Developmental regulation of polyglutamylated α- and β-tubulin in mouse brain neurons

Polyglutamylation is an important posttranslational modi-fication of tubulin that is very active in nerve cells, where it accounts for the main factor responsible for tubulin het-erogeneity. In the present work, we have analyzed quantitative and qualitative changes in glutamylated α- and β-tubulin occurring during neuronal differentiation in culture. Glutamylated α- and β-tubulin both markedly accumulate during this process with a time course remark-ably similar to that observed in vivo during brain development. However, the characteristics of the glutamylation of the two subunits are not exactly the same. Glutamylated α-tubulin is already abundant in very young neurons and displays, at this stage, a wide range of its degree of glutamylation (1 to 6 glutamyl units present in the lateral polyglutamyl chain), which remains unchanged during the entire period of the culture. Glutamylated β-tubulin is present at very low levels in young neurons and its accumulation during differentiation is accompanied by a progressive increase in its degree of glutamylation from 2 to 6 glutamyl units. Posttranslational incorporation of [3H]glutamate into α- and β-tubulin decreases during differentiation, as well as the rate of the reverse deglu-tamylation reaction, suggesting that accumulation of glu-tamylated tubulin is accompanied by a decrease in the turnover of glutamyl units onto tubulin. Neuronal differentiation is also accompanied by an increase of other posttranslationally modified forms of tubulin, including acetylated and non-tyrosinatable α-tubulin, which can occur in combination with polyglu-tamylation and contributes to increase the complexity of tubulin in mature neurons.