The ubiquitinproteasome system and its role in the

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 E 1 - ubiquitin activating enzyme Non specific E 2 - ubiquitin conjugating enzyme E 3 - ubiquitin protein ligase Substrate specific Transfers the ubiquitin to a lysine residue of the substrate. E 4 Synthesis of a polyubiquitin chain on the substrate by conjugation of additional ubiquitin molecules through lysine 48.

The poly-ubiquitin chain 1 2 U 5 U 4 U 3 U 2 U 1 1 - poly-ubiquitinated protein 2 - poly-ubiquitinated protein incubated with de-ubiquitinating enzyme

The proteasome • A 20 S barrel-like core with proteolytic activity • Two 19 S 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 26 S proteasome.

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

APC/C - the mitotic E 3 • APC/C =Anaphase Promoting Complex/ Cyclosome • 1500 KDa Multi-subunit complex (~12 subunits). • Known to be active from metaphase through G 1. • 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 G 1/S S G 2

Some roles of ubiquitin-mediated protein degradation • control of cell division • signal transduction • regulation of gene expression • immune response • embryonic development • apoptosis