TTLL3 is a monoglycylase which modifies alpha- and beta-tubulin, generating side chains of glycine on the gamma-carboxyl groups of specific glutamate residues within the C-terminal tail of alpha- and beta-tubulin. It is involved in the side-chain initiation step of the glycylation reaction by adding a single glycine chain to generate monoglycine side chains. It is not involved in elongation step of the polyglycylation reaction.