Nucleo-Cytoplasmic Exchange

ID #2434

In lecture 25, we talked about how an mRNA molecule can get out of the nucleus. We discussed that it attaches to a protein that displays not only an NLS but an NES so that it can get into and out of the nucleus. After getting the mRNA molecule into the cytoplasm, why does the protein not just stay attached to the exportin that is already going back into the nucleus as opposed to detaching and reattaching to an importin?

That way would certainly be efficient, but cells don't always evolve to do things in the most efficient way. Sometimes (as we'll see for the final) this is because it provides a way to regulate the overall process. 
In addition, exportin and a mRNA export factor going back into the nucleus as one unit would make it more difficult to recycle the exportins. Remember, while exportins are in the cytoplasm, they use GTP to recycle back into a conformation that can bind another protein and transport it out of the nucleus. 
Basically, if what you describe was the case, it would be difficult for the exportin to release the mRNA export factor within the nucleus (once the pair returned) and then bind another mRNA export factor + mRNA. For that matter, it would make it more difficult for the exportins to bind any other proteins in the nucleus that need to be exported. 

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