Presentation on theme: "Protein Folding and Processing The classic principle of protein folding is that all the information required for a protein to adopt the correct three-dimensional."— Presentation transcript:
Protein Folding and Processing The classic principle of protein folding is that all the information required for a protein to adopt the correct three-dimensional conformation is provided by its amino acid sequence. Molecular chaperones are proteins that facilitate the folding of other proteins. Two specific families of chaperone proteins act in a general pathway of protein folding in both prokaryotic and eukaryotic cells – Heat shock proteins and Chaperonins. Unfolded polypeptide chains are shielded from the cytosol within the chamber of the chaperonin.
Action of chaperones during translation and Transport chains that are still being translated on ribosomes, thereby preventing incorrect folding or aggregation of the amino-terminal portion of the polypeptide before synthesis of the chain is finished. Chaperones also stabilize unfolded polypeptide chains during their transport into subcellular organelles.
The role of N -linked glycosylation in ER protein folding. 3
Protein folding in the cell Basics - cell compartments, molecular crowding: cytosol, ER, etc. Folding on the ribosome - co-translational protein folding Molecular chaperones - concepts, introduction - intramolecular chaperones - chemical chaperones - protein chaperones
Folding in vitro vs. in vivo folding by dilution in buffer protein denatured in a chaotrope folded protein in vitroin vivo folding folded protein
Problem: non-native proteins non-native proteins expose hydrophobic residues that are normally buried within the ‘core’ of the protein these hydrophobic amino acids have a strong tendency to interact with other hydrophobic (apolar) residues - especially under crowding conditions intramolecular misfolding X X X X intermolecular aggregation X X X X X X incorrect molecular interactions & loss of activity incorrect molecular interactions & loss of activity exposed hydrophobic residues 3-10
EukaryotesArchaeaBacteria --Trigger Factor NAC - Hsp70 system[Hsp70 system]Hsp70 system prefoldin - chaperonins (group II) chaperonins (Group I) small Hsps [small Hsps] Hsp90-[Hsp90] AAA ATPases --SecB --[PapD/FimC] Hip, Hop, Bag, clusterin, cofactors A-E, calnexin, calreticulin, etc. etc. -- Overview of chaperone families: Distribution
IRE-1 XBP-1 The Unfolded Protein Response (UPR) The UPR occurs when proteins are misfolded in the endoplasmic reticulum (ER). Reducing agents, such as DTT, interfere with disulfide bond formation while drugs can interfere with glycosylation; both agents cause proteins to misfold in the ER thus triggering the UPR. The product of the ire-1 gene is the sensor of misfolded proteins and when activated removes an intron from the pre mRNA from the xbp-1 gene. Active xbp-1 protein (from spliced mRNA) activates the genes that code for ER chaperones, such as hsp-4. Hsp4 (grp78) grp170
PROTEIN TURNOVER AND AMINO ACID CATABOLISM Degradation of proteins 1) dietary proteins - amino acids - pepsin in stomach - pancreatic proteases - aminopeptidase N -other peptidases 2) endogenous proteins - protein turnover: synthesis, degradation, resynthesis - damaged proteins - half-lives of proteins: depend on amino-terminal residues
Cellular Protein Degradation Lysosomal Nonspecific Endocytosis Foreign proteins Energy favorable to degrade proteins Non-lysosomal Specificity, requires ATP Conditions of stress Ubiquitin-proteosomal pathway 26S proteosome Role in cellular processes/signaling
Protein turnover; selective degradation/cleavage Individual cellular proteins turn over (are degraded and re- synthesized) at different rates. E.g., half-lives of selected enzymes of rat liver cells range from 0.2 to 150 hours. N-end rule: On average, a protein's half-life correlates with its N- terminal residue. Proteins with N-terminal Met, Ser, Ala, Thr, Val, or Gly have half lives greater than 20 hours. Proteins with N-terminal Phe, Leu, Asp, Lys, or Arg have half lives of 3 min or less. PEST proteins having domains rich in Pro (P), Glu (E), Ser (S), Thr (T), are more rapidly degraded than other proteins.
Ubiquitinylation – Proteosome Degradation E3 determines protein substrate
Ubiquitination 1) ubiquitin - a 8.5 kd protein (76 residues) - formation of an isopeptide bond with ε-amino group of lysine of the proteins - a tag for destruction - polyubiquitin: a strong signal for degradation 2) enzymes for ubiquitination - E1 (ubiquitin-activating enzyme) - E2 (ubiquitin-conjugating enzyme) - E3 (ubiquitin-protein ligase) - variation: E3 > E2 > E1: more finely tuned substrate discrimination - HPV (human papilloma virus) activates a specific E3 enzyme: tumor suppressor protein p53
Regulation of ubiquitination: Some proteins regulate or facilitate ubiquitin conjugation. Regulation by phosphorylation of some target proteins has been observed. E.g., phosphorylation of PEST domains activates ubiquitination of proteins rich in the PEST amino acids. Glycosylation of some PEST proteins with GlcNAc has the opposite effect, prolonging half-life of these proteins.
19S and 20S Proteasome Subunits Characteristics 20S Subunit Barrel Contains 6 proteolytic sites 2x Tryptic 2x Chymotryptic 2x Peptidylglutamyl- peptidase Linearized protein required 19S Subunit Base and Lid Contains subunits with known and unknown functions Tetra-Ub (K48) recognition Deubiquitination activity Protein unfolding activity (Chaperone function)
Ubiquitin AA Sequence MQIFV K TLTG KTITLEVEPS DTIENVKA K I QDKEGIPPDQ QRLIFAG K QL EDGRTLSDYN IQ K ESTLHLV LRLR GG 48 63 6
Transmembrane Proteins Regulated by Ub-dependent Sorting In metazoans: Neurotransmission:Ion channels: AMPA glutamate receptorsENaC Glycine receptorsClC-5 Cell-cell contacts:Immune molecules E-cadherindownregulated by viruses: OccludinMHC class I B7-2 Developmental patterning:ICAM-1 DeltaCD4 Notch Roundabout
Poly-Ub Chains Ub K K K48 Linkage K63 Linkage K63 K48 Peters, J.M. 1998 Ubiquitin and the Biology of the Cell Signal to proteosome K48, Ub 4 Cell Signaling K63
ENaC function Major ion channel that controls salt and fluid resorption in the kidney Mutations in the PPXY motif cause accumulations of channels at the cell surface and result in Liddle’s syndrome, and inherited form of hypertension
ENac surface Stability Nedd 4 (HECT ligase) - negatively regulates ENaC surface stability Nedd4 WW domains bind PPXY motif of ENaC subunits Nedd4 also interacts with serum and glucocorticoid-regulated kinase (SGK) SGK contains two PPXY motifs that bind to Nedd4 WW domains SGK-dependent Nedd4 P inhibits the Nedd4-ENaC interaction therefore, Nedd4 P increases ENaC at the cell surface