Transcript Slide 1
Molecular Chaperones The Hsp70 Super-family Pathways For Protein Folding Protein Folding in Cells Occurs in a Crowded Environment where Misfolding & Aggregation are Favored Hsp 70 Family Proteins Assist Protein Folding in an Energy Dependent Process A. Hsp70 domain structure & Cochaperone sites B. Hsp70 chaperoning Cycle The Hsp 70 Super-Family Contains Diverse Members With Related Domain Structures Protein Folding in Eukaryotes Involves MultiChaperone Systems Hsp90 assisted Hsp60 assisted The Hsp70 Super Family Contains Distinct Sub-Families The C. elegans Hsp70 Chaperones • Cytoplasmic & Nuclear – Hsp70 – Hsp-1 – Hsp110 homologue • Endoplasmic Reticulum – Hsp-3 – Hsp-4 – Grp170 homologue • Mitochondria – Hsp-6 • Hsp-5 is a pseudogene Hsp 70 Family in Drosophila 70 kDa Hsps Locus Heat-shock-protein70Ba 87B12-87B12 Heat-shock-protein70Bb 87B14-87B14 Hsp70Bbb Heat-shock-protein70Bc 87B14-87B14 87B14-87B1587A3-87A3 Hsc70-3 Heat shock protein cognate (Grp78) Large MW Hsp70s Hsc70 Cb (hsp110) 70C-b 87A3 Hsp70 Family Members are Found in All Major Cellular Compartments • Cytoplasm & Nucleus – Hsp70 (multiple paralogs) – Hsp110 (Multiple Paralogs) • Mitochondria & Chloroplasts – Hsp70 • Endoplasmic Reticulum – Grp78 (hsp 70 multiple paralogs) – Grp170 Cells Respond to Stress by Increased Synthesis of Molecular Chaperones • Misfolded proteins in the cytoplasm & nucleus induce the heat shock response, synthesis of additional heat shock proteins - Hsp’s • Misfolded proteins in the endoplasmic reticulum induce the unfolded protein response, synthesis of additional ER chaperones - Grp’s The Unfolded Protein Response (UPR) Hsp4 (grp78) grp170 XBP-1 IRE-1 • 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. Cellular Stresses Promote Misfolding and Aggregation of Proteins • • • • • • • High temperatures Heavy metals Alcohol Anoxia Osmotic stress Energy starvation Reducing agents The Structure of Yeast Hsp110 Hsp110 is a Nucleotide Exchange Factor for Hsp70 Summary Efficient folding of many newly synthesized proteins depends on assistance from molecular chaperones, which serve to prevent protein misfolding and aggregation in the crowded environment of the cell. Nascent chains binding chaperones, including trigger factor, Hsp70, and prefoldin, stabilize elongating chains on ribosomes in a Non-aggregated state. Folding in the cytosol is achieved either on controlled chain release from these factors or after transfer of newly synthesized proteins to downstream chaperones, such as the chaperonins. These are large, cylindrical complexes that provide a central compartment for a single protein chain to fold unimpaired by aggregation. Understanding how the thousands of different proteins synthesized in a cell use this chaperone machinery has profound implications for biotechnology and medicine. -