The ubiquitin-proteasome system and its role in the cell

cycle

The cell cycle

How is the cell cycle regulated?

All events need to happen in the right order.Each event needs to happen once and only once!

• protein synthesis

• post-translational modifications

• protein degradation

How are proteins degraded?

• by the ubiquitin-proteasome system

Nobel prize in chemistry, 2004

Avram Hershko Aaron Ciechanover Irwin Rose

What is ubiquitin?

• 76 amino acids polypeptide• very highly conserved from plants to eukaryotes• ubiquitous (= widely distributed) to all cells• active in different cellular processes like DNA repair, cellular trafficking, endocytosis.

Its main role is in tagging proteins for degradation by poly-ubiquitin chains.

Tagging proteins with ubiquitin

Synthesis of a polyubiquitin chain on the substrate by conjugation of additional ubiquitin molecules through lysine 48.

E4

E1 - ubiquitin activating enzyme Non specific

E2 - ubiquitin conjugating enzyme

E3 - ubiquitin protein ligase Substrate specificTransfers the ubiquitin to a lysine residue of the substrate.

The poly-ubiquitin chain

1 - poly-ubiquitinated protein2- poly-ubiquitinated protein incubated with de-ubiquitinating enzyme

U5U4U3

U1

U2

1 2

The proteasome

• A 20S barrel-like core with proteolytic activity

• Two 19S regulatory subunits responsible for ubiquitin chain recognition, removal of ubiquitin and protein unfolding.

The whole proteasome (core+regulatory units) is often referred to as the 26S proteasome.

Degradation in the protasome

The poly-ubiquitinated protein is recognised by the 26S proteasome, unfolded and degraded. The ubiquitin molecules are recycled.

APC/C - the mitotic E3

• APC/C =Anaphase Promoting Complex/ Cyclosome

• 1500 KDa Multi-subunit complex (~12 subunits).

• Known to be active from metaphase through G1.

• Regulates chromatide separation and exit from mitosis.

Oscillation patterns of two APC/C substrates

Housekeeping gene

substrate 1

substrate 2

MG1

G1/S S G2

Cells were synchronised in mitosis (M) with nocodazole. They were released into fresh medium and samples were taken every two hours.

Some roles of ubiquitin-mediated protein degradation

• control of cell division• signal transduction• regulation of gene expression• immune response• embryonic development• apoptosis