CAPABILITY OF TUBULIN AND MICROTUBULES TO INCORPORATE AND TO RELEASE TYROSINE AND PHENYLALANINE AND THE EFFECT OF THE INCORPORATION OF THESE AMINO ACIDS ON TUBULIN ASSEMBLY

Abstract

Abstract— Incorporation of [ 14 C]tyrosine into the C‐terminal position of α‐tubulin of rat brain cytosol was 10‐fold higher for non‐assembled than for assembled tubulin. The incorporation into tubulin from disassembled microtubules was higher than into non‐assembled tubulin; therefore, the low incorporation into microtubules was not due to a lower acceptor capacity of their tubulin constituent. [ 14 C]Tyrosine was released from assembled and non‐assembled [ 14 C]tyrosinated tubulin by the action of an endogenous carboxypeptidase. Release from non‐assembled tubulin was shown by incubating a tubulinyl‐[ 14 C]tyrosine preparation in the presence of CaCl 2 at a concentration that abolished microtubule formation. Release from microtubules was inferred from the observation that the percentages of [ 14 C]tyrosine released and the decrease of the specific radioactivity of the recovered microtubules were practically identical and did not change after a 10‐fold dilution of the incubated microtubules. [ 3 H]Phenylalanine was released from a preparation of tubulinyl‐[ 3 H]phenylalanine also by an enzymatic activity. The capacity of a tubulin preparation to incorporate tyrosine was increased 43% by pre‐treatment with endogenous carboxypeptidase. Tubulin tyrosinated in vitro was assembled to the same extent as native tubulin. After a mixture of tubulinyl‐[ 14 C]tyrosine and tubulinyl‐[ 3 H]phenylalanine was partially assembled, the ratio of 14 C/ 3 H found in the microtubules was the same as in the non‐assembled tubulin fraction.