Turnover of nascent proteins in HeLa-S3 cells and the quasi-linear incorporation kinetics of amino acids

The turnover of nascent proteins in HeLa S-3 cells was estimated simultaneously with synthesis by the difference between the expected accumulation of radioactivity, based on rate determinations from very short pulse treatments, from that observed by continuous exposure for 100 min. The results were similar to those from conventional pulse-chase analysis, but not identical for reasons which are discussed. They confirm that extensive protein turnover accompanies synthesis in the growing cell, and therefore net synthesis is much lower than gross synthesis. Cells retained all proteins synthesised during heat treatment at 45 °C, indicating the complete inhibition of protein degradation, cells retaining all proteins synthesised. This particular perturbation experimentally validated the deficit method, and its potential usefulness in future turnover studies.