-1 Dyf phenotype. Our screen should identify molecules upstream and downstream of MT glutamylation.
Our data suggest that, in cilia, MT glutamylation is part of a Tubulin Code that regulates ciliary transport of molecular motors and sensory receptors and controls axonemal structure.
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