Bidirectional traffic occurs continuously between the cytosol and the nucleus. The many proteins that function in the nucleus—including histones, DNA polymerases, RNA polymerases, transcriptional regulators, and RNA-processing proteins—are selectively imported into the nuclear compartment from the cytosol, where they are made. At the same time, almost all RNAs—including mRNAs, rRNAs, tRNAs, miRNAs, and snRNAs—are synthesized in the nuclear compartment and then exported to the cytosol. Like the import process, the export process is selective; mRNAs, for example, are exported only after they have been properly modified by RNA-processing reactions in the nucleus. In some cases, the transport process is complex. Ribosomal proteins, for instance, are made in the cytosol and imported into the nucleus, where they assemble with newly made ribosomal RNA into particles. The particles are then exported to the cytosol, where they assemble into ribosomes. Each of these steps requires selective transport across the nuclear envelope.
nuclear pore Complexes perforate the nuclear envelope
Large and elaborate nuclear pore complexes (NPCs) perforate the nuclear envelope in all eukaryotes. Each NPC is composed of a set of approximately 30 different proteins, or nucleoporins. Reflecting the high degree of internal symmetry, each nucleoporin is present in multiple copies, resulting in 500–1000 protein molecules in the fully assembled NPC, with an estimated mass of 66 million daltons in yeast and 125 million daltons in vertebrates (Figure 12–8). Most nucleoporins are composed of repetitive protein domains of only a few different types, which have evolved through extensive gene duplication. Some of the scaffold nucleoporins (see Figure 12–8) are structurally related to vesicle coat protein complexes, such as clathrin and COPII coatomer (discussed in Chapter 13), which shape transport vesicles; and one protein is used as a common building block in both NPCs and vesicle coats. These similarities suggest a common evolutionary origin for NPCs and vesicle coats: they may derive from an early membrane-bending protein module that helped shape the elaborate membrane systems of eukaryotic cells, and in present-day cells stabilize the sharp membrane bends required to form a nuclear pore.
The nuclear envelope of a typical mammalian cell contains 3000–4000 NPCs, although that number varies widely, from a few hundred in glial cells to almost 20,000 in Purkinje neurons. The total traffic that passes through each NPC is enormous: each NPC can transport up to 1000 macromolecules per second and can transport in both directions at the same time. How it coordinates the bidirectional flow of macromolecules to avoid congestion and head-on collisions is not known.
Each NPC contains aqueous passages, through which small water-soluble molecules can diffuse passively. Researchers have determined the effective size of these passages by injecting labeled water-soluble molecules of different sizes into the cytosol and then measuring their rate of diffusion into the nucleus. Small molecules (5000 daltons or less) diffuse in so fast that we can consider the nuclear envelope freely permeable to them. Large proteins, however, diffuse in much more slowly, and the larger a protein, the more slowly it passes through the NPC. Proteins larger than 60,000 daltons cannot enter by passive diffusion. This size cut-off to free diffusion is thought to result from the NPC structure (see Figure 12–8). The channel nucleoporins with extensive unstructured regions form a disordered tangle (much like a kelp bed in the ocean) that restricts the diffusion of large macromolecules while allowing smaller molecules to pass.