Variation of tubulin half-life during the cell cycle in the synchronous plasmodia of Physarum polycephalum.

The half-life of tubulin has been studied during the cell cycle of the myxomycete Physarum, using a specific quantitative immunological method. In asynchronous microplasmodial and amoebal cultures the apparent half-life of tubulin was greater than 15 h. In contrast, in the naturally synchronous plasmodia, in which tubulin exhibits a cyclic synthesis, we have shown for the first time that tubulin half-life was not constant through the cell cycle. There was no tubulin degradation before mitosis, while tubulin half-life was reduced to about 1 h during the postmitotic period. Such a variation of stability through the cell cycle has not been observed in the case of thymidine kinase, another protein showing a cyclic synthesis in Physarum plasmodia. The decrease of tubulin half-life that occurred during the S-phase immediately following mitosis (no G1-phase in Physarum) was independent of the increase of growth temperature between 22 and 32 degrees C, in contrast with the half-life of thymidine kinase which decreased in the same conditions. Tubulin did not completely disappear after mitosis. A 20% residual amount of tubulin persisted from late S-phase to early G2-phase. We have shown that tubulin degradation was not modified by actinomycin D or cycloheximide but was prevented when DNA synthesis was inhibited by fluorodeoxyuridine and hydroxyurea. In contrast, inhibition of S-phase did not modify the half-life of thymidine kinase. These results indicate that: 1) during the cell cycle, the pool of tubulin is regulated not only at the transcriptional and translational levels but also by a cell cycle-dependent degradative process.(ABSTRACT TRUNCATED AT 250 WORDS)