Nuclear proteins having a molecular mass more than approximately 40 KDa must be actively imported through nuclear pore complexes. What is the name given to the stretch of amino acid sequence (located in the protein that needs to be transported) that allows the selective transport of proteins into the nucleus? Based on what we learned in the class, name three important proteins that are required for the import of proteins into the nucleus, and briefly describe how they function.

Nuclear Localization Signal (NLS).

1. Importin

2. Nucleoporin

3. RanGTP

Explanation: The specific amino acid sequence motif that allows the selective transport of proteins having a molecular mass more than approximately 40 KDa into the nucleus is called Nuclear Localization Signal (NLS).

These proteins enter the nucleus through the Nuclear Pore Complex (NPC). The transit of the protein from the cytoplasm to the nucleus requires binding to a receptor called Karyopherin or Importin β, this receptor binds to the protein by direct recognition of the NLS sequence or through an adapter protein, Importin α (depending on the NLS sequence).

The passage of the complex through the NPC needs, in turn, the binding of the Importin to the Nucleoporins, proteins that form the nuclear pore.

Once the complex passes through the pore, the receptor-bound protein is released by the interaction of Importin with RanGTP, an enzyme with guanosine triphosphate (GTP) hydrolytic activity.