Chapter 31 . Folding, Processing and DegradationChapter 31Completing the Protein Life Cycle:Folding, Processing,and Degradation. . . . . . . . . . . . . . . . . . . . . . . .Chapter OutlineProtein folding: ChaperonesTF (Trigger factor in E. coli), NAC (in eukaryotes) bind to nascent chain on ribosomeHsp70 (DnaK): Binds to nascent polypeptide chainDnaJ (Hsp40): Binds to unfolded proteins and passes them to Hsp70 (DnaK)GrpE: ADP/ATP exchange on DnaKHsp60: ChaperoninsGroup I in eubacteriaGroES-GroELGroup II in archaea and eukaryotesCCT (triC) a GroEL analogPrefoldin (GimC)Hsp90: foldosomePost-translational processingProteolytic cleavage of pro-enzymesProtein translocationCharacteristics of translocation systemsProteins made as preproteins with signal peptidesSpecific protein receptors exist on target membraneMovement catalyzed by complex structures: Translocons: ATP (or GTP) drivenProteins generally maintained in loosely folded conformations for translocation competenceProkaryotic translocationN-terminal leader sequenceN-terminus of leader sequence: Basic amino acidsCentral domain hydrophobicC-terminus: Nonhelical structureLeader peptidase: Removes leader sequenceEukaryotic translocation and protein sortingSecreted and membrane proteins synthesized on ER-localized ribosomesCytoplasmic ribosome initiates translationN-terminal signal sequence detected by signal recognition particle (SRP)SRP/ribosome complex binds to docking protein: ER membrane protein241
Chapter 31 . Folding, Processing and DegradationRibosome delivers peptide to transloconSignal peptidase cleaves leader sequenceMembrane proteins carry 20-residue stop transfer sequenceRetrograde transport : Sec61p: Moves protein from ER back to cytosol Mitochondrial protein importN-terminal sequence 10 to 70 residues longoForm amphiphilic -helixoBinds to TOM (mitochondria outer membrane translocase)Outer membrane proteinoSAM (sorting and assembly complex) passes to TOMInner membrane proteinoTOM to TIM22Matrix proteinoTOM to TIM23ChloroplastsTOCs and TICsProtein degradation: Ubiquitination most common pathway in eukaryotesUbiquitin: Conserved 76-residue proteinE1: Ubiquitin-activation protein: Attaches to C-terminal Gly of ubiquitinE2: Ubiquitin-carrier protein: Accepts ubiquitin from activator protein: Carriedon cysteine residueE3: Ubiquitin-protein ligase: Binds target protein: Protein ubiquitinated on amino groupsProteins with acidic N-terminiN-termini altered by Arg-tRNAPEST sequences: Target proteins for degradationProteasomes20S proteasomes26S proteasomes HtrA proteaseFunctions as chaperone at low temperature (20°C)Switch from chaperone to protease function as temperature increasesDegPChapter ObjectivesProtein FoldingProtein folding starts before the polypeptide is released from the ribosome. In many cases,folding is assisted by molecular chaperones.