Perhaps the most startling feature of the nuclear envelope are the very large, basket-like transport structures called the nuclear pores (figure 4). These structures have a molecular weight of 125 million daltons, making them thirty times larger than a ribosome. Composed of 100 to 200 different proteins collectively called nucleoporins, each nuclear pore pierces through both membranes of the nuclear envelope and probably opens into the interchromatin space of the nucleus. Some nucleoporins are structural components of the nuclear pore; others facilitate transport. Each mammalian cell nucleus contains 3,000 to 5,000 of these pores. The large number is needed to transport the tremendous quantity of proteins, enzymes, RNAs, factors, and complexes in and out of the nucleus to maintain its function and integrity. Small molecules, ions, and proteins up to 45,000 daltons passively diffuse through the pores. However, the vast majority of material transported is through a highly controlled process called "gating," which is responsible for keeping complexes such as the ribosomes in the cytoplasm from entering the nucleus.
Some proteins require multiple crossings through the nuclear pore. Ribosomal proteins are first made in the cytoplasm, transported into the nucleus, assembled into ribosome subunits by the nucleolus, and then transported back out into the cytoplasm. Viruses infect nuclei by taking advantage of the presence of nuclear pores. Some can be transported intact, while others "dock" on the cytoplasmic side of the pore and inject their DNA into the nucleus through the pore's opening. Each nuclear pore can both import and export material in one of two ways.
Any protein transported in or out of the nucleus must contain a nuclear localization signal, which is a specific sequence of four to eight amino acids that triggers either nuclear import or export. Each nuclear pore contains nucleoporins that recognize either the import or export signal, called importins or exportins, respectively. Importins, located on the cytoplasmic side of the nuclear pore, bind their import "cargo" and flip or slide it to the inside of the nucleus. They then move back into their original position, ready to "transport" their next "cargo." The opposite happens on the side of the nuclear pore facing the interior of the nucleus. Here, exportins bind proteins within the nucleus carrying the export signal and flip or slide them through the pore and into the cytoplasm. RNA molecules and complexes can also move through the pores, but only if the importins and or exportins recognize them as cargo.
Diane C. Rein
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