Regulation of Nuclear Protein Sorting

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.

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.

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.

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.

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.

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.

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.