Polyglutamylase which modifies tubulin, generating polyglutamate side chains of variable lengths on the gamma-carboxyl group of specific glutamate residues within the C-terminal tail of tubulin (PubMed:17499049, PubMed:20530212). Preferentially mediates ATP-dependent polyglutamate long side-chain elongation over the initiation step of the polyglutamylation reaction (PubMed:17499049, PubMed:20530212). Preferentially modifies the alpha-tubulin tail over a beta-tail (PubMed:17499049). Required for CCSAP localization to both spindle and cilia microtubules (By similarity). Promotes tubulin polyglutamylation which stimulates spastin/SPAST-mediated microtubule severing, thereby regulating microtubule functions (PubMed:20530212). ATP + L-glutamate + L-glutamyl-[protein] = ADP + gamma-L-glutamyl-L-glutamyl-[protein] + H(+) + phosphate (L-glutamyl)(n)-gamma-L-glutamyl-L-glutamyl-[protein] + ATP + L-glutamate = (L-glutamyl)(n+1)-gamma-L-glutamyl-L-glutamyl-[protein] + ADP + H(+) + phosphate Highly expressed in brain, kidney, liver, lung, muscle and testis (PubMed:17499049). Expressed in heart, spleen and trachea (PubMed:17499049). In the brain, expressed in ependymal cilia, cortex, corpus callosum and striatum (PubMed:23897886). The flexible c-MTBD (cationic microtubule binding domain) region mediates binding to microtubules. It is positively charged and becomes ordered when bound to microtubules: it interacts with a negatively charged patch on tubulin. The presence of positive charges in the c-MTBD region is essential for proper binding. Gln-252 is the main determinant for regioselectivity, which segregates between initiases and elongases in all tubulin--tyrosine ligase family. A glutamine residue at this position is found in elongases TTLL6, TTLL9, TTLL11, TTLL13, TTLL10 and favors glutamate-chain elongation, whereas an arginine residue is found in initiases TTLL2, TTLL4, TTLL5, TTLL3, TTLL8 and favors initiation. Belongs to the tubulin--tyrosine ligase family. Truncated N-terminus. Truncated C-terminus.