In general, the dynamics underlying the receptor cycle of Pex5p seem to be of importance because the import and export rates of Pex5p have to be balanced [ 55].
As a consequence, the control of import and export rates is one mechanism by which the glycerol content inside of the cell can be altered.
Sequence variation generates different binding affinities, which correlate to protein import and export rates and therefore nuclear concentration.
These data derived from HeLa cells indicate that the nuclear import and export rates of RPL27-GFP are different.
Here, we present a model for extracting the import and export rates from FRAP experiments of STAT5B-GFP in the steady state of unstimulated NIH3T3-EpoR cells.
Therefore, we focused on the nuclear import and export rates of unphosphorylated STAT5 with the goal to generate rates for the steady state in unstimulated cells that can be set to a fixed value in a larger pathway model.
Our results provide a procedure to link directly measured import and export rates of unphosphorylated STAT5B with data indirectly describing the nucleocytoplasmic cycling of activated STAT5B generated by biochemical experiments.
Biochemical data describing the phosphorylation dynamics of the pathway components after stimulation in combination with mathematical modeling can then serve to indirectly determine nuclear import and export rates for phosphorylated STAT5.
To determine the import and export rates α imp and α exp 39 FRAP data sets generated from cells expressing varying concentrations of STAT5B-GFP were fitted with eq. (23) described in the Methods section.
Previously, rapid nucleocytoplasmic cycling of activated STAT5 has been identified as the step most sensitive to perturbation within the core module of the JAK2/STAT5 pathway by mathematical modeling based on biochemical data [ 18], but import and export rates could not be measured experimentally.
The mathematical model was based on a "conveyor-belt" [19] type of differentiation and allowed the study of cell fluxes, residence times, and rates of import, export, proliferation, and death across cell compartments for thymocytes and recent thymic emigrants.
