Question 1

How does phosphorylation mask nuclear localization signals on cargo proteins?

Accepted Answer

Phosphorylation causes conformational changes in cargo proteins, preventing them from fitting into the binding site of nuclear receptors. This mechanism, observed in the viral transcriptional factor v-Jun, effectively masks the nuclear localization signal and retains the cargo in the cytosol, blocking its transport to the nucleus.

Question 2

What role do inhibitory proteins play in regulating nuclear protein import?

Accepted Answer

Inhibitory proteins bind directly to nuclear localization signals on cargo, interfering with nuclear receptor recognition. For example, I-kappaB binds to NF-kappaB's NLS, preventing its nuclear import. This mechanism allows cells to regulate gene expression by controlling when transcription factors can enter the nucleus.

Question 3

How do cytoskeletal proteins control the size of molecules entering the nucleus?

Accepted Answer

Cytoskeletal proteins modulate nuclear pore complex diameter by constricting or dilating the pore opening. As a tight aqueous pore, the NPC expands and contracts to allow selective molecules to squeeze through, regulating which macromolecules can enter the nucleus based on their size.

Question 4

What is the role of SREBP in cholesterol-regulated nuclear transport?

Accepted Answer

SREBP is a precursor transcription factor bound to SCAP in the ER membrane. At low cholesterol, SCAP changes conformation, transporting the complex to the Golgi where proteases cleave SREBP's cytosolic domain. The active transcription factor then enters the nucleus to activate sterol regulatory genes.

Question 5

How does cargo retention prevent unnecessary nuclear transport?

Accepted Answer

Cargo proteins can bind cytosolic factors like 14-3-3 protein or be tagged for degradation, inhibiting their transport across the nuclear envelope. This retention mechanism prevents unnecessary movement of proteins between the nucleus and cytoplasm, allowing cells to control which proteins access the nuclear environment.

Question 6

What determines whether a protein is transported as an active or inactive precursor?

Accepted Answer

Some proteins like p105 are secreted as inactive precursors in the cytoplasm. Phosphorylation signals their cleavage to release active forms, such as P50, which can then be transported to the nucleus. This mechanism allows cells to regulate gene expression by controlling when precursor activation occurs.

Question 7

How does modifying nuclear receptor affinity enhance protein import to the nucleus?

Accepted Answer

Phosphorylation of importin alpha by casein kinase II enhances its binding affinity for importin beta, promoting nuclear import of cargo. Alternatively, increased importin beta levels outcompete other nuclear receptors for binding sites on nuclear pore complexes, accelerating substrate trafficking into the nucleus.

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