How are proteins targeted to the nucleus?

Proteins are targeted to the nucleus primarily through the use of nuclear localization signals (NLS), which are specific amino acid sequences that signal the cellular machinery to transport the protein into the nucleus. These NLS are recognized by importin proteins, which bind to the protein carrying the NLS and facilitate its passage through the nuclear pore complexes that span the nuclear membrane. The importin-protein complex then undergoes a series of interactions that allow it to translocate into the nucleus, where the importin is released and the protein can perform its nuclear functions.

This targeting mechanism is crucial for cellular regulation, as it ensures that only those proteins with the appropriate signals can gain access to the nucleus, allowing for controlled gene expression, replication, and repair processes. The specificity of importins for NLS allows for the selective transport of a variety of nuclear proteins, encompassing transcription factors, histones, and other essential nuclear components.

In contrast, the other options do not accurately describe the process of nuclear targeting or imply mechanisms that would lack the precision necessary for cellular function.