Synthesis and accumulation of nuclear and cytoplasmic proteins during interphase in mouse fibroblasts in vitro

The synthesis and accumulation of protein in the cell nucleus and cytoplasm during interphase growth of the mouse fibroblasts in vitro (L-cells) has been studied by a combination of quantitative cytophotometric, autoradiographic and time lapse cinematographic methods.The nuclear dry mass of the average cell of the population remained constant or nearly so during the G1 period and increased predominantly during the S and G2 periods by a factor of approximately 2. The onset of this increase occurred at the same time or slightly after the initiation of the DNA synthesis in the cell. The cytoplasmic mass increased, contrary to that of the cell nucleus, predominantly during the G1 period. However, a slight increase of cytoplasmic mass also occurred towards the end of interphase. The rate of protein synthesis in the cytoplasm increased during the whole of interphase. Despite the fact that the rate of protein synthesis was higher in the cytoplasm during the S period than in the G1 period, less protein was accumulated in the cytoplasm at that time than during the G1 period. These results demonstrate a synthesis of nuclear proteins in the cytoplasm and a transportation of these proteins to the cell nucleus during the S period.