Multiple forms of tubulin in polytomella and chlamydomonas: evidence for a precursor of flagellar α-tubulin

The quadriflagellate alga polytomella agilis contains several α-tubulins with distinct isoelectric points (McKeithan, T.W., and J.L. Rosenbaum, 1981, J. Cell Biol., 91:352-360). While α-3 is the major component in flagella, α-1 predominates in cytoskeletal microtubules. For determination of whether the differences in α- tubulins are due to distinct genes or to posttranslational modification of a common α-tubulin precursor, poly A+ RNA was isolated from deflagellated and control (nonregenerating) cells and translated in vitro. Approximately twice as much α-1 was synthesized using RNA from deflagellated as compared to control cells; however, there was no detectable synthesis in vitro of α-3 in either. These results suggest that α -3 tubulin is formed in vivo by posttranslational modification of a form co- migrating with, and possibly identical to, cytoskeletal α-tubulin. In the related alga chlamydomonas reinhardii deflagellation greatly stimulates synthesis of tubulin and tubulin mRNA. As in polytomella, the principal α-tubulin synthesized both in vivo and in vitro following deflagellation in chlamydomonas is more basic than the major α-tubulin and appears to correspond to α-1 tubulin in polytomella. The conversion of α-1 to α-3 receives additional support from in vivo labeling and pulse-chase experiments. In addition, in both polytomella and chlamydomonas some conversion of α-1 to α-3 appears to occur even when protein synthesis is inhibited.