Monoglutamylase which modifies tubulin, adding a single glutamate on the gamma-carboxyl group of specific glutamate residues of target proteins. Involved in the side-chain initiation step of the polyglutamylation reaction but not in the elongation step. Preferentially modifies beta-tail tubulin over the alpha-tubulin. Involved in side-chain glutamylation of tubulin in sensory cilia. Together with ttll-5 and ttll-11, required for male mating