Nuclear Structure and Components

Nuclear Structure

• The nucleus of eukaryotic cells is enclosed by a double membrane, continuous with the rough endoplasmic reticulum (RER).
• The nuclear envelope displays a cytosolic face with ribosomes and a luminal face, connected to the RER lumen.
• The space between the inner and outer nuclear membranes is the perinuclear space.
• The nuclear envelope is perforated by nuclear pore complexes (NPCs), protein-lined channels.
• The nuclear envelope is reinforced by a network of intermediate filaments, composed of lamins (A, B, and C), forming the nuclear lamina.
• The nuclear lamina provides structural support and shape to the nucleus.

Nuclear Components

• The nucleoplasm, within the nucleus, houses genetic material (DNA) and the nucleolus.
• The nucleolus, a non-membrane-bound structure, is the site of ribosome biogenesis.
• DNA is associated with histones and non-histone proteins, forming chromatin.
• Heterochromatin, a condensed and tightly packed form of chromatin, is peripherally located.
• Euchromatin, a less condensed and dispersed chromatin form, is centrally situated.

Chromatin Structure

• DNA, combined with histones and non-histone proteins, forms chromatin.
• Euchromatin's open structure enables active transcription and gene expression.
• Heterochromatin's compact structure inhibits transcription and maintains inactivity.

Nuclear Transport

• The nuclear envelope's permeability allows small molecules and ions to pass through by passive diffusion.
• Active transport is required for large molecules like proteins and RNA to traverse the nuclear envelope through nuclear pores.
• Specialized proteins, importins and exportins, facilitate this active transport, which needs energy.

Importin-mediated Nuclear Import

• Importin proteins bind to nuclear localization signals (NLS) on proteins destined for the nucleus.
• Importin's alpha subunit recognizes the NLS.
• A complex forms with the cargo protein, importin, and the GTPase Ran, traversing the nuclear pore complex.
• Inside the nucleus, RanGTP causes importin release of the cargo protein.
• Importin-RanGTP complex returns to the cytoplasm, where RanGAP hydrolyzes GTP to GDP, releasing importin.
• The free importin is now ready to bind to another NLS-containing protein.

Exportin-mediated Nuclear Export

• Exportins bind to nuclear export signals (NES) on proteins destined for the cytoplasm.
• An exportin-cargo complex, aided by RanGTP, passes through the NPC.
• RanGTP is crucial as a molecular switch in the export process.
• Outside the nucleus, RanGAP converts RanGTP to RanGDP, releasing the cargo and exportin.
• The exportin-RanGDP complex returns to the nucleus for further transport.

Importance of Nuclear Transport

• Nuclear import and export ensure the proper delivery of proteins and RNAs to their cellular destinations.
• Accurate nuclear transport is vital for gene expression regulation, cell signaling, and controlling the cell cycle.

Key Takeaways

• The nucleus is a crucial and dynamic organelle in cell function, with the organization of chromatin and meticulous molecular transport through the nuclear envelope essential for the nucleus's integrity and for controlling cellular activities.