19. MODULE 19.pdf

MODULE 19 Protein Degradation – Overview proteins “live” for minutes to days the levels of proteins are determined by their rates of synthesis and degradation rapidly degraded proteins are usually regulatory or damaged proteins in eukaryotic cells, there are two pathways of protein...

MODULE 19 Protein Degradation – Overview proteins “live” for minutes to days the levels of proteins are determined by their rates of synthesis and degradation rapidly degraded proteins are usually regulatory or damaged proteins in eukaryotic cells, there are two pathways of protein degradation, by proteasomes and lysosomes Autophagy a process, by which a cell’s own organelles or proteins are degraded in lysosomes supports the turnover of organelles protects against intracellular threats (e.g., bacteria, abnormal protein aggregates) it is also activated by ER stress and when cells require more amino acids and energy it removes neoplastic cells Types of Autophagy three types: macroautophagy, microautophagy, chaperone-mediated autophagy macroautophagy: encapsulation of cargo into double-membrane vesicles (autophagosomes); these mature and fuse with lysosomes to form autophagolysosomes microautophagy: the lysosome directly engulfs portions of the cytoplasm chaperone-mediated autophagy: chaperone proteins bind to cargo and transport it across the lysosomal membrane for degradation Ubiquitin-Proteasome System proteins with short half-lives are usually ubiquitinated and degraded by proteasomes (e.g., cell cycle regulatory proteins, unfolded/misfolded proteins) monoubiquitination contributes to various cellular functions polyubiquitination targets proteins to proteasomes for degradation and contributes to other processes (e.g., DNA damage response, mitochondrial maintenance, T Cell Receptor signaling, and NF-κB signaling) MODULE 19 Ubiquitin a small protein, the encoding genes are in tandem repeats possibly due to the heavy transcription demands contains several lysine residues, to which another ubiquitin can be ligated in a chain, the first ubiquitin is covalently bound to the target protein chains, in which each additional ubiquitin is linked to lysine 48 of the previous ubiquitin, have a role in proteasome targeting Ubiquitin Machinery Ubiquitin-activating enzyme (E1) hydrolyzes ATP to form a link between the enzyme active site and ubiquitin (A). The activated ubiquitin is transferred to E2 ubiquitin-conjugating enzyme (B). E2 enzymes with E3 protein ligases attach ubiquitin to lysine residues of proteins (C). E3 ligases provide specificity, as each enzyme modifies only a subset of substrate proteins. Proteasomes protein complexes that degrade normal and damaged proteins via proteolysis that requires ATP components in the Ubiquitin-Proteasome System and the protein quality control selectively degrade intracellular proteins with a short half-life; these proteins must be first ubiquitinated proteasome inhibitors have anti-cancer activity because they induce apoptosis by disrupting the degradation of pro-growth cell cycle proteins Structure and Mechanism of the Proteasome the multi-subunit 26S proteasome recognizes, unfolds, and degrades polyubiquitinated proteins into peptides a ubiquitinated protein is recognized by the 19S regulatory particle in an ATP- dependent binding step the proteins are deubiquitinated and at least partially unfolded before entering the core; the unfolding requires energy from ATP the proteins enter the interior of the 20S particle to come in contact with the proteolytic active sites there MODULE 19 Putting It Together the Ubiquitin-Proteasome System and the lysosomes are major pathways for protein degradation autophagosomes, formed in the process of macroautophagy, are intermediates in the lysosomal protein degradation pathway distinct types of ubiquitin conjugation mark proteins for different fates proteins with short half-lives are usually polyubiquitinated and degraded by proteasomes; long-lived proteins are typically degraded by lysosomes monoubiquitination of proteins may play a role in processes other than degradation the ubiquitination machinery consists of ubiquitin-activating enzymes (E1), ubiquitin- conjugating enzymes (E2), and ubiquitin protein ligases (E3) ubiquitination deficiencies are associated with pathologies the 26S proteasome has a barrel-like structure; it has a 20S subunit that forms a hollow core and two 19S subunits on both ends of the 'barrel'; it requires ATP for the unfolding of the proteins