Polyglutamylase which modifies both tubulin and non-tubulin proteins, generating alpha-linked polyglutamate side chains on the gamma-carboxyl group of specific glutamate residues of target proteins (PubMed:17499049, PubMed:21074048, PubMed:20530212, PubMed:26829768, PubMed:32747782). Preferentially mediates ATP-dependent long polyglutamate chain elongation over the initiation step of the polyglutamylation reaction (PubMed:17499049, PubMed:21074048, PubMed:20530212, PubMed:26829768, PubMed:32747782). Preferentially modifies the alpha-tubulin tail over a beta-tail (PubMed:17499049, PubMed:20530212, PubMed:21074048, PubMed:32747782). Promotes tubulin polyglutamylation which stimulates spastin/SPAST-mediated microtubule severing, thereby regulating microtubule functions (PubMed:20530212). Mediates microtubule polyglutamylation in primary cilia axoneme which is important for ciliary structural formation and motility (PubMed:22246503). Mediates microtubule polyglutamylation in motile cilia, necessary for the regulation of ciliary coordinated beating (PubMed:23897886). Polyglutamylates non-tubulin protein nucleotidyltransferase CGAS, leading to CGAS DNA-binding inhibition, thereby preventing antiviral defense response (PubMed:26829768). 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 kcat is 0.73 min(-1) with glutamylated tubulin as substrate (PubMed:32747782). kcat is 0.04 min(-1) with non-glutamylated tubulin as substrate (PubMed:32747782). Found in a complex with CEP41. CEP41 is required for its transport between the basal body and the cilium axoneme. Highly expressed in testis (PubMed:17499049). Expressed in brain, heart, kidney, liver, lung, muscle and trachea (PubMed:17499049). In the brain, specifically expressed in ependymal cilia (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-180 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.