Total Tubulin and Its Aminoacylated and Non-aminoacylated Forms During the Development of Rat Brain

Abstract
The amount of total tubulin in the soluble fraction of rat brain was measured by a method based on the purification of tubulin previously labeled by incorporation of [14C]tyrosine in the C terminus of its .alpha.-chain. The tubulin content decreased from 20.1-1.30 nmol/mg protein when the animals passed from 4-30 days old and then remained practically constant. The amounts of aminoacylated and non-aminoacylated tubulin present in the soluble brain extracts were determined from the incorporation of [14C]tyrosine into the free acceptor sites of tubulin preparations that were preincubated or without carboxypeptidase A to eliminate tyrosine and phenylalanine from the C terminus of the .alpha.-chain of tubulin. The values were corrected for the inactivation of tubulin to accept [14C]tyrosine that occured during the isolation and incubation of the soluble fractions. The ratio non-aminoacylated/aminoacylated tubulin increased from 1.62 .+-. 0.03 in the 4 day old rats to 2.11 .+-. 0.17 in the 120 day old rats. The aminoacylatable tubulin, i.e., the sum of aminoacylated and non-aminoacylated tubulin, decreased from 1.71-0.75 nmol/mg protein from 4-30 day old rats, respectively, and then remained practically constant. The amount of aminoacylatable tubulin was lower than total soluble tubulin. A fraction of tubulin apparently exists that cannot accept tyrosine. This non-aminoacylatable tubulin fraction increases with the age of the animal. In the 120 day old rats, this tubulin species accounts for 48% of the total soluble tubulin.