Tubulin tyrosinolated in vivo can be different from that tyrosinolated in vitro

Tubulin can be tyrosinolated, in the presence of ATP, by tubulin-tyrosine ligase, and tyrosine can be released by the same enzyme in the presence of ADP plus inorganic phosphate. There is however a 'non-substrate' component of tubulin which can not be tyrosinolated or detyrosinolated by this enzyme. Tubulin tyrosinolated in vivo was found to be the non-substrate species in HeLa cells, and the substrate species in cultured neuronal cells. In this respect HeLa tubulin resembled membrane-associated tubulin from brain, and neuronal cell tubulin resembled brain cytosolic tubulin.