Transcript The ubiquitin-proteasome system and its role in the cell

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
Avram Hershko
Aaron Ciechanover
Nobel prize in chemistry, 2004
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
E1 - ubiquitin activating enzyme
Non specific
E2 - ubiquitin conjugating enzyme
E3 - ubiquitin protein ligase
Substrate specific
Transfers the ubiquitin to a
lysine residue of the substrate.
E4
Synthesis of a polyubiquitin chain on the substrate by conjugation of
additional ubiquitin molecules through lysine 48.
The poly-ubiquitin chain
1 2
U5
U4
U3
U2
U1
1 - poly-ubiquitinated protein
2- poly-ubiquitinated protein incubated with
de-ubiquitinating enzyme
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
Cells were synchronised in mitosis (M) with nocodazole.
They were released into fresh medium and samples were taken
every two hours.
M
substrate 1
substrate 2
Housekeeping gene
G1
G1/S
S
G2
Some roles of ubiquitin-mediated
protein degradation
• control of cell division
• signal transduction
• regulation of gene expression
• immune response
• embryonic development
• apoptosis