Polyglycylase which modifies both tubulin and non-tubulin proteins, generating polyglycine side chains of variable lengths on the gamma-carboxyl groups of specific glutamate residues of target proteins (PubMed:19524510, PubMed:18331838, PubMed:19427864). Involved in the elongation step rather than the initiation step of the polyglycylation reaction (PubMed:19524510, PubMed:18331838, PubMed:19427864). Polyglycylates alpha-tubulin and beta-tubulin (PubMed:19524510, PubMed:19427864). Polyglycylates non-tubulin proteins such as nucleosome assembly protein NAP1 (PubMed:18331838). (glycyl)(n)-glycyl-L-glutamyl-[protein] + ATP + glycine = (glycyl)(n+1)-glycyl-L-glutamyl-[protein] + ADP + H(+) + phosphate Highly expressed in testis (PubMed:17499049). Expressed in brain, heart, kidney, liver, lung, muscle and trachea (PubMed:17499049). Gln-307 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.