Nuclear import

Proteins that will be sent to the nucleus must have a nuclear localization sequence, found anywhere in the primary structure, to go there. Importin binds to the NLS on the cargo protein, and enters the nucleus through the nuclear pore complex. Once inside Ran-GEF switches out a GDP molecule for GTP, and this complex binds to importin, stimulating the release of the cargo protein inside the nucleus. The importin-Ran complex then leaves the cell, and GAP helps hdrolyze the GTP back into GDP. This stimulates teh release of importin so the cycle can then start over with a new cargo protein. The protein that entered the nucleus cannot now leave the nucleus unless it contains a nuclear export signal as well.