ADP-ribosylatable content of elongation factor-2 changes during cell cycle of normal and cancerous human cells

The amount of protein elongation factor EF-2 that can be inactivated by diphtheria toxin-mediated ADP-ribosylation, a measure of its active content, decreases by 45% and 66% in G1-arrested normal human fibroblasts and in HeLa cells respectively. On restimulation of cells with fresh serum, the amounts of ADP-ribosylatable EF-2 begin to increase within 4 h. Whereas the level of active EF-2 returns to normal (exponential phase of growth) in 20 h in the case of fibroblasts, only 47% recovery was observed for HeLa cells during this period. The apparent long half-lives of EF-2 mRNA and protein indicate possibilities of posttranslational ADP-ribosylation and de-ADP-ribosylation as the regulators of the amounts of active EF-2 during human cell cycle.