Processes of the Golgi Apparatus

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  1. Constitutive Pathway
    Newly synthesized molecules are not concentrated; there is no post-Golgi storage pool and the transport vesicle has a short transient time from the Golgi to the surface; usually occurs with non-polarized cells
  2. Regulated Secretion
    The secretory products are condensed in electron-dense core forms; the dense core vesicles accumulate in the cytoplasm because they can have a half-life of many hours and fusion requires altering the level of an intracellular messenger, such as calcium
  3. pathway to lysosomes
    • 1)      Lysosomal proteins are synthesized on bound ribosomes and carried to Golgi
    • 2)      It is separated from other proteins in the Golgi by the addition of a phosphate group to its mannose sugars of the N-linked carbohydrate chains which is added during its progression to the trans face
    • a.       These phosphorylated mannose residues are sorting signals
    • 3)      Lysosomal enzymes carrying these mannose 6-phosphate receptors are recognized and captured by mannose 6-phosphate receptors, which are integral membrane proteins that span the TGN membranes
    • 4)      Lysosomal enzymes are escorted from the TGN by a family of adaptor proteins called GGAs, which will bind to both the receptors and clathrin, allowing concentration of the receptors in the membrane, which hold the lysosomal enzymes in the luminal side. This increases concentration of both the receptors and enzymes in clathrin-coated vesicles
    • a.       The receptors have a dual role of binding to both the GGA adaptors and the lysosomal enzymes
    • 5)      Transport in clathrin coated vesicles occurs
    • a.       Arf1 facilitates clathrin-coat formation
    • 6)      Clathrin-coat is lost and MPRs dissociate from lysosomal enzymes before fusing with other organelles
  4. Glycosylation
    • -          As movement through the stacks occurs, most mannose residues are removed from the core oligosaccharides, and other sugars are added by glycosyltransferases
    • -          Sequence depends on spatial arrangement of glycosyltransferases
    • -          Sialic acid, for example, gets placed at the terminal end of the polypeptide
    • -          Also additions of phosphates (such as with lysosomal enzymes) and sulfates can occur
  5. Targeting to a specific compartment:
    Step One
    • -          Movement towards a specific target compartment
    • o   May travel long distances but is aided by tracks 
  6. Targeting to a specific compartment:
    Step Two
    • -          Tethering to the target membrane
    • o   Two types of tethering proteins:
    • §  Rod, shaped fibrous proteins that act as bridges
    • §  Multiprotein complexes that bring the membranes into closer proximity
    • o   Specificity is due to a family of small G proteins called Rabs, which give a particular surface identity
    • They also play a role in recruiting specific cytosolic tethering proteins to specific membrane surfaces and membrane trafficking
  7. Targeting to a specific compartment:
    Step Three
    • -          Docking
    • o   Carried out by SNARES, which contain a SNARE motif that will allow the formation of a complex between the two membranes
    • §  V-Snares on the vesicle membrane interact with the T-Snares of the target membrane
  8. Targeting to a specific compartment:
    Step Four
    • -          Fusion
    • o   The v and t- SNAREs are capable of pulling two lipid bilayers together with sufficient force to cause them to fuse
    • o   Other view is that it remains in an arrested state by interaction with accessory proteins, docked and ready to discharge their contents once receiving an activating signal
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Processes of the Golgi Apparatus
2013-11-12 16:33:48
Cell Bio Processes

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