Chap 11 P 2

Protein Folding in the Endoplasmic Reticulum (ER)

  • Protein Folding and Processing:

    • Occurs during translocation across the ER membrane or within the ER lumen.
    • Main role of lumenal ER proteins:
    • Assist in folding.
    • Aid in the assembly of newly translocated polypeptides.

  • Chaperones and Protein Misfolding:

    • Hsp70 (BiP):
    • Binds to unfolded polypeptide chains during their membrane passage.
    • Mediates folding and assembly of multisubunit proteins.

Disulfide Bond Formation

  • Importance in Protein Folding:
    • Formation of disulfide bonds is crucial.
    • Cytosol: Reducing environment, cysteine residues are in reduced form ($—SH$).
    • ER: Oxidizing environment promotes formation of disulfide bonds ($S—S$), facilitated by protein disulfide isomerase.

Glycosylation in the ER

  • N-linked Glycosylation:
    • Occurs on specific asparagine residues during translocation into the ER.
    • Synthesis: Oligosaccharide synthesized on a dolichol lipid carrier.
    • Functions:
    • Prevents protein aggregation.
    • Provides signals for subsequent protein sorting.

Misfolded Proteins and ERAD

  • Removal of Misfolded Proteins:
    • Identified and exported from the ER via ER-associated degradation (ERAD).
    • Degraded by the ubiquitin-proteasome system.
    • Chaperones in the ER lumen assist in folding upon exiting the translocon.
    • Correctly folded proteins exit the ER; improperly folded ones are targeted back to cytosol.

Unfolded Protein Response (UPR)

  • Activates upon accumulation of unfolded proteins:

    1. Leads to ER expansion and increase in chaperone production.
    2. If folding does not normalize, cell may undergo programmed cell death.

  • Signaling Mechanism: Unfolded proteins activate 3 receptors in the ER membrane:

    • IRE1: Cleaves pre-mRNA of transcription factor XBP1, stimulating transcription of UPR genes.
    • ATF6: Cleaved to release active transcription factor ATF6.
    • PERK: Phosphorylates translation factor eIF2, inhibiting general translation to reduce protein influx into ER.

Smooth Endoplasmic Reticulum (SER)

  • Functionality of SER:
    • Syntheses of membrane lipids in association with existing membranes.
    • Major sites for lipid synthesis, including:
    • Types of Lipids:
      • Phospholipids, glycolipids, and cholesterol.
    • Phospholipids synthesized in the cytosol from water-soluble precursors (glycerol).

Golgi Apparatus

  • Functions:

    • Processes and sorts proteins from the ER for transport to endosomes, lysosomes, plasma membrane, or secretion.

  • Structure:

    • Flattened membrane-enclosed sacs (cisternae) and associated vesicles, compartmentalized into:
    • Cis compartment: Receives molecules from ERGIC.
    • Medial and Trans compartments: Main modification sites.
    • Trans-Golgi Network: Sorting and distribution center.

  • Transport Mechanism:

    • Proteins move through the Golgi from the cis to trans face in vesicles, and the topological orientation of membrane proteins and lipids is maintained throughout the transport process.

Protein Export and Retrieval

  • Export Signals: Proteins targeted for export have distinct peptide and carbohydrate signals directing packaging into transport vesicles.
    • Unmarked proteins can follow a default pathway to the Golgi.
  • Retrieval Signals:
    • Resident ER proteins recognized and transported back via specific sequences (KDEL or KKXX) at the carboxy terminus.
  • Vesicles bud from ERES, fuse into the ER-Golgi intermediate compartment (ERGIC), and continue to the Golgi apparatus for further processing and sorting.