Which proteins bind to nuclear localization signals (NLS) on newly synthesized proteins to facilitate their transport into the nucleus?

• A. Ubiquitin ligases
• B. Exportins
• C. Chaperonins
• D. Importins

Answer: (D)

Verified step by step guidance

1. Understand the role of nuclear localization signals (NLS): NLS are specific amino acid sequences on proteins that act as 'zip codes,' directing the protein to the nucleus. These signals are recognized by transport proteins.
2. Learn about the function of importins: Importins are a type of transport protein that specifically bind to NLS on newly synthesized proteins. They facilitate the transport of these proteins through the nuclear pore complex into the nucleus.
3. Differentiate importins from other proteins: Ubiquitin ligases are involved in tagging proteins for degradation, exportins are responsible for transporting molecules out of the nucleus, and chaperonins assist in protein folding. None of these directly bind to NLS for nuclear import.
4. Understand the transport process: Once importins bind to the NLS on a protein, they guide the protein through the nuclear pore complex. This process is energy-dependent and often involves interactions with Ran, a small GTPase.
5. Conclude that importins are the correct answer: Based on their specific role in recognizing NLS and facilitating nuclear import, importins are the proteins responsible for this process.