Final answer:
Upon the ubiquitylation and degradation of the fluorescent protein's high-affinity nuclear binding partner, the fluorescent protein accumulates in the cytosol due to the absence of its retention mechanism in the nucleus.
Step-by-step explanation:
If we consider a fluorescent protein that has both a nuclear localization and nuclear export signal, which is normally found both in the nucleus and in the cytosol at comparable concentrations, and has a high-affinity binding protein in the nucleus, the behavior of this protein can be altered by modifications to its associated proteins. In this scenario, the binding protein in the nucleus is ubiquitylated and degraded after the activation of a signaling pathway. This ubiquitylation acts as a signal for the protein degradation via the proteasome, an organelle responsible for degrading proteins.
When the nuclear binding protein is degraded, the equilibrium that was established between the nuclear and cytosolic concentrations of the fluorescent protein may shift. Given that the high-affinity binding partner in the nucleus is no longer available to bind to the fluorescent protein, one would expect the fluorescent protein to accumulate less in the nucleus. Thus, the correct answer is that the fluorescent protein accumulates in the cytosol (2), as now there is less retention of the protein in the nucleus due to the absence of its binding partner.