Chlamydomonas a-Tubulin Is Posttranslationally in the Flagella during Flagellar Assembly Modified

The principal a-tubulin within Chlamydomonas reinhardti i flagellar axonemes differs from the major c~-tubulin in the cell body. We show that these two isoelectric variants of a-tubulin are related to one another since posttranslational modification of the cell body precursor form converts it to the axonemal form. During flagellar assembly, precursor a-tubulin enters the flagella and is posttranslationally modified within the flagellar matrix fraction prior to or at the time of its addition to the growing axonemal microtubules. Experiments designed to identify the nature of this posttranslational modification have also been conducted. When flagella are induced to assemble in the absence of de novo protein synthesis, tritiated acetate can be used to posttranslationally label atubulin in vivo and, under these conditions, no other flagellar polypeptides exhibit detectable