Cell Biology - The Nucleus and Nuclear Transport

Flashcards generated from lecture notes covering cell biology, the eukaryotic nucleus, nuclear envelope, nuclear pore complexes, nucleoporins, and Ran-dependent nuclear import/export systems.

Eukaryote

An organism whose cells contain a membrane-bound nucleus and other membrane-bound organelles, such as mitochondria, endoplasmic reticulum, and the Golgi apparatus.

Nucleus

A membrane-bound organelle in eukaryotic cells that contains DNA, serving as the repository of genetic information and the control centre of the cell.

Nucleolus

A prominent structure located inside the nucleus where ribosomes are assembled and ribosomal RNA ($rRNA$) is transcribed.

Nuclear Envelope

A double membrane enclosing the nucleus that separates it from the cytoplasm, featuring an outer membrane continuous with the rough endoplasmic reticulum.

Chromatin

A complex of DNA and histone proteins found in eukaryotic cells whose primary function is to package DNA molecules into more compact, denser structures.

Nuclear Lamina

A supportive fibrous meshwork made up of nuclear lamins located beneath the inner nuclear membrane that maintains nuclear shape, anchors chromatin, and regulates nuclear integrity.

Nuclear Lamins

intermediate filament proteins (major types being A-type and B-type) that polymerise to form the nuclear lamina. and provide structural support, organise DNA, controls gene expression, and ensure correct functions in cell growth and division

Nuclear Pore Complex (NPC)

A large molecular structure with eightfold rotational symmetry, composed of approximately 30 different nucleoporins, that regulates transport in and out of the nucleus.

Nucleoporins

Proteins (abbreviated as Nups) that serve as the structural building blocks of the nuclear pore complex, featuring repeat regions rich in specific amino acids such as FG repeats.

FG Repeats

made up of amino acids: Phenylalanine and glycine repeat regions found on nucleoporins

forms a mesh-like gel within the central channel of the nuclear pore complex to create a selective barrier.

Nuclear Localisation Signal (NLS)

A short amino acid sequence of (lysine and arginine)

that targets a protein for import into the nucleus.

Nuclear Export Signal (NES)

A specific amino acid sequence or tag required for macromolecules to be recognized by export receptors and transported out of the nucleus.

Karyopherins

A family of nuclear transport receptors that includes importins (which bring proteins into the nucleus) and exportins (which take proteins out).

Importins

Nuclear transport receptors that bind cargo proteins containing a Nuclear Localisation Signal (NLS) in the cytoplasm and deliver them into the nucleus.

Exportins

Nuclear transport receptors that bind cargo molecules containing a Nuclear Export Signal (NES) inside the nucleus and transport them to the cytoplasm.

Adaptor Proteins

Proteins that bridge the gap between a cargo protein's Nuclear Localisation Signal (NLS) and a nuclear import receptor when the cargo does not fit the receptor directly.

RanGTP

A GTP-binding protein present in both the nucleus and cytosol that provides energy and directionality for transport across the nuclear pore complex.

Ran-GEF

Guanine Exchange Factor located on chromatin inside the nucleus that converts Ran-GDP into Ran-GTP to maintain high nuclear Ran-GTP levels.

Ran-GAP

GTPase Activating Protein located in the cytoplasm that stimulates GTP hydrolysis on Ran-GTP to convert it to Ran-GDP, maintaining high cytoplasmic Ran-GDP levels.

Steady-State Localisation

The net location of a protein within the cell (nucleus vs. cytosol), determined by the relative rates of its nuclear import and export.

Laminopathies

A group of human diseases caused by mutations in lamin genes, including muscular dystrophies, cardiomyopathies, lipodystrophies, and premature ageing syndromes.

describe the process of nuclear export of a cargo protein (7 marks)

1. Cargo protein with NLS in the Cytosol

2. Importin binds to the NLS on the cago protein

3. this complex moves through the NPC

4. RAN-GTP binds to the importin, causing the cargo protein to be released in the nucleus

5. The RAN-GTP and importin pass through the NPC out of the nucleus into the cytosol

6. RAN-GAP (different one) stimulates the hydrolysis of RAN-GTP into RAN-GDP

7. Importin is able to be re-used

where are high concentrations of Ran-GTP most likely to be found in?

the nucleus

Where are high concentrations of Ran-GDP most likely to be found in?

the cytosol

Label this diagram of the nucleus

what signal is associated with nuclear import?

Nuclear localization signal

what signal is associated with nuclear export?

Nuclear export signal

A cargo protein that lacks an NLS will….

stay in the cytosol

A cargo protein that lacks NES will…

stay in the nucleus

what forms the selective mesh within an NPC?

FG-repeat nucleoporins

what forms the mesh in the inner nuclear membrane? and what is it made of?

the nuclear lamina

the nuclear lamina is made of interconnected lamins (proteins)

label this nucleus

how do small molecules of 5KDa or below move through the nuclear envelope?

passive diffusion through the FG repeats of the NPC

how do large proteins of 40KDa or below move through the nuclear envelope?

require Importin and NLS, and Exportin and NE.

how do 30 nm ribosome or above move through the nuclear envelope?

they can’t unless they have an Export signal

roughly how many macromolecules does the NPC transport per second?

1000 macromolecules per second