Nuclear Import

Within the cell, the movement of proteins is very regulated. Each protein must have specific signal sequences in order to get sent to where they are supposed to go. Proteins that have a nucleus signal sequence will get imported in from the cytoplasm, and the signal sequence is not cleaved off once the protein enters the nucleus. Nuclear import is a process that involves the importin enzyme and the protein also folds as it enters the nucleus. First, importin binds to the cargo protein and moves through the nuclear protein channel into the nucleus. Once inside, GEF substitues GDP for GTP in the GTPase complex which binds to the cargo protein and importin, stimulating the release of the cargo protein. The rest of the complex then exits the cell through a separate channel. Once back in the cytoplasm, GAP substitutes GTP for GDP which initiates the release of the GTPase from importin. The cycle can then start again.