MM9 - Dynamic Regulation of Protein (ii)

Ubiquitin-Proteasome System (UPS)

ubiquitin

tag for degradation

signals destruction

small highly conserved compact protein

added to lysine residues on protein, then more join to form poly-ubiquitin chain

protein stability determined by its ability to be ubiquitinated

proteasome

recognises poly-ubiquitin chain

large multi-subunit protein

barrel shaped

in nucleus or cytosol

all subtypes have same composition as they all have the same function

degrades target proteins to short peptides

ubiquitination = multistep process...

1) ubiquitin activated via ubiquitin-activating enzyme E1 (uses ATP)

2) ubiquitin transferred to ubiquitin-conjugating enzyme E2

3) ubiquitin transferred to target protein via E3 ligase

4) E3 recognises target protein

adds specificity + complexity

7 alpha type proteins @ ends of barrel

7 beta-type proteins in 2 central beta rings

targeted proteins enter through opening @ top, peptides exit @ bottom

cleave targeted proteins @ different locations

trypsin cleaves after arginine/lysine

chymotrypsin cleaves after aromatic AAs (tyrosine/phenylalanine

caspase-like cleaves after aspartic/glutamic acid

continuously recycled

mono, multi + polyubiquitinated proteins can be degraded lysosomally

only poly-ubiquitinated proteins proteins can be degraded proteolytically (chain needed

19S subunit (cap complex) regulates ubiquitin accessibility to proteasome

protein brought to 20S subunit to be degraded (ATP needed for unfolding of primary structure)

E3 ligases + disease

Parkinson's

loss of function mutation in Parkin E3 residue

AR, early onset

loss of ligase activity

also plays role in sporadic cases

parkin over expression protects against toxins

Tumorigenesis

MDM2 = E3 ligase that inhibits + degrades tumour suppressor P53

increased MDM2 in many cancers

UPS inhibitors

used to treat multiple myeloma (cancer of plasma cells)

e.g. Bortezomib

NF-kappa-beta

TF, NB in regulating cell proliferation

turned on by degradation of inhibitor kappa-beta

cancer treatment: block inhibitor-kappa-beta degradation, stops cell proliferation