In general, a protein has to display a distinct signal for it to be transported through the nuclear pore. Proteins with a nuclear localization signal are transported in, while proteins with a nuclear export signal are transported out. Mobile transporter proteins, usually mediating either export or import, recognize the appropriate targeting sequence and then interact with structural elements of the nuclear pore. This causes the pore to expand, allowing the active transport of the transporter-protein complex. The precise mechanism by which large molecules move through the nuclear pore is not yet fully understood. However, the discovery of the role played by the GTPase Ran in nuclear transport has given valuable insight into outer ring
Spokes outer ring
ribosome nucleolus ribosome nucleolus
rough endoplasmic reticulum cytosol
euchromatin heterochromatin nuclear pore inner nuclear membrane intermembrane space outer nuclear membrane nuclear lamina rough endoplasmic reticulum cytosol
Figure 10.2. The nuclear pore.
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