Tubulin, but not microtubules, is the substrate for tubulin:tyrosine ligase in mature avian erythrocytes

Chicken erythrocytes, which contain a marginal band of microtubules, were used to study the influence of the aggregation state of tubulin on the post-translational incorporation of tyrosine into the alpha-tubulin subunit. We found that the incorporation of [14C]tyrosine occurs almost exclusively into the nonassembled tubulin pool. The marginal band was practically not labeled. The low incorporation into microtubules was not due to the lack of tubulin with acceptor capacity since after cold-induced disassembly, an additional amount of [14C]tyrosine could be incorporated. 14C-Tyrosinated tubulin of the nonassembled pool could not be incorporated into microtubules of the marginal band after prolonged incubation at 37 degrees C or when the marginal band was regenerated after cold-induced depolymerization. In erythrocytes, tubulin:tyrosine ligase behaved as a soluble entity when the cells were lysed under microtubule-preserving conditions.