Once at the correct address, the protein enters the organelle. A sorting signal directs the protein to the organelle in which it is required. Nuclear localization signals are not removed from the proteins inside the nucleus. Nuclear localization signals are needed to repeatedly reimport nuclear proteins after they have been released into the cytosol during mitosis, when the nuclear envelope breaks down. Removing an ER signal sequence from an ER protein leaves it in the cytosol.
Proteins enter the nucleus through nuclear pores .
The nuclear envelope consists of an inner and an outer membrane. The inner face of the membrane is lined by the nuclear lamina. Transport through the nuclear pores occurs in both directions. The signal sequence consists of one or two short stretches of several positively charged lysines or arginines. Nuclear import receptors bind to the nuclear localization signal and direct the proteins to the nuclear pore.