What is required for proteins to be imported into the nucleus?

Proteins that need to enter the nucleus must contain a nuclear localization signal (NLS). This signal is a specific sequence of amino acids that is recognized by importins, a type of transport protein. The importins facilitate the transport of the cargo protein through the nuclear pore complexes that span the nuclear envelope. Once inside, the NLS ensures that the protein is correctly targeted to the nuclear compartment, allowing it to perform its functions, such as DNA binding or regulatory roles in gene expression.

In contrast, the other options are not involved in the import of proteins into the nucleus. A nuclear export signal (NES) is needed for proteins exiting the nucleus, whereas a signal recognition particle (SRP) aids in the targeting of proteins to the endoplasmic reticulum in the secretory pathway. Translation initiation factors are involved in the formation of the ribosome complex to begin protein synthesis, and do not play a role in nuclear import.