The proper sorting of proteins to the cell nucleus has been shown to play a key role in the regulation of cell growth and development. When chemically coupled to the large fluorescent protein B-phycoerythrin, synthetic peptides derived from the amino acid sequence of the SV40 Large T antigen specifically target the protein conjugate to the nucleus. Transport of such conjugates across the nuclear envelope was demonstrated after microinjection into cultured cells or in an in vitro import assay using rat liver nuclei. Transport was time, temperature and energy dependent; only conjugates containing the localization sequence were properly transported. Monoclonal antibodies have been generated which bind specifically to this sequence. These antibodies bind to intact T antigen, and other nuclear proteins, suggesting that the sequence is normally exposed on the outer surface of molecules to be transported into the nucleus. A nuclear localization signal has also been identified in the primary structure of the human glucocorticoid receptor. A synthetic peptide of 10 residues derived from the sequence of this receptor are sufficient to direct transport of large molecules into the nucleus. Attempts are being made to interrupt nuclear targeting of the glucocorticoid receptor. The nuclear pore complex traverses the nuclear envelope and mediates uptake to the nucleus. The laboratory has demonstrated that proteins bearing cytoplasmically oriented, O-linked G1cNAc are components of the nuclear pore complex. The nuclear pore glycoproteins can be selectively labelled using the actin wheat germ agglutinin. This lectin reversibly blocks both import into the nucleus and the export of RNA from the nucleus to the cytoplasm. These findings raise the exciting possibility that cytoplasmic glycosylation may be involved in the assembly or functioning of the nuclear pore. Monoclonal antibodies have been raised against these nuclear pore glycoproteins thus allowing their purification. Sequence information from the major nuclear pore protein was obtained and this protein has been molecularly cloned. The structure and function of the nuclear pore protein is currently being probed using recombinant DNA technology.
