Enzyme which specifically adds tyrosine to the α chain of tubulin

A posttranslational modification of tubulin with potential significance for the regulation of microtubule assembly or function has been revealed by the discovery of an enzyme (tubulin--tyrosine ligase) which can add a tyrosine residue to the alphachain, apparently through peptide bond linkage to a C-terminal glutamate or glutamine. The ability to add tyrosine in the presence of ATP and to release it agiain in the presence of ADP and inorganic phosphate (or arsenate) appear to be functions of the same enzyme, as judged by the relative rates at which these reactions are catalyzed over a 20-fold enzyme purification. The apparent size of the enzyme from bovine brain is 150 000 daltons in extracts and after ammonium sulfate fractionation, but 35 000 after elution from anion-exchange columns. Addition of pure dimeric tubulin to the latter species converts it back to the larger form, which is apparently a stoichiometric 1:1 complex of tubulin and the 35 000-dalton enzyme. Tubulin--tyrosine ligase is specific for tubulin; other proteins with C-terminal glutamate or glutamine do not substratesmate or glutamine do not act as substrates or inhibitors. It is less specific for tyrosine; five out of six tyrosine dipeptides were inhibitors and competed with tyrosine.