Importin 13: a novel mediator of nuclear import and export

Importin beta -related receptors mediate translocation through nuclear pore complexes. Co-operation with the RanGTPase system allows them to bind and subsequently release their substrates on opposite sides of the nuclear enve lope, which in turn ensures a directed nucleocytoplasmic transport. Here we identify a novel family member from higher eukaryotes that functions prima rily, but not exclusively, in import. It accounts for nuclear accumulation of the SUMO-1/sentrin-conjugating enzyme hUBC9 and mediates import of the R BM8 (Y14) protein, and is therefore referred to as importin 13 (Imp13). Une xpectedly, Imp13 also shows export activity towards the translation initiat ion factor eIF1A and is thus a case where a single importin beta -like rece ptor transports different substrates in opposite directions. However, Imp13 operates differently from typical exportins in that the binding of eIF1A t o Imp13 is only regulated indirectly by RanGTP, and the cytoplasmic release of eIF1A from Imp13 is triggered by the loading of import substrates onto Imp 13.