Differences in the way a mammalian cell and yeast cells coordinate cell growth and cell-cycle progression

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Abstract
It is widely believed that cell-size checkpoints help to coordinate cell growth and cell-cycle progression, so that proliferating eukaryotic cells maintain their size. There is strong evidence for such size checkpoints in yeasts, which maintain a constant cell-size distribution as they proliferate, even though large yeast cells grow faster than small yeast cells. Moreover, when yeast cells are shifted to better or worse nutrient conditions, they alter their size threshold within one cell cycle. Populations of mammalian cells can also maintain a constant size distribution as they proliferate, but it is not known whether this depends on cell-size checkpoints. We show that proliferating rat Schwann cells do not require a cell-size checkpoint to maintain a constant cell-size distribution, as, unlike yeasts, large and small Schwann cells grow at the same rate, which depends on the concentration of extracellular growth factors. In addition, when shifted from serum-free to serum-containing medium, Schwann cells take many divisions to increase their size to that appropriate to the new condition, suggesting that they do not have cell-size checkpoints similar to those in yeasts. Proliferating Schwann cells and yeast cells seem to use different mechanisms to coordinate their growth with cell-cycle progression. Whereas yeast cells use cell-size checkpoints, Schwann cells apparently do not. It seems likely that many mammalian cells resemble Schwann cells in this respect.