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. Preferentially mediates ATP-dependent polyglutamate long side- chain elongation over the initiation step of the polyglutamylation reaction. Preferentially modifies the alpha-tubulin tail over a beta- tail (By similarity). Required for CCSAP localization to both spindle and cilia microtubules (PubMed:22493317). Promotes tubulin polyglutamylation which stimulates spastin/SPAST-mediated microtubule severing, thereby regulating microtubule functions (By similarity). . This is the function of TTLL11 (tubulin tyrosine ligase like 11, Ensembl gene identifier ENSG00000175764).