In eukaryotes, the nuclear membrane provides a physical barrier to the
passive diffusion of macromolecules from and into the cytoplasm. Nucleocytoplasmic
traffic occurs through highly specialized structures known as nuclear pores, and
involves the participation of a special class of transport proteins. Active transport across
the nuclear pores is an energy-dependent process that relies on the activity of Ran-
GTPases both in the nuclear and cytoplasmic compartments.
Nuclear import of proteins is an essential step in regulating gene expression and the
replication cycle of several viruses. In this review, the key mechanisms, pathways, and
models underlying the transport of proteins across nuclear pores are analysed.
Keywords: Nuclear pore complex, nucleoporin, nuclear localization signal,
protein import, importin, karyopherin.
