Quantitative Studies on Cell Proteins in Suspension Cultures

The protein per cell decreases during a 96‐hour growth of Chang's and HeLa cells cultivated in suspension. This decrease is very high for the cytoplasmic proteins, while the nuclear proteins appear to be more stable. The content of cell proteins is reestablished during a phase of intensive enrichment which occurs during the first 30–60 min of the newly inoculated culture. The rate of protein labelling in this phase appears to be greater in the nuclei than in the cytoplasm. Both the level of rapid accumulation and the decrease of the protein content with time appear to be a function of cell density. The rapid protein accumulation occurs when this density in the culture is lowest. With time and increase of the cell density in the culture, the protein content per cell goes down. In non‐synchronized cells this decrease is steady. In “naturally” synchronized cells it shows itself in an oscillatory fashion. The artificial thymidine synchronization of HeLa suggests that the fluctuating protein production in the “naturally” synchronized cultures of these cells is linked with the periodical doubling of the cell number during the growth cycle which includes, for HeLa, at least three cell cycles.