Protein Targeting

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  • Protein Targeting
    • Targeting
      • Proteins move into outwards unadorned cells. Needs to be controlled by the ER vesicles and golgi
      • A short sequence of amino acids acts as an address label recognised by a sorting system. These can fold to be next to each other e.g. nuclear localisation sequence which allows entrance to the nucleus
        • Nuclear localisation sequence interacts with receptor proteins on nuclear pore which interacts with complex on fibril allowing protein to enter. Active process
      • Cytoplasmic proteins don't have address sequences (default situation)
      • Transfer to organelles depends on recognition of the address information by translocation proteins
        • Specific targeting; protein and translator complex docks with proteins on target organelle membrane. Protein release through a pore and enters the organelle. Translocationprotein may also act as a chaperone ensuring correct folding
    • Where are proteins made?
      • Ribosomes in the cytosol (make mitochondrial, nuclear, peroxisomal, cytosolic proteins)
      • Ribosomes on rough ER (make er, golgi, plasma membrane, secretory and cytosomal proteins)
    • Mitochondrial import
      • Double membrane crossed by unfolded proteins. Use 2 translocator proteins. Process; protein attaches to outer membrane. Another complex needed
    • Rough ER
      • Proteins destined for membranes of the er, golgi, plasma and secretory proteins must be inserted into or through the er membrane to reach ultimate destination
        • Some proteins stay in the ER e.g. BIP which remains and assists translocation using protein disulphide isomerase
      • Such proteins contain a signal sequence. A signal recognition particle directs er signal sequences to a specific receptor on the rough er
      • Proteolytic cleavage of the signal allows luminal release
      • For proteins that enter the secretory  pathway all the modifications occur either during translocation across the ER membrane or within the lumen
      • Stop transfer signals permit integration into the membrane. Combination of start and stop signals determine orientation of multipass membrane proteins
      • ER lumen on outside of cell. Membrane trafficking protein is responsible for delivering proteins to cell surface. Some proteins return to the er from the golgi after modification
    • Excocytosis
      • Involves transport to membrane via microtubules moving vesicles. Matching signal molecules allow docking and fusion. Can be regulative or constitutive (occurs all the time)


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