Microtubules form diverse structures, despite being composed of alpha and beta tubulins that vary only in their C-terminal tails. Post-translational modifications (PTMs), most of which can be added to the tubulin C-terminal tail, are thought to increase tubulin diversity, creating a "Tubulin Code" that regulates microtubule-based motor transport, as well as microtubule stability and structure (Verhey and Gaertig 2007). Polyglutamylation is a reversible tubulin modification regulated by tubulin tyrosine ligase-like (TTLL) proteins that initiate and elongate glutamate side-chains on tubulin C-terminal tails, and cytosolic carboxypeptidases (CCPs) that cleave glutamate residues, reducing or removing polyglutamylation. Although molecules that write or erase the Tubulin Code, such as TTLL glutamylases and CCP deglutamylases, have been identified, we lack an understanding of how the Tubulin Code is written, read and interpreted by cells. Cilia provide an ideal model to study PTMs. The axoneme is composed of highly post-translationally modified microtubules that display a diversity of structures and perform a variety of functions. In C. elegans sensory cilia,
ccpp-1 and
ttll-11 regulate microtubule deglutamylation and glutamylation (O'Hagan et al 2011; Kimura et al. 2010; Chawla et al 2016). To discover effectors of the tubulin code, which may read or interpret tubulin PTMs, we are using genetic and biochemical methods. From a forward genetic screen, we isolated mutants that suppressed the progressive degeneration of cilia characteristic of
ccpp-1 mutants. We are currently mapping and cloning these genes. We are also using biochemical approaches to uncover both tubulin and non-tubulin substrates of
ccpp-1 and
ttll-11. Molecular motors are also influenced by glutamylation, and therefore must read and interpret the Tubulin Code. To understand cell-specific effects of glutamylation on ciliary transport, we are also testing how loss of glutamylase and deglutamylase enzymes alters ciliary velocity and distribution of molecular motors in vivo. Our findings on
ccpp-1 and ttll-mediated pathways should further our understanding of the basic mechanisms that modify and stabilize cilia, and how defects in these pathways might contribute to human conditions such as male infertility, polycystic kidney disease, impaired vision, and abnormal neurodevelopment.