Lecture 11

Cards (22)

  • Co-translational targeting
    Essential for directing newly synthesized proteins to the ER, setting the stage for their eventual roles in the cell
  • Signal Peptide (SP)
    • Nascent proteins destined for the secretory pathway are marked by an N-terminal signal peptide
    • Signal peptide is typically 15-30 amino acids long, featuring a positively charged amino acid near the N-terminus and a hydrophobic stretch that is recognized by the signal recognition particle (SRP)
  • Mechanism of SP recognition and ER docking
    1. Signal peptide is first recognized by SRP in the cytosol, which pauses translation
    2. SRP then targets the ribosome-nascent chain complex to the ER membrane by binding to the SRP receptor
    3. This interaction guides the ribosome to the ER translocon, a channel through which the nascent protein enters the ER lumen
    4. Upon docking, SRP is released and recycled, and the translocon opens to allow the protein to enter the ER
  • Signal peptide cleavage and protein maturation
    1. Once the nascent protein begins to enter the ER, the signal peptidase (SPase) cleaves off the signal peptide, allowing the rest of the protein to be translated directly into the ER lumen
    2. In some cases, the signal peptide is further processed by signal peptide peptidase (SPPase), which breaks it down into smaller fragments
  • Protein folding in the ER
    • ER chaperones, such as BiP and calnexin, play critical roles in ensuring proper protein folding and preventing aggregation under the crowded conditions of the ER lumen
  • Secretory pathway progression
    1. After successful entry and folding within the ER, proteins are packaged into vesicles and transported to the Golgi apparatus
    2. Here, they undergo further modifications and sorting for their final destinations, which could be incorporation into the plasma membrane, secretion outside the cell, or delivery to lysosomes
  • Regulatory mechanisms ensure precise protein sorting and modification, while cellular checkpoints prevent errors that could lead to diseases
  • Misregulation of these processes can lead to various diseases, highlighting the importance of understanding co-translational targeting mechanisms for potential therapeutic interventions
  • ER functions
    • The ER lumen is a central site for protein folding, facilitated by various chaperones
    • Two primary modifications in the ER are N-glycosylation and disulfide bond formation
  • Chaperones in the ER and cytosol
    • Hsp70 Family (Hsc70, Hsp70)
    • Hsp90
    • BiP (GRP78)
    • GRP94
  • Specialized ER chaperones
    • Protein Disulfide Isomerase (PDI)
    • ERp57 and Calreticulin (CRT)
    • Calnexin (CNX)
    1. glycosylation process
    1. N-glycosylation starts with the covalent addition of an oligosaccharide tree (core N-glycan) from a lipid carrier to a target protein by the membrane-integral enzyme oligosaccharyltransferase (OST)
    2. As proteins move through the secretory pathway, the core N-glycans are modified to form complex N-glycans
    3. The initial core N-glycan is composed of 3 glucose, 9 mannose, and 2 N-acetyl glucosamine units
    4. The removal of two glucose residues from the core N-oligosaccharide in the ER facilitates interactions with chaperones like calreticulin required for efficient folding
  • Disulfide bond formation
    1. Disulfide bonds form when two cysteine residues are brought close together during protein folding, catalyzed by PDI and its relatives
    2. PDI is also involved in shuffling and breaking disulfide bonds until the protein reaches a stable conformation
  • ER-associated protein degradation (ERAD)
    1. Misfolded proteins in the ER are targeted for degradation through the ERAD pathway
    2. This involves marking the proteins with ubiquitin, unfolding them, and then feeding them through a complex known as the DISLOCON for degradation in the cytosol
  • Major Histocompatibility Complex (MHC) Class I assembly
    1. BiP and other chaperones aid in the assembly of MHC Class I molecules
    2. This process is crucial for the immune system's ability to present peptide antigens to T-cells
    3. The assembly involves several steps and components like tapasin and the TAP transporter, ensuring that peptide fragments are properly inserted into the MHC molecule
    1. glycosylation
    Involves the attachment of sugars to the oxygen atoms of amino acids and takes place primarily in the ER and Golgi apparatus of eukaryotes, as well as in Archaea and Bacteria
  • Types of O-glycosylation
    • O-N-acetylgalactosamine (O-GalNAc)
    • O-mannose
    • O-fucose and O-glucose
  • Proteolytic cleavage
    This modification process is crucial for activating certain proteins within the secretory pathway
  • Proteolytic cleavage examples
    • Albumin
    • Digestive enzymes
    • Hormones like glucagon and insulin
    • Neurotransmitters like enkaphalin
    • Proteins that could otherwise aggregate, like mature collagen
  • Alzheimer's disease and APP processing
    • Amyloid precursor protein (APP) processing involves β-secretase (BACE) and γ-secretase cuts, which can lead to the release of Aβ peptide fragments that aggregate and form plaques in the brain
    • The cutting by α-secretase can prevent the release of the toxic Aβ fragment, with ongoing research focusing on the regulation of these secretases to control Aβ buildup
  • Addition of lipids
    Lipid addition assists in targeting and maintaining proteins within membranes
  • Types of lipid addition
    • GPCRs (G-protein-coupled receptors) typically have a myristic acid lipid anchor
    • GPI-linked proteins feature a glycophosphatidylinositol membrane anchor that is linked via sugars to the protein, aiding in their attachment to the membrane