How proteins are transported from cytoplasm to the nucleus

Nuclear proteins are transported actively through nuclear pores by a selective, mediated process. The process is mediated by a nuclear localization signal (NLS), and can be divided into at least two steps, (a) targeting to the pores and (b) translocation through the pores. The first step involves the formation of a stable complex containing a nuclear protein, termed nuclear pore-targeting complex (PTAC), in the cytoplasm. The second step, translocation, requires at least two soluble factors, a small GTPase Ran and its interacting protein p10/nuclear transport factor 2 (NTF2), along with nuclear pore complex components. These findings have been generally obtained by using the NLS of SV40 large T-antigen, and data concerning the detailed mechanism are now accumulating. Transport pathways other than for the SV40 T-antigen, for example, extracellular signal-dependent nuclear protein import pathway, have also recently been studied. Considering all these observations, one should be able to attain an understanding of the mechanism of intracellular information transduction between cytoplasm and the nucleus.