What is a NUCLEAR LOCALIZATION SIGNAL?

The nuclear pore complex mediates transportation of macromolecules larger than approximately 50 kDa into the nucleus.  Not all proteins larger than this size can enter the nucleus, though.  Therefore, some proteins must contain a specific signal that allows them to be recognized and transported into the nucleus.

How does a protein get through the nuclear pore complex:

    1.  Proteins could interact directly with the NPC and then be transported to the 

         nucleus without the assistance of other factors

    2.  Proteins could interact with another protein(s), and these proteins would interact with the NPC to  

         mediate transport

It has been shown that cytosolic proteins are necessary for the transport of proteins into the nucleus (I don't know if this is true in ALL cases?).  The nuclear localization signal is the specific protein sequence that is both necessary and sufficient for these interactions to occur, allowing transport of the protein. 

(Macara 2001)

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