The Transport of Molecules between the Nucleus and the Cytosol

The nuclear envelope encloses DNA and defines the nuclear compartment.
- Consists of two concentric membranes penetrated by nuclear pore complexes.
- Inner and outer nuclear membranes maintain distinct protein compositions despite being continuous.

Contains specific proteins that act as binding sites for chromatin and the protein meshwork of the nuclear lamina.
Provides structural support for the inner membrane.

Surrounded by the inner nuclear membrane, continuous with the endoplasmic reticulum (ER).
Studded with ribosomes engaged in protein synthesis.
Proteins produced are transported into the perinuclear space, which is contiguous with the ER lumen.

Bidirectional Traffic: Continuous movement between the cytosol and the nucleus.
- Proteins functioning in the nucleus, such as histones, DNA, and RNA polymerases, are imported from the cytosol.
- tRNAs and mRNAs are synthesized in the nucleus and exported to the cytosol.
Nuclear Pore Complexes: Large structures penetrating the nuclear envelope that facilitate transport.
- Composed of approximately 50 different proteins, termed nucleoporins.
- Molecular mass of each complex is approximately 125 million daltons.
- Active nuclei contain more pore complexes, generally 3000-4000 per envelope in mammalian cells.

During DNA synthesis, approximately 10^6 histone molecules must be imported every 3 minutes.
Average transport rate is about 100 histone molecules per minute through each pore complex.
Heavy ribosomal subunits transport roughly 6 subunits per minute from the nucleus to cytosol.

Each pore has aqueous channels for small water-soluble molecule diffusion.
- Molecules 5000 daltons or less diffuse rapidly.
- 17,000 dalton proteins take 2 minutes to equilibrate; those above 60,000 daltons can hardly enter.
- Pore complex has diffusion equivalent to a 9 nm diameter, 15 nm long channel.

Specific signals present in nuclear proteins dictate nuclear import.
NLS can be signal sequences or signal patches rich in positively charged amino acids (lysine and arginine).
- Variability exists across different nuclear proteins in NLS sequences.
NLS guides nuclear proteins, even when attached to small peptides on cytosolic proteins.

Use of gold particles coated with NLS to track nuclear import via electron microscopy, indicating dilation during transport.
Macromolecules maintain folded conformations during transport via nuclear pores, contrasting with organelle transport where proteins unfold.

Nuclear Import Receptors: Recognize nuclear localization signals for transporting proteins.
- Utilize a variety of receptors specialized for structurally similar NLS-proteins.
Import receptors bind to FG-repeats (phenylalanine-glycine rich) of nucleoporins, aiding transport by repeated attachment and detachment along tracks.
Adaptors may assist in bridging import receptors and NLS, suggesting common evolution between these proteins.

Nuclear export mimics import process with nuclear export signals and corresponding receptors.
- Nuclear export receptors are related to nuclear import receptors and share evolutionary origins.
Gold spheres coated with RNA quickly transported into the cytosol demonstrate bidirectional pore functionality.

Ran regulates directionality in nuclear transport.
Exists in two forms: Ran-GDP and Ran-GTP, maintained in gradient across the nucleus and cytosol by GAP (GTPase-activating protein) and GEF (guanine exchange factor).
Binding of Ran-GTP in the nucleus promotes cargo release from import receptors.

Control of nuclear localization and export signals can modulate protein location between nucleus and cytosol.
- The balance of import and export rates dictates protein localization.
Example: Gene regulatory proteins concealed in the cytosol until stimulation occurs, permitting nuclear transport.

The nuclear lamina, composed of nuclear lamins, provides structural integrity.
- During mitosis, lamins depolymerize under phosphorylation influence, allowing envelope disassembly.
Nuclear envelope reforms as dephosphorylated lamins reattach to chromatin post-mitosis.

NLS are not cleaved from proteins after transport, necessitating repeated import.
RNA export mechanisms similarly employ shuttling proteins with nuclear export signals guiding RNA out of the nucleus.