The Nuclear Envelope and traffic between the nucleus and cytoplasm.

Chapter 9, pages 355-370.

In the next few lectures we will turn our attention to the internal compartments of the cell and the processes by which proteins are delivered from their cytoplasmic site of synthesis to each compartment. The most prominent of the subcellular compartments is the nucleus. As the "Command Central" of the cell, where the cell's instructions reside in the DNA, the nucleus must be able to communicate with the rest of the cell, even as it is separated from the cytoplasmic components by membranes that enclose it. In the following section, we will consider how this is accomplished.

What separates the nucleus from the rest of the cell?
The nucleus is separated from the rest of the cell by the nuclear envelope.





What is the nuclear envelope made of?
The nuclear envelope is made up of two membranes (each of these has a regular phospholipid bilayer structure, like other membranes). The two membranes are separated by a space that is connected to the lumen (interior) of the endoplasmic reticulum.

See Figure 9.1. 




Are the inner and outer membrane connected to other cellular structures?
The outer membrane of the nuclear envelope is continuous with the endoplasmic reticulum. The inner membrane of the nuclear envelope has an underlying meshwork of filaments called the nuclear lamina.

See Figure 9.5 




What is the nuclear lamina made of?
The lamina is made of proteins called lamins.






What is the function of the nuclear lamina?
The lamina is thought to provide structural support to the nuclear envelope, and to provide a place for chromatin to attach during interphase.





Can molecules pass through the nuclear envelope?
Since the nuclear envelope is made up of two phospholipid bilayers, the only molecules that can easily cross the membranes are small, nonpolar ones.





So how do large, polar molecules like RNAs get out of the nucleus after transcription and processing and how do transcription factors that are made in the cytosol get into the nucleus to regulate transcription ?
Large molecules get across the nuclear envelope at special places called nuclear pore complexes.





What's a nuclear pore complex?
The nuclear pore complex is a channel through which molecules like RNA and protein can travel from nucleus to cytosol or the other way around. It is made up of a large number of proteins (50-100 different polypeptides).





How do molecules get through the nuclear pore complex?
Relatively small molecules can just diffuse through the nuclear pore complex, while larger ones are transported in a regulated manner. The nuclear pore channel can open and close (it has been compared to an iris diaphragm in a camera) to allow molecules through or to restrict their passage.

See Figure 9.6




What targets a protein to the nucleus?
Specific amino acid sequences in the protein target it to the nucleus. These amino acid sequences are called nuclear localization signals. These amino acid sequences are internal rather than being found at the N-terminus of the protein.





What do these amino acid sequences look like?
Most nuclear localization signals are short stretches of basic amino acids like lysine and arginine. Sometimes these basic amino acids are not in one long stretch, they are in two halves, separated by other amino acids. This is the case for the nuclear localization signal of nucleoplasmin, where a lysine-arginine pair is separated from four lysines by a ten amino acid stretch. Nuclear localization signals that are in two parts are called bipartite nuclear localization signals.

See Figure 9.9 




How do proteins with a nuclear localization signal get into the nucleus?
There are two steps in this process:
1. The nuclear localization signal on a protein is recognized by another protein called an importin. The importin binds to the nuclear localization signal and brings the protein to be transported to the nuclear pore. The importin, with its "cargo" moves through the nuclear pore into the nucleus.
2. In the nucleus, a protein called Ran, which has a GTP molecule bound to it, binds to the importin with its cargo. This makes the importin release the transported protein into the nucleus. The importin, still bound to Ran-GTP returns to the cytosol, where Ran-GTP is released from the importin and the GTP bound to Ran is hydrolyzed to GDP, In this way, the importin is brought back to the cytosol where it can pick up another protein destined for the nucleus. (Ran-GDP can be sent back to the nucleus by piggy-backing on to a protein called NTF2 or nuclear transport factor 2).

See Figure 9.11

Please note that the figure in the third edition of the text shows Ran-GDP/importin bound to a cargo protein in the cytosol (figure 8.9). This is not accurate. Refer to Figure 9.11 from the 4th edition (link above).



Are proteins sent out of the nucleus, too?
Yes, some proteins are exported out of the nucleus.





How is this done?
Exporting a protein from the nucleus is like importing in reverse.
Here's how it works.
Proteins to be exported have a nuclear export signal
These nuclear export signals are recognized by proteins called exportins.
Ran/GTP favors the formation of complexes of exportins and the proteins they are transporting (remember that Ran/GTP destabilized the complex between importin and the transported protein).
Once the protein-exportin-Ran/GTP complex is in the cytosol, GTP is hydrolysed to GDP, and the transported protein is released.

See Figure 9.12. 




How are RNAs transported from the nucleus?

Most RNAs go out of the nucleus, rather than coming into it. RNAs are generally transported as RNA-protein complexes, in which the proteins carry the nuclear export signals. Just as for proteins, alone, these complexes are recognized and bound by exportins and escorted out of the nucleus by Ran/GTP.
-Ribosomal RNAs are a special case, in that they are assembled with ribosomal proteins into ribosomal subunits before being exported from the nucleus.

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Copyright © 2009 Indira Rajagopal