Which of the following results provided evidence of a discrete nuclear localization signal somewhere on the nucleoplasmin protein?
a. The nucleoplasmin protein was small and easily slipped through the nuclear pore complex.
b. After cleavage of the nucleoplasmin protein, only the tail segments appeared in the nucleus.
c. Removing the tail from the nucleoplasmin protein allowed the core segment to enter the nucleus.
d. The SRP bound only to the tail of the nucleoplasmin protein, not the core segment.

Verified step by step guidance

1. Understand the concept of a nuclear localization signal (NLS): A nuclear localization signal is a sequence of amino acids that 'tags' a protein for import into the cell nucleus by nuclear transport. Proteins with an NLS can be recognized by nuclear transport receptors and transported through the nuclear pore complex.
2. Analyze the options provided: Each option describes a different experimental observation related to the nucleoplasmin protein. The goal is to identify which observation suggests the presence of a discrete NLS on the protein.
3. Consider option b: 'After cleavage of the nucleoplasmin protein, only the tail segments appeared in the nucleus.' This suggests that the tail segments contain the NLS, as they are the only parts of the protein that enter the nucleus after cleavage.
4. Evaluate the significance of option b: The fact that only the tail segments appear in the nucleus after cleavage indicates that these segments have the necessary signal for nuclear entry, providing evidence of a discrete NLS located on the tail.
5. Conclude the analysis: Based on the evidence provided by option b, the presence of a discrete nuclear localization signal on the nucleoplasmin protein is supported by the observation that the tail segments, which contain the NLS, are able to enter the nucleus.

Key Concepts

Nuclear Localization Signal (NLS)

A Nuclear Localization Signal (NLS) is a sequence of amino acids that 'tags' a protein for import into the cell nucleus by nuclear transport. Proteins with an NLS are recognized by nuclear transport receptors, which facilitate their passage through the nuclear pore complex. Understanding NLS is crucial for identifying how proteins are selectively transported into the nucleus.

Nucleoplasmin Protein Structure

Nucleoplasmin is a pentameric protein involved in chromatin assembly and is known for its role in nuclear transport. It consists of distinct segments, typically a core and a tail, each potentially having different functions or signals. The structure of nucleoplasmin is essential for understanding how its segments contribute to its localization and function within the cell.

Nuclear Pore Complex (NPC)

The Nuclear Pore Complex (NPC) is a large protein complex that spans the nuclear envelope, acting as a gateway for the transport of molecules between the nucleus and the cytoplasm. It allows the passage of small molecules by passive diffusion, while larger molecules require active transport facilitated by signals like the NLS. Understanding NPC function is key to comprehending how proteins like nucleoplasmin are transported into the nucleus.