ran-GDp is imported into the nucleus by its own import receptor, which is specific for the GDp-bound conformation of ran. The ran-GDp receptor is structurally unrelated to the main family of nuclear transport receptors. however, it also binds to fG-repeats in npC channel nucleoporins.
654 Chapter 12: intracellular Compartments and protein sorting
This gradient of the two conformational forms of Ran drives nuclear transport in the appropriate direction. Docking of nuclear import receptors to FG-repeats on the cytosolic side of the NPC, for example, occurs whether or not these receptors are loaded with appropriate cargo. Import receptors, facilitated by FG-repeat binding, then enter the channel. If they reach the nuclear side of the pore complex, Ran-GTP binds to them, and, if the receptors arrive loaded with cargo molecules, the Ran-GTP binding causes the receptors to release their cargo (Figure 12–13A). Because the Ran-GDP in the cytosol does not bind to import (or export) receptors, unloading occurs only on the nuclear side of the NPC. In this way, the nuclear localization of Ran-GTP creates the directionality of the import process.
Having discharged its cargo in the nucleus, the empty import receptor with Ran-GTP bound is transported back through the pore complex to the cytosol. There, Ran-GAP triggers Ran-GTP to hydrolyze its bound GTP, thereby converting it to Ran-GDP, which dissociates from the receptor. The receptor is then ready for another cycle of nuclear import.
Nuclear export occurs by a similar mechanism, except that Ran-GTP in the nucleus promotes cargo binding to the export receptor, rather than promoting cargo dissociation. Once the export receptor moves through the pore to the cytosol, it encounters Ran-GAP, which induces the receptor to hydrolyze its GTP to GDP. As a result, the export receptor releases both its cargo and Ran-GDP in the cytosol. Free export receptors are then returned to the nucleus to complete the cycle (Figure 12–13B).
Transport Through npCs Can Be regulated by Controlling access to the Transport machinery
Some proteins contain both nuclear localization signals and nuclear export signals. These proteins continually shuttle back and forth between the nucleus and the cytosol. The relative rates of their import and export determine the steady-state localization of such shuttling proteins: if the rate of import exceeds the rate of export, a protein will be located mainly in the nucleus; conversely, if the rate of export exceeds the rate of import, a protein will be located mainly in the cytosol. Thus, changing the rate of import, export, or both, can change the location of a protein.
Figure 12–13 How GTP hydrolysis by Ran in the cytosol provides directionality to nuclear transport.
movement through the npC of loaded nuclear transport receptors occurs along the fG-repeats displayed by certain npC proteins. The differential localization of ran-GTp in the nucleus and ran-GDp in the cytosol provides directionality (red arrows) to both nuclear import (a) and nuclear export (B). ran-Gap stimulates the hydrolysis of GTp to produce ran-GDp on the cytosolic side of the npC (see figure 12–12).
Some shuttling proteins move continuously into and out of the nucleus. In other cases, however, the transport is stringently controlled. As discussed in Chapter 7, cells control the activity of some transcription regulators by keeping them out of the nucleus until they are needed there (Figure 12–14). In many cases, cells control transport by regulating nuclear localization and export signals—turning them on or off, often by phosphorylation of amino acids close to the signal sequences (Figure 12–15).