Lecture 13: The Nucleus

Function of the nucleus

To house and protect the cell's genetic material (DNA).

Main components of the nucleus

Nuclear envelope, chromosomes (chromatin), nuclear matrix, nucleolus, and nucleoplasm.

Chromosomes in the nucleus

Highly extended nucleoprotein fibers called chromatin.

Nuclear matrix

A fibrillar protein network that helps organize nuclear content.

Nucleolus

Synthesis of ribosomal RNA (rRNA) and ribosome assembly.

Nucleoplasm

The fluid substance inside the nucleus, similar to cytoplasm.

Structure of the nuclear envelope

A double bilayer membrane (~10-15 nm apart) separating nucleus from cytoplasm.

Nuclear pore sites

Fused regions of the envelope that form nuclear pores.

Nuclear lamina

A mesh-like structure that supports the envelope, helps chromatin attach, and is involved in transcription and replication.

Nuclear lamina during cell division

It is broken down by phosphorylation.

Progeria

A disease caused by mutations in nuclear lamin; cells lack proper lamina structure.

Nuclear pore complex

~30 different proteins called nucleoporins, present in 8 copies each (octagonal symmetry).

Control of nuclear entry and exit

Via signals like Nuclear Localization Signals (NLS) and Nuclear Export Signals (NES).

Nuclear Localization Signal (NLS)

A sequence of basic amino acids (usually C-terminal) that directs proteins into the nucleus.

NLS is 'necessary and sufficient'

Necessary = required for import; Sufficient = enough to cause import on its own.

Testing for necessary NLS

Remove it and see if nuclear import still occurs.

Testing for sufficient NLS

Add it to a cytoplasmic protein and see if it enters the nucleus.

Nuclear Export Signal

A leucine-rich sequence that directs proteins out of the nucleus.

Proteins required for nuclear import

Importin, Ran, and the nuclear pore complex.

Role of importin

It binds to NLS-containing proteins and escorts them into the nucleus.

Ran and its active form

A small GTPase; active when bound to GTP.

First step of nuclear import

NLS-containing protein binds importin.

After importin binds the cargo protein

The complex docks to cytoplasmic filaments of the pore.

Movement of cargo-importin complex through the pore

It hops from one binding site to another.

Cause of cargo release in the nucleus

Ran-GTP binds importin, triggering cargo release.

Ran-GTP localization

There is more Ran-GTP in the nucleus than in the cytoplasm due to the localization of regulatory proteins (Ran GEF inside nucleus).

Importin after cargo release

Ran-GTP escorts importin back to the cytoplasm.

Ran-GTP recycling in the cytoplasm

Ran hydrolyzes GTP to GDP, causing it to release importin.

NTF2

A protein that escorts Ran-GDP back into the nucleus.

Ran-GAP

Stimulates GTP hydrolysis in cytoplasm (Ran -> GDP).

Ran-GEF

Located in nucleus; exchanges GDP for GTP on Ran.

Protein mediating nuclear export

Exportin.

Requirements for nuclear export

Exportin, cargo with NES, and Ran-GTP.

Release of cargo in the cytoplasm during export

Ran-GTP is hydrolyzed by Ran-GAP, leading to dissociation of the complex.

Ran-GDP after export

It returns to the nucleus with NTF2 for reactivation.