Other transcription regulators are bound to inhibitory cytosolic proteins that either anchor them in the cytosol (through interactions with the cytoskeleton or specific organelles) or mask their nuclear localization signals so that they cannot interact with nuclear import receptors. An appropriate stimulus releases the gene regulatory protein from its cytosolic anchor or mask, and it is then transported into the nucleus. One important example is the latent gene regulatory protein that controls the expression of proteins involved in cholesterol metabolism. The protein is made and stored in an inactive form as a transmembrane protein in the ER. When a cell is deprived of cholesterol, the protein is transported from the ER to the Golgi apparatus where it encounters specific proteases that cleave off the cytosolic domain, releasing it into the cytosol. This domain is then imported into the nucleus, where it activates the transcription of genes required for both cholesterol uptake and synthesis (Figure 12–16).
As we discuss in detail in Chapter 6, cells control the export of RNAs from the nucleus in a similar way. snRNAs, miRNAs, and tRNAs bind to the same family of nuclear export receptors just discussed, and they use the same Ran-GTP gradient to fuel the transport process. By contrast, the export of mRNAs out of the nucleus uses a different mechanism. mRNAs are exported as large assemblies, which can be as large as 100 million daltons (see Figure 6–37) and can contain hundreds of proteins of a few dozen different types. These mRNA ribonucleoprotein complexes (mRNPs) first dock at the nuclear side of the NPC, where they are extensively remodeled. Although Ran-GTP is indirectly involved in the export (because it imports the proteins that bind to the mRNA molecules), the translocation across the NPC is thought to be driven by ATP hydrolysis. How export directionality is assured is unclear. It is likely that the many accessory proteins tethered to the NPC’s nuclear and cytoplasmic fibrils have important roles in remodeling the mRNPs as they pass through the pores, in particular stripping away nuclear proteins as the mRNPs exit on the cytosolic side of the NPC, thereby ensuring that transport is unidirectional. Upon entry into the cytosol, these nuclear mRNP
Figure 12–14 The control of nuclear transport in the early Drosophila embryo. The embryo at this stage is a syncytium, shown here in cross section, with many nuclei in a common cytoplasm, arranged around the periphery, just beneath the plasma membrane. The transcription regulatory protein Dorsal is produced uniformly throughout the peripheral cytoplasm, but it can act only when inside the nuclei. The Dorsal protein has been stained with an enzyme-coupled antibody that yields a brown product, revealing that Dorsal is excluded from the nuclei at the dorsal side (top) of the embryo but is concentrated in the nuclei toward the ventral side (bottom) of the embryo. The regulated traffic of Dorsal into the nuclei controls the differential development between the back and belly of the animal. (Courtesy of siegfried roth.)
Figure 12–15 The control of nuclear import during T cell activation. The nuclear factor of activated T cells (nf-aT) is a transcription regulatory protein that, in the resting T cell, is found in the cytosol in a phosphorylated state. When T cells are activated by foreign antigen (discussed in Chapter 24), the intracellular Ca2+ concentration increases. in high Ca2+, the protein phosphatase calcineurin binds to nf-aT and dephosphorylates it. The dephosphorylation exposes nuclear import signals and blocks a nuclear export signal. The complex of nf-aT and calcineurin is therefore imported into the nucleus, where nf-aT activates the transcription of numerous genes required for T cell activation.