Nucleoplasmin is a nuclear protein. This protein was divided into two segments and linked to the same large cytoplasmic protein, generating two fusion proteins. After injecting these fusion proteins into a cell, one of the proteins was found in the nucleus and the other in the cytoplasm. Which of the following conclusions can be drawn from these results?

a. The cytoplasmic protein contains a nuclear localization signal. b. Nucleoplasmin does not have a nuclear localization signal. c. One of the fusion proteins entered the nucleus by passive transport. d. Only one of the two fusion proteins possesses a nuclear localization signal.

d. Only one of the two fusion proteins possesses a nuclear localization signal.

Explanation:

A nuclear localization signal or nuclear localization sequence is the sequence present in the nuclear proteins. This sequence targets the nuclear proteins into the nucleus. The nuclear localization signals are recognized by specific importins that complex with the nuclear proteins. The nuclear protein-importin complex binds to the nuclear pore and is allowed to enter the nuclear via specific nuclear membrane proteins.

Importins are dissociated from the nuclear proteins in the nucleus and are transported back to the cytoplasm to serve in the transport of other nuclear proteins. A protein without the nuclear localization signal cannot enter the nucleus as importins would not complex with those proteins. The fusion protein that remained in the cytoplasm did not have the nuclear localization signal while the one that entered the nucleus contained it.