Endoplasmic Reticulum


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  • Endoplasmic Reticulum and Golgi Apparatus
    • endoplasmic reticulum
      • appearance
        • contiguous tubes and sacs
        • er lumen (space) bounded by er membrane
      • Rough ER
        • ribosomes
          • secretory proteins
          • transmembranous proteins
          • ER and golgi proteins
        • production of secreted proteins
          • mRNA coding for cytosolic proteins translated in cytocol
            • by polyribosomes
          • common pool of ribosomal subunits in cytosol
          • mRNA encoding ER trageted protein translated by polyribosomes attached to membrane
        • targeting protein production
          • signal peptide binds to signal recognition particle, pauses translation
            • hydrophobic amino acid
          • SRP interacts with SRP receptor on ER membrane
          • translation and passage through protein translocator in ER membrane
          • SRP and SRP receptor recycled
        • soluble proteins
          • translocator opens it's pore allows transfer of polypeptide chain across bilayer
          • pore closes after protein completely transfered
        • transmembranous proteins
          • similar to soulube protein transfer, except contains a stop codon
          • translocator discharges protein horizontally
        • N-linked glycosylation
          • precursor ogliosaccharride is assembled onto phosphodolicol
          • transferred to peptide chain by enzyme
      • membrane synthesis
        • phospholipid synthesis
        • assembled in outer layer, by membrane bound enzymes
        • scramblase (phospholipid translocator), causes layers to equilate
      • Smooth ER
        • no ribosomes
        • transitional between er and golgi
        • steroid hormone synthesis
        • detoxification of drugs in the liver
          • lipophilic drugs oxidised, increasing solubility
          • may be further modified in cytosol before excretion
    • Sarcoplasmic reticulum
      • calcium store in the muscles
      • action potential depolarises membrane
      • voltage gates open
      • ca2+ enters cytosol
      • stimulates muscle contraction
      • ca2+ pumps restore levels
    • Golgi Apparatus
      • appearance
        • stack of membrane bound compartments (cisternae)
        • cis face is nearest the ER, vesicles from ER dock here
        • trans face, vesicles depart for cell surface
      • Functions
        • secretory proteins sorted, modified and dispatched
        • site of carbohydrate synthesis
          • glycoproteins and proteoglycans
        • prominant in secretory cells
      • glycolysation
        • protein marker
        • protector
        • cellular marker
        • regulatory roles
    • Protein Folding
      • 2 of 3 terminal glucose molecules rmoved
      • calnexin is a chaperone molecule that recognises single glucose of incompletely folded protein, prevents export to golgi
      • partialy folded proteins have another glucose added by glycosyl transferase, try again
      • misfolded proteins are sent to cytoplasm for degradation
      • correctly folded sent to golgi
      • travel to golgi
        • proteins need exit signals
        • COPII interacts with cytosolic tails of receptor, causing a vesicle to bud off
        • vesicles shed COPII coat and undetgo homolytic fission (join other vesicles)
        • moves along microtubules to golgi
      • travel from golgi
        • vesicles coated in COPI bud from the golgi
        • ER has proteins with retrieval signals that interact with COPI
  • lysosomal proteins
    • ribosomes
      • secretory proteins
      • transmembranous proteins
      • ER and golgi proteins
  • hydrophobic signal moves horizontally, is degraded
    • soluble proteins
      • translocator opens it's pore allows transfer of polypeptide chain across bilayer
      • pore closes after protein completely transfered

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