Very small gold beads are injected into a cell, then an hour is allowed to pass.

Question

Very small gold beads are injected into a cell, then an hour is allowed to pass. Under examination, the gold beads are found in both the cytoplasm and the nucleus. In a second experiment where larger gold beads are injected, they are only found in the cytoplasm. In a third experiment, NLS tags are added to the large gold beads, and after an hour beads are seen in both the cytoplasm and nucleus. From this experiment one could conclude that

a) the nuclear localization sequence is lysine rich

b) the nuclear localization mechanism is nonspecific as long as the tag is present

c) the gold beads have an inherent import signal

d) nuclear import mainly relies on diffusion to get molecules into the nucleus

Explanation / Answer

Diffusion of macromolecules into nucleus mainy occurs through passive diffusion. The transport molecules through nuclear pore complex (NPC) depends on the diameter of particles. Molecules larger than 25 - 40nm diameter are transported to NPC and transport of such molecules require a nuclear localization signal (NLS). The NLS tagged molecules are transported to karyoplasm. Above experiment model is designed to show the entry of hepatitis B virus (HBV) into nucleus. The virus particle contains large diameter. Gold particles without NLS could enter only cytoplasm but gold particles of large size tagged to NLS could enter karyoplasm also. From this observation, it can be concluded that gold beads have an inherent import signal that binds with cargo protein receptor to enter intop nucleus. Option c is correct.

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