Nuclear import signals

Importins are transport proteins at the nuclear pore complex, needed for the selective import of proteins into the nucleus. They recognize nuclear localization signal sequences of cargo proteins. 

Sequence of amino acids that determine the transport of proteins into the nucleus. Although there is no clear consensus, nuclear localization signals tend to be rich in positively charged residues, which allow interaction with proteins from the nuclear import machinery (i.e., importins). 

Proteins similar to importins, referred to as ex-portins, are involved in export of many macromolecules from the nucleus. Cargo molecules for export carry nuclear export signals (NESs). Ran proteins are involved in this process also, and it is now established that the processes of import and export share a number of common feamres. 

Unfortunately, many other nuclei of importance in chemistry, such as and 0, have nuclear spin values of 0 and hence do not give nuclear resonance signals in a magnetic field. 

Hebert E. Improvement of exogenous DNA nuclear importation by nuclear localization signal-bearing vectors a promising way for non-viral gene therapy Biol Cell 2003 2 95(2) 59-68. 

The nuclear envelope is perforated with huge macromolecular assemblies of 30 different proteins that form nuclear pore complexes with a central channel of 25-30 nm in diameter. This channel allows proteins smaller than 30 kDa to passively traverse the outer and inner nuclear membranes. Larger proteins are actively transported across the nuclear envelope and contain nuclear localization signal (NLS) sequence motifs. These signals consist of one or two clusters of four or five basic residues localized usually within the polypeptide chain. The import of proteins with NLS through the channel is facilitated by the carrier heterodimer of importin-a ( > (Gorlich and Kutay 1999 Pemberton and Paschal... 

In most multicellular eukaryotes, the nuclear envelope breaks down at each cell division, and once division is completed and the nuclear envelope reestablished, the dispersed nuclear proteins must be reimported. To allow this repeated nuclear importation, the signal sequence that targets a protein to the nucleus—the nuclear localization sequence, NLS—is not removed after the protein arrives at its destination. An NLS, unlike other signal sequences, may be located almost anywhere along the primary sequence of the protein. NLSs can vary considerably, but many consist of four to eight amino acid residues and include several consecutive basic (Arg or Lys) residues. 

Jain AK, Bloom DA, Jaiswal AK. 2005. Nuclear import and export signals in control of Nrf2. J Biol Chem 280 29158-29168. 

Theodore M, Kawai Y, Yang J, Kleshchenko Y, Reddy SP, Villalta F, Arinze IJ. 2008. Multiple nuclear localization signals function in the nuclear import of the transcription factor Nrf2. J Biol Chem 283 8984—8994. 

Collas, P. and Alestrom, P. (1996) Nuclear localization signal of SV40 T antigen directs import of plasmid DNA into sea urchin male pronuclei in vitro. Mol. Reprod. Dev., 45, 431 —438. 

Michael, W.M., Eder, P.S. and Dreyfuss, G. (1997) The K nuclear shuttling domain a novel signal for nuclear import and nuclear export in the hnRNP K protein. EMBO J., 16, 3587-3598. 

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