Mitosis and protein synthesis 4 turnover of short- and long-lived proteins during the cell cycle of synchronized L-132 and HeLa S-3 cells.

Turnover rates of short- and long-lived proteins throughout the cell cycle were measured in two established mammalian cell lines, L-132 and HeLa S-3, using both selection and induction synchronization. Short-lived, newly synthesized proteins turned over at the same rate during all stages of the cell cycle when this rate was expressed as a percentage of total labelling of proteins present at the start of the chase. Since fewer proteins were made in M-phase, the absolute turnover rate was probably reduced to a small degree during division itself, indicating a close co-ordination between synthesis and degradation. In contrast, long-lived proteins showed a considerable reduction in their rate of turnover specifically during M-phase. One possible explanation for the reduced degradation of long-lived protein is the suppression of lysosomal activity during division, with no apparent effect on the turnover of short-lived proteins which is due to a non-lysosomal system.