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

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