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. Involved in the elongation step rather than the initiation step of the polyglycylation reaction. Polyglycylates alpha-tubulin and beta-tubulin. Polyglycylates non-tubulin proteins such as nucleosome assembly protein NAP1. (glycyl)(n)-glycyl-L-glutamyl-[protein] + ATP + glycine = (glycyl)(n+1)-glycyl-L-glutamyl-[protein] + ADP + H(+) + phosphate Gln-310 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.