Question

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

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

Answer 1

Option D, where a protein is imported on its own without the need for an import receptor, is the scenario that does not normally occur at a nuclear pore complex. Proteins generally require nuclear localization signals and associated receptors to be transported into the nucleus.

Step-by-step explanation:

The scenario that does NOT normally occur on a nuclear pore complex is:

D. A protein is imported on its own through the pore, without the need for a separate import receptor.

Proteins require a nuclear localization signal (NLS) to be recognized and transported into the nucleus. This sequence usually consists of positively charged amino acids such as lysine and proline. The NLS binds to nuclear transport receptors, usually importins, which facilitate the protein travel across the nuclear envelope and into the nucleus. This process is an illustration of active transport and is ATP-dependent, as it occurs against a concentration gradient.

Common scenarios in the nuclear pore complex include a protein complex being imported with one subunit containing an NLS, simultaneous import and export of NLS and NES-containing proteins through the same pore, and the export of nuclear import receptors post-import. These all rely on nuclear transport receptors to mediate the movement across the nuclear envelope.