Does a nuclear localizing signal always assure protein transport into the nucleus in transfected cells?

A decade ago Lanford and Butel demonstrated that proteins required a specific amino acid sequence in order to be routed from the cell cytosome to the nucleus. Using recombinant DNA and gene transfection techniques, Kalderon et al and Lanford and colleagues found that the addition of a specific amino acid sequence, the nuclear localizing signal (NLS), to a protein normally secreted or localized within the cytosome resulted in its transport from its cytosomal site of synthesis to the nucleus. In such studies, immunofluorescence microscopy is routinely used to evaluate the intracellular localization of the protein encoded by the transgene in intact transfected cells. The nucleus is brightly fluorescent in cells transfected with genes for a protein with an associated NLS. In contrast, proteins without an NLS remain in the cytosome. Generally, in transfected cells, the nuclear association of the fluorescent-labeled antibody directed against a protein of interest is interpreted to mean that this protein is localized within the nucleus.