Abstract
The import of proteins into the nucleus is mediated by nuclear localization sequences (NLSs), which are short polypeptide regions having a high content of basic amino acids (for reviews, see Roberts, 1989; Garcia-Bustos et al., 1991; Raikhel, 1992). NLSs are found at many locations within proteins but are not removed following translocation as are signals for chloroplastic (Keegstra, 1989), mitochondrial (Hartl and Neupert, 1990) and vacuolar (Chrispeels and Raikhel, 1992) import. The best studied NLSs are those that resemble the import signal of the simian virus 40 (SV40) large T-antigen (PKKKRKV; Kalderon et al., 1984; Lanford and Butel, 1984). Two additional classes of NLSs have also been identified. These are exemplified by the yeast mating type α-2 (Mat α-2) NLS (Hall et al., 1984), which possesses hydrophobic and basic amino acids (KIPIK), and bipartite NLSs which are composed of two short regions of basic amino acids separated by approximately ten residues (for review, see Dingwall and Laskey, 1991).
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