The nuclear pore-targeting complex binds to nuclear pores after association with a karyophile.

We recently showed that a karyophilic protein forms a stable complex, termed nuclear pore-targeting complex (PTAC), with cytoplasmic components prior to nuclear pore-binding. In this study, we cloned a cDNA encoding a 97 kDa of PTAC (PTAC97). Recombinant PTAC97 completely reconstitutes the nuclear binding-step in conjunction with a 58 kDa component of PTAC (PTAC58) in the semi-intact cell-free transport assay. Biochemical analysis reveals that PTAC58 binds to a karyophilic protein, and PTAC97 is associated with PTAC58 in a 1:1 molar ratio. A complex of PTAC97 and PTAC58 targets nuclear pores, depending on the presence of a karyophile. These in vitro results suggest that the first step in nuclear import occurs through the targeting-complex formation of a karyophile with PTAC58 bound to PTAC97.