Variations in the cell‐free translating apparatus of cultured animal cells as a function of time during cell growth

Abstract

Vero M₃ cells, a line derived from the kidney of an African Green Monkey, display certain alterations in their protein synthetic apparatus as a function of time during a growth cycle. (Growth cycle here refers to exponential growth of unsynchronized cells in culture and their subsequent passage into the stationary phase.) The capacity of cytoplasmic extracts of these cells to promote endogeneous mRNA‐mediated polypeptide synthesis or poly U‐mediated polyphenylalanine synthesis declines from the second day after the initiation of the growth cycle. The ribosome sedimentation profile indicates that after the second day of growth a decrease also occurs in the total amount of ribosomes per cell, and that a shift occurs from predominantly polyribosome structures to predominantly subunits and monoribosomes structures. The activity of the translation factor, elongation factor 1, also progressively decreases after the second day of growth. Furthermore, when crude factor preparations from cells in the second day of growth (Exponential phase) and from cells in the fifth day of growth (Stationary phase) are compared for leucyl‐tRNA synthetase and prolyl‐tRNA synthetase activities, it is found that the extracts from fifth‐day cells have significantly less activity. The activity of another enzyme, acid phosphatase, remains relatively unaffected as a function of time during the cell growth cycle. When HeLa S₃ plating cells are grown under the same conditions, they do not display the same responses.