Question

Which of the following scenarios does NOT normally occur on a nuclear pore complex?

1) A protein complex is imported into the nucleus, with ONLY one of its subunits containing a nuclear localization signal (NLS).
2) In a single pore, an NLS-containing protein is imported, while at the same time a nuclear export signal (NES)-containing protein is exported.
3) A nuclear import receptor is exported from the nucleus through the pore.
4) A protein is imported on its own through the pore, without the need for a separate import receptor.
5) All of the above are possible scenarios and occur naturally.

Answer 1

The scenario where a protein is imported through a nuclear pore complex without the need for a separate import receptor is not typical. Normally, proteins with a nuclear localization signal require a nuclear import receptor for their transport into the nucleus.

Step-by-step explanation:

The scenario which does NOT normally occur on a nuclear pore complex is the fourth one, where a protein is imported on its own through the pore, without the need for a separate import receptor. Proteins that are imported into the nucleus typically require a nuclear localization signal (NLS), a sequence of amino acids such as -pro-pro-lys-lys-arg-lys-val-, which enables them to bind to a nuclear transport receptor. These receptors are essential for the active transport of proteins into the nucleus, facilitated by ATP hydrolysis as the proteins enter the nucleoplasm against a concentration gradient.

Scenarios such as a protein complex being imported where only one subunit has an NLS, simultaneous import and export of NLS- and NES-containing proteins through a single pore, and the export of a nuclear import receptor, all represent normal functions of the nuclear pore complex.