Tubulin tyrosination in Crithidia: modifying enzymes and modification states of tubulin

An enzyme that adds C-terminal tyrosine to tubulin has been identified in Crithidia fasciculata. It tyrosinates Crithidia, but not brain, tubulin and is specific for the alpha chain. Crithidia cells could not be shown to fix tyrosine in the absence of protein synthesis, which is consistent with the pattern of distribution of C-terminal tyrosine in tubulin from different subcellular compartments of this protozoan. Terminal tyrosine was present in about 5% of flagellar alpha chain from cells in stationary phase and 20% from cells from midlog phase; none was detected in tubulin from cytosol or the subpellicular corset. In contrast to mammalian cells, in which a higher state of tyrosinolation characterizes recently assembled or unstable microtubules, terminal tyrosine was present only in the most stable polymer, the flagellar doublet microtubules.