Answer:
Nuclear Localization Signal
Step-by-step explanation:
If a protein that is assembled in the cytoplasm has to function or be localized to the nucleus, the type of additions to the carboxyterminal end or accessories for mitosis will serve no purpose if the protein cannot get in. For this reason, proteins that are made to be moved inside the nucleus must have a protein signal to enter.
The nucleus is the "brain" of the cell and it must control what leaves and enters. A protein signal allows the protein to enter inside the nuclear pore. Since the nucleus is restrictive on the components that are allowed to enter or not, this signal is critical.