Abstract
It has been observed that the growth of the nucleus and the cytoplasm is
coordinated during cell growth, resulting in a nearly constant nuclear-to-cell
volume ratio (N/C) throughout the cell cycle. Previous studies have shown that
the N/C ratio is determined by the ratio between the number of proteins in the
nucleus and the total number of proteins in the cell. These observations
suggest the importance of the nucleocytoplasmic transport process in nuclear
size by regulating protein concentrations in the nucleus and cytoplasm. This
paper combines a biophysical model of Ran-mediated nucleocytoplasmic transport
and a simple cell growth model to provide insights into several key aspects of
the N/C ratio homeostasis in growing cells. Our model shows that the
permeability of the nuclear envelope needs to grow in line with the cell to
maintain a nearly constant N/C ratio, that several parameters involved in the
nucleocytoplasmic transport mechanism and gene translation significantly affect
the N/C ratio, and that Ran may potentially compensate for the lack of NTF2 in
the nucleocytoplasmic transport mechanism to maintain a viable N/C ratio.
However, this compensation is possible only if RanGDP is allowed to translocate
through the nuclear envelope independently of NTF2.