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Lecture 9-2

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Lecture 9-2

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    1. Topic 9-2 1

    2. Topic 9-2 2 RER functions Synthesis of Integral Membrane Proteins Membrane proteins translocated into the ER membrane cotranslationally Contain one or more hydrophobic domains Block movement into the ER lumen Stop-transfer sequences > 15 aa hydrophobic or uncharged

    3. Topic 9-2 3 RER functions Synthesis of Integral Membrane Proteins If SRP signal sequence = stop sequence termed a stop anchor sequence Membrane alignment determined by +ve charged aa residues More +ve end faces cytosol

    4. Topic 9-2 4 RER functions Synthesis of Integral Membrane Proteins N- or C- terminus faces cytosol

    5. Topic 9-2 5 RER functions Synthesis of Integral Membrane Proteins multi-membrane-spanning proteins Sequential TM segments have opposite orientations

    6. Topic 9-2 6 Membranes arise from pre-existing membranes Membrane components ER other compartments Its proteins and lipids are modified Membranes in each compartment have unique character

    7. Topic 9-2 7 Membranes arise from pre-existing membranes Membranes show asymmetry maintained through all compartments Synthesis of Membrane Lipids Entirely within the ER Exceptions sphingomyelin, glycolipids (Golgi) Mitochondrial lipids

    8. Topic 9-2 8 Synthesis of Membrane Lipids Phospholipid synthesizing enzymes Integral proteins of the ER membrane Active sites facing cytosol Newly synthesized phospholipids Inserted into the cytosolic side Some moved to the opposite leaflet enzymes flippases Lipids carried in transport vesicles as part of membrane

    9. Topic 9-2 9 Membranes of different organelles Markedly different lipid composition Organelles contain enzymes modify lipids During budding some types preferentially included Phospholipid-transfer proteins Transfer specific phospholipids from one membrane compartment to another

    10. Topic 9-2 10 Transitional elements of the RER Devoid of ribosomes Exit site for budding transport vesicles After they bud transport vesicles fuse Larger vesicles Interconnected tubules The endoplasmic reticulum Golgi intermediate compartment (ERGIC)

    11. Topic 9-2 11 Structure Characteristic morphology Flattened disc-like cisternae Associated vesicles and tubules Cisternae in a stack A few to several thousand

    12. Topic 9-2 12 Structure Several compartments Cis face toward ER Medial Trans face

    13. Topic 9-2 13 Structure Cis Interconnecting tubules the cis Golgi network CGN Sorting station Proteins sent back to ER Others allowed to proceed to next station

    14. Topic 9-2 14 Structure Trans Trans Golgi network TGN Sorting station Proteins to plasma membrane Other intracellular destinations

    15. Topic 9-2 15 Function Newly synthesized proteins enter at the cis face Sequentially modified Length trimmed Oligosaccharides modified Glycoproteins and Glycolipids Synthesis of complex polysaccharides

    16. Topic 9-2 16 Most vesicles covered by a protein coat Coated vesicles Protein coats two functions Cause the membrane to curve Mechanism for selective transport Protein coats have specific affinity for cytosolic domains of integral proteins in the donor membrane

    17. Topic 9-2 17 Most vesicles covered by a protein coat Several classes of coated vesicles COPII-coated vesicles Move materials forward to ERGIC COPI-coated vesicles Move materials in a retrograde manner Clathrin-coated vesicles TGN Endosomes, lysosomes, endocytic pathway

    18. Topic 9-2 18 COPII-coated vesicles Select and concentrate transport components Integral membrane proteins of the ER Enzymes acting at a later stage Membrane proteins docking and fusion Membrane proteins binding soluble cargo secretory proteins

    19. Topic 9-2 19 COPI-coated vesicles Proteins of ER have C-terminal retrieval sequence Specific receptors Soluble ER proteins typically have - KDEL KDEL receptor cytosolic domain binds to the COPI coat

    20. Topic 9-2 20 COPI-coated vesicles Integral membrane proteins of ER SRP receptor KKXX

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