Nuclear pore complexes (NPC) are protein pores found in the nuclear membrane, allowing the transport of mRNA and small proteins out of the nucleus into the cytoplasm. This allows transcription to occur during protein synthesis. NPCs allow selectivity in nuclear export and import. Pre-mRNA cannot be transported out of the nucleus, until it has been modified by the spliceosome. This is done by depositing protein onto the mRNA to be exported, after it has undergone splicing, enabling it to pass through the pore.
Nuclear import and export requires a number of factors, which have been identified through fractionation of cytosolic extracts. Ran, karyopherins, and GTP/GDP are all important components in allowing nuclear transport through NPCs (and there are likely others that are yet to be identified). Karyopherins (importins, exportins) bind directly to the cargo molecule on the nuclear localisation sequence or nuclear export signal, allowing it to pass through the NPC.