Phosphorylation of Elongation Factor-2 And Activity Of Ca²⁺/Calmodulin-Dependent Protein Kinase III During The Cell Cycle

Phosphorylation of the eukaryotic elongation factor 2 (eEF-2) blocks the elongation step of translation and stops overall protein synthesis. Although the overall rate of protein synthesis in mitosis reduces to 20% of that in S phase, it is unclear how the protein translation procedure is regulated during the cell cycle, especially in the stage of peptide elongation. To delineate the regulation of the elongation step through eEF-2 function, the changes in phosphorylation of eEF-2, and in activity of corresponding $Ca^{2+}$/calmodulin (CaM)-dependent protein kinase III (CaMK-III) during the cell cycle of NIH 3T3 cells, were determined. The in vivo level of phosphorylated eEF-2 showed an 80% and 40% increase in the cells arrested at G1 and M, respectively. The activity of CaMK-III also changed in a similar pattern, more than a 2-fold increase when arrested at G1 and M. The activity change of the kinase during one turn of the cell cycle also demonstrated the activation at G1 and M phases. The activity change of cAMP-dependent protein kinase (PKA) was reciprocal to that of CaMK-III. These results indicated: (1) the activity of CaMK-III was cell cycle-dependent and (2) the level of eEF-2 phosphorylation followed the kinase activity change. Therefore, the elongation step of protein synthesis might be cell cycle dependently regulated.