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. Step 1: Understand the question. The question is asking which of the given options provides evidence that there is a discrete nuclear localization signal (NLS) on the nucleoplasmin protein. The NLS is a specific sequence of amino acids that directs a protein to the nucleus.
2. Step 2: Review each option carefully. Option a suggests that the nucleoplasmin protein is small and can easily pass through the nuclear pore complex. This doesn't provide evidence of a NLS because size alone doesn't determine whether a protein can enter the nucleus.
3. Step 3: Consider option b. This option states that 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 that enter the nucleus.
4. Step 4: Look at option c. This option suggests that removing the tail from the nucleoplasmin protein allows the core segment to enter the nucleus. This doesn't provide evidence of a NLS because it suggests that the tail was preventing the core from entering the nucleus, not that the core has a NLS.
5. Step 5: Finally, consider option d. This option states that the SRP (signal recognition particle) only binds to the tail of the nucleoplasmin protein, not the core segment. This doesn't provide evidence of a NLS because the SRP is involved in targeting proteins to the endoplasmic reticulum, not the nucleus. Therefore, the correct answer is option b.

Key Concepts

Nuclear Localization Signal (NLS)

A Nuclear Localization Signal (NLS) is a specific amino acid sequence that enables proteins to be recognized and transported into the nucleus by the nuclear transport machinery. This signal is essential for proteins that function within the nucleus, as it directs their movement through the nuclear pore complex, which selectively allows molecules to enter the nucleus.

Nuclear Pore Complex (NPC)

The Nuclear Pore Complex (NPC) is a large protein structure embedded in the nuclear envelope that regulates the transport of molecules between the nucleus and the cytoplasm. It allows small molecules to pass freely while requiring specific signals, like the NLS, for larger proteins to be actively transported into the nucleus, ensuring proper cellular function and regulation.

Protein Segmentation and Function

Proteins can have distinct functional regions or segments, often referred to as domains. In the case of nucleoplasmin, the core segment may perform a different function than the tail segment. Understanding how these segments interact with cellular components, such as the nuclear pore complex or signal recognition particles (SRP), is crucial for determining their localization and role within the cell.