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55-140 (56)
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University of Windsor
Biological Sciences

CHAPERONES, QUALITY CONTROL, TURNOVER CHAPERONES: bind to and facilitate the correct FATE of another protein chaperone binding can affect: folding assembly compartmentalization degradation **do not accelerate correct folding, but DECELERATE incorrect folding** differentiates chaperones from isomerases (folding proteins) also known as HSP’s – (heat shock proteins) upregulated during heat shock (to prevent misfolding) bind newly translated peptides off the ribosome  prevents aggregation involved in mitochondrial transport (easier when protein is unfolded) HSP70 binding cycle: PREVENTS AGGREGATION 1. low affinity binding (hydrophobic stretch recognition) “open” 2. ATP hydrolysis  high affinity binding “closed” 3. ADP removed  chaperone disassociates HSP60: double doughnut structure very large (1/2 size of ribosome) central cavity provides shielded environment for folding “protein shield” also ATPase activity HSP70 binding tends to precede HSP60 folding HSP 90: conformational maturation of steroid hormone receptors and signaling kinases **aggregation can cause disease by inducing conformational changes** TURNOVER: misfolded proteins must be eliminated Regulation: compartmentalization selective activation turnover is studied by pulse (5 minutes)-chase (hours); half-lives determined LIPID DEGRADATION: glycerophospholipids can be degraded/remodeled throughout the cell sphingolipids – centralized degradation (lysosome) cholesterol – not degraded; its levels are regulated by controlling synthesis PROTEIN DEGRADATION - disposal of damaged proteins - metabolic control - cell differentiation - cell cycle / proliferation control - antigen presentation PROTEASOME – cytosolic degradation (as opposed to intra-organelle) PROTEASOME SYSTEM: 30% of newly made proteins fail to fold properly!! 4 concentric rings + 2 caps (for regulation) complete proteasome = 26S inner rings (B-chains) contain proteolytic activity narrow entrance  can’t insert folded proteins UBIQUITIN TARGETING: ubiquitin chain tagging (covalent attacthement) require E1, E2, E3 enzymes (each level is more specific than one before it) E1 – activates ubiquitin E2 – binds E1: ubiquitin and takes it from E1 E3 – recognizes substrate; transfers ubiquitin to it E2/E3 = “ubiquitin ligase” SUBSTRATE RECOGNITION: “N-END” rule - certain AA’s are stabilizing, others are destabilizing destabilizing AA’s are found in compartmentalized proteins likely to play a
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