The Periodic Synthesis of S-Adenosylmethionine:Protein Methyltransferases during the HeLa S-3 Cell Cycle

Abstract

Abstract The activities of three S-adenosylmethionine:protein methyltransferases (protein methylase I, II, and III) were measured throughout the mitotic cycle of synchronized HeLa S-3 cells in an attempt to determine the biological function of these enzymes. 1. The activities of protein methylase I and II, which catalyze the methylation of protein-arginine residues and carboxyl groups, increase more or less continuously until they have doubled by the end of G2 phase. These increases were not obviously correlated with major cell cycle events such as DNA replication or histone synthesis and modification. 2. The activity of protein methylase III (S-adenosylmethionine:protein-lysine methyltransferase) remains relatively constant through G1 and early S phase of the cell cycle, then begins increasing until it reaches over three to four times its initial level by the end of G2. The increase in protein methylase III activity is temporally correlated with the increased mono- and dimethylation of the e-amino groups of histone lysine residues. 3. By inhibiting protein and RNA synthesis it was determined that this increase in protein methylase III activity in late S and G2 of the HeLa cell cycle was probably due to the de novo synthesis of enzyme molecules of those times. 4. The same 3- to 4-fold increase in the activity of protein methylase III occurs at the usual time after mitosis even if DNA and histone synthesis were prevented by treatment of the cells with cytosine arabinoside. In such treated cells, the enzyme apparently overmethylated the already mono- and dimethylated lysine residues of previously synthesized histone molecules to the di- and trimethyllysine state. These data suggest that protein methylase III synthesis is not triggered by the appearance of its normal substrate but rather is coupled with some other unknown cell cycle event.