Nuclear localization signal
A nuclear localizing sequence (NLS) is an amino acid sequence which acts like a 'tag' on the exposed surface of a protein. This sequence is used to target the protein to the cell nucleus through the Nuclear Pore Complex and to direct a newly synthesized protein into the nucleus via its recognition by cytosolic nuclear transport receptors. Typically, this signal consists of a few short sequences of positively charged lysines or arginines. Different nuclear localized proteins may share the same NLS. An NLS has the opposite function of a nuclear export signal, which targets proteins out of the nucleus.
Types of nuclear localization signals
The first NLS to be discovered is the sequence PKKKRKV in SV40 large T antigen  . The NLS of nucleoplasmin, KR[PAATKKAGQA]KKKK, is the prototype of ubiquitous bipartite signal: two clusters of basic amino acids, separated by a spacer of about 10 amino acids . Both signals are recognized by importin α. Importin α contains a bipartite NLS itself, which is specifically recognized by importin β. The latter can be considered the actual import mediator.
Chelsky et al. proposed the consensus sequence K-K/R-X-K/R for monopartite NLSs . A Chelsky sequence may, therefore, be part of the downstream basic cluster of a bipartite NLS.
There are many other types of NLS, such as the acidic M9 domain of hnRNP A1, the sequence KIPIK in yeast transcription repressor Matα2, and the complex signals of U snRNPs. Most of these NLSs appear to be recognized directly by specific receptors of the importin β family without the intervention of an importin α-like protein .
Mechanism of nuclear import
Proteins gain entry into the nucleus through the nuclear envelope. The nuclear envelope consist of concentric membranes, the outer and the inner membrane. These are the gateways to the nucleus. The envelope consist of pores or large nuclear complexes.
A protein translated with a NLS will bind strongly to importin (aka karyopherin), and together, the complex will move through the nuclear pore. At this point, Ran-GTP will bind to the importin-protein complex, and its binding will cause the importin to lose affinity for the protein. The protein is released, and now the Ran-GTP/importin complex will move back out of the nucleus through the nuclear pore. A GTPase activating protein (GAP) in the cytoplasm hydrolyzes the Ran-GTP to GDP, and this causes a conformational change in Ran, ultimately reducing its affinity for importin. Importin is released and Ran-GDP is recycled back to the nucleus where guanine exchange factor (GEF) exchanges its GDP back for GTP.
Exporting proteins out of the nucleus is programmed by a nuclear export signal.
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Gorlich, D. (1997). Nuclear protein import. Current Opinion in Cell Biology, 9(3), 412-419.
C. Patrick Lusk, Gunter Blobel, and Megan C. King (2007). Highway to the inner nuclear membrane: rules for the road. Nature Reviews Molecular Cell Biology 8, 414-420 (May 2007).