Section 7

Regulated transport of transcription factors and protein kinases This mechanism allows signals received at the cell surface to be transmitted to the nucleus.

By phosphorylation at a serine residue adjacent to its nuclear localization signal Adequate phosphate levels keep Pho4 in the cytoplasm, while low phosphate levels allow its nuclear import.

It forms an inactive complex with an inhibitory protein (IκB) in the cytoplasm This prevents NF-κB from being transported to the nucleus.

Mediates the export of tRNAs and miRNAs from the nucleus to the cytoplasm Specific exportins are responsible for this transport.

By a distinct mechanism independent of Ran and karyopherins This involves a set of proteins known as the exporter complex.

Remodels mRNA folding, causing dissociation of the exporter complex This releases the mRNA into the cytoplasm.

Regulate the recycling of importins and Ran Importins are recycled after each round of import, and Ran is regenerated by Ran GEF.

Approximately 22 nucleotides They play a role in RNA interference and gene expression regulation.

Blocks transcription of the inactive X chromosome This is part of the dosage compensation mechanism in female cells.

• SINEs • LINEs These elements move to new genomic sites by reverse transcriptase.

Nucleosome A nucleosome consists of DNA wrapped around a histone core.

• 147 base pairs of DNA • Two molecules each of H2A, H2B, H3, and H4 histones • One molecule of H1 This structure is fundamental to chromatin organization.

• 40S: Small subunit • 60S: Large subunit These subunits are assembled from pre-rRNA and ribosomal proteins.

Regulates cell cycle progression and apoptosis p53 is crucial for responding to DNA damage.

TRUE Loss of p53 leads to increased mutation frequencies and genomic instability.

• Regulates cell survival • Induces apoptosis via proapoptotic Bcl-2 family members (PUMA, Noxa) • Responds to DNA damage p53 is crucial for eliminating cells with potentially deleterious mutations, such as those that could develop into cancer.

FALSE Cells without p53 fail to undergo apoptosis when exposed to DNA-damaging agents, contributing to tumor resistance to chemotherapy.

Target proteins for export from the nucleus These signals are often rich in hydrophobic amino acids, particularly leucine.

• Importins • Exportins Together, they are known as karyopherins and facilitate the transport of proteins across the nuclear envelope.

• Promotes stable complexes with exportins and cargo proteins • Hydrolyzed to Ran/GDP in the cytoplasm This process facilitates the release of cargo proteins into the cytoplasm.

• Small polar molecules • Ions • Macromolecules (proteins and RNAs) The nuclear pore complex is essential for regulating the traffic of these molecules between the nucleus and cytoplasm.

By passive diffusion Molecules less than ~40 kDa can diffuse freely in either direction.

• Composed of about 30 different proteins (nucleoporins) • Contains a central channel with eightfold symmetry • Anchored within the nuclear envelope The nuclear pore complex plays a fundamental role in eukaryotic cell physiology.

• Proteins with FG repeats • Form selective barriers for macromolecule movement FG-NUPs are crucial for controlling the transport of proteins and RNAs across the nuclear envelope.