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The ubiquitin-proteasome system and its role in the cell
cycle
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The cell cycle
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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
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How are proteins degraded?
• by the ubiquitin-proteasome system
Nobel prize in chemistry, 2004
Avram Hershko Aaron Ciechanover Irwin Rose
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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.
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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.
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The poly-ubiquitin chain
1 - poly-ubiquitinated protein2- poly-ubiquitinated protein incubated with de-ubiquitinating enzyme
U5U4U3
U1
U2
1 2
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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.
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Degradation in the protasome
The poly-ubiquitinated protein is recognised by the 26S proteasome, unfolded and degraded. The ubiquitin molecules are recycled.
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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.
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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.
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Some roles of ubiquitin-mediated protein degradation
• control of cell division• signal transduction• regulation of gene expression• immune response• embryonic development• apoptosis