Molecular mechanisms of protein translocation through nuclear pores

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Most karyophilic proteins accumulate rapidly into the nucleus through nuclear pores by selective mediated mechanisms. The selective nuclear import of karyophilic proteins is directed by short amino acid sequences termed nuclear localization signals (NLSs). The process of the mediated import of karyophilic proteins has been considered to involve at least two steps ; NLS-dependent binding to the cytoplasmic face of nuclear pores, followed by translocation through the nuclear pore complex.The digitonin-permeabilized cell-free transport system facilitates the identification of soluble factors involved in nuclear import. Using this in vitro system, we obtained the following data.1.We found that a karyophilic protein forms a stable complex with cytoplasmic factors to target nuclear pores in active nuclear transport process. We termed this as nuclear pore-targeting complex (PTAC).2.We identified two essential components of PTAC and isolated their cDNA clones. Based on their calculated molecular masses, we named them as PTAC58 and PTAC97.3.PTAC58 was found to have specific NLS-binding activity. A portion of cytoplasmically injected antibodies against PTAC58 migrated rapidly into the nucleus, indicating dynamic movement of this protein across the nuclear envelope.4.It was shown that PTAC97 is associated with PTAC58 in a 1 : 1 molar ratio.5.A complex of PTAC58 and PTAC97 targets nuclear pores, depending on the presence of a karyophile.These results indicate that the first step in nuclear import occurs through the targeting-complex formation of a karyophile with PTAC58 bound to PTAC97.