Protein import and export into and out of the cell nucleus requires a signal in the protein to be transported. Furthermore, several factors are involved in recognition of the signal, delivery of the protein to the nuclear pore complex (NPC) and its subsequent translocation across the NPC. We have biochemically analyzed the mechanism of nuclear protein import and export using all recombinant factors from Saccharomyces cerevisiae. Using solution binding assays, GTP hydrolysis and exchange assays, as well as surface plasmon resonance we have identified various interactions between different components of the transport machinery. We propose a model for the mechanism for nuclear protein import and export. Both nuclear protein import and export share certain features, i.e. docking of the transport cargo at the NPC via a karyopherin, release of the cargol karyopherin complex, and subsequent recycling of the transport factors for another round of function. However, although the proteins of the general transport machinery involved are the same for nuclear protein import and export, some transport factors play different roles in import and in export
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