What is the primary function of a nuclear localization signal?

The primary function of a nuclear localization signal (NLS) is to direct a protein into the nucleus. This signal is a peptide sequence within the protein that is recognized by transport proteins known as nuclear transport receptors or importins. These importins facilitate the movement of the protein across the nuclear pore complex, allowing it to enter the nucleus where it can perform its functions, such as regulating gene expression or participating in DNA replication and repair.

The presence of an NLS is crucial for proteins that need to localize to the nucleus since the nuclear envelope acts as a barrier, separating the nuclear contents from the cytoplasm. Without this specific targeting mechanism, proteins would not be able to efficiently access the nucleus to carry out their roles, which is vital for multiple cellular processes.