The regulation of protein transport to the nucleus by phosphorylation.

Since the identification over 10 years ago of the sequence responsible for the nuclear localization of the simian virus 40 (SV40) large tumour antigen (T-ag) and the demonstration of its ability to target heterologous, normally non-nuclear, proteins to the nucleus, research in the field of nuclear transport has largely revolved around the idea of nuclear localization signals (NLSs) being exclusively responsible for protein targeting to the nucleus. NLSs have been defined for a variety of nuclear proteins, and specific NLS receptors or NLS binding proteins (NLSBPs) have been identified and purported to play an essential role in the transport process. It has become increasingly clear over the last few years, however, that NLS function can be regulated, with phosphorylation as the main mechanism controlling NLS-dependent nuclear localization of a number of proteins, including transcription factors (TFs) such as nuclear factor (NF) KB, c-rel, dorsal, the a-interferon-stimulated gene factor 3 (ISGF-3), the yinterferon activated factor (GAF), the nuclear factor of activated T-cells (NF-AT) and the yeast TF SWI5. Nuclear translocation of various TFs and oncogene products accompanies changes in the differentiation or metabolic state ofeukaryotic cells precisely, indicating that nuclear protein import is a key control point in the regulation of gene expression. This review seeks to expound the considerable evidence presently available for the regulated nuclear localization of proteins. This will include analysis showing that even the nuclear localization of the archetypal NLS-containing T-ag is subject to regulation, whereby phosphorylation sites for casein kinase II (CKII) and the cyclin-dependent kinase (cdk) cdc2 in the vicinity of the NLS determine both the rate and the absolute amount of T-ag that is maximally accumulated in the nucleus in an NLSdependent fashion. The regulatory module for regulated T-ag transport, i.e. the CcN motif, comprising the phosphorylation sites together with the NLS, has been identified in a variety of other nuclear-localized proteins. It appears to be a special type of phosphorylation-regulated NLS (prNLS), where kinases can specifically regulate the nuclear import of particular proteins through phosphorylation at site(s) close to their respective NLSs. The review will also describe the mechanisms by which phosphorylation regulates nuclear protein import, including cytoplasmic retention factors, intraT and inter-molecular NLS masking and direct NLS masking by phosphorylation.