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4. Structure/Function of the Organelles - Synthesis

Explore the structure, function, and transport processes within the endomembrane system of cells, including protein synthesis, secretion, and movement. Learn about the roles of the endoplasmic reticulum (ER) and Golgi complex, as well as the different pathways involved in protein transport.

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4. Structure/Function of the Organelles - Synthesis

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  1. 4. Structure/Function of the Organelles - Synthesis Topic 4

  2. Protein Synthesis and Transport within the Cell • The Endomembrane System • Membrane organelles – a dynamic, integrated network • Materials are shuttled – small transport vesicles • Bud from a donor membrane compartment • Move in a directed manner • Pulled by motor proteins • Tracks – microtubules of the cytoskeleton • At their destination – fuse with membrane of acceptor compartment • Repeated cycles of budding and fusion Topic 4

  3. Protein Synthesis and Transport within the Cell • The Endomembrane System Topic 4

  4. Protein Synthesis and Transport within the Cell • The Endomembrane System • Typical biosynthetic pathway • Proteins synthesised in the endoplasmic reticulum • Modified through the Golgi complex • Transported to destination • Also a secretory pathway Topic 4

  5. Protein Synthesis and Transport within the Cell • The Endomembrane System • Secretory activities – two types • Constituitive • Transported from site of synthesis and secreted continuously • Regulated • Materials to be secreted are stored in large, densely packed, membrane-bound secretory granules • Discharged following specific stimulus • Examples: endocrine cells, acinar cells, nerve cells Topic 4

  6. Protein Synthesis and Transport within the Cell • The Endomembrane System • Materials moved out by secretory pathways • Materials moved in by endocyte pathway • From outside the cells to endosomes / lysosomes • Secreted Proteins – targeted by sorting signals • In the aa sequence or • Attached oligosaccharide Topic 4

  7. Protein Synthesis and Transport within the Cell • The Endomembrane System • Autoradiography • Example: Acinar cells – synthesis of digestive enzymes Topic 4

  8. Protein Synthesis and Transport within the Cell • The Endomembrane System • Autoradiography • Cells overlayed with photographic film • Exposed to radioisotopes in the sample • Radiolabeled aa Topic 4

  9. Protein Synthesis and Transport within the Cell • The Endomembrane System • Autoradiography • Tissues – washed and transferred to unlabelled aa • “pulse-chase” Topic 4

  10. Protein Synthesis and Transport within the Cell • The Endomembrane System • Biochemical analysis of subcellular fractions • Technique to homogenize cells • Cytoplasmic membranes fragmented and form vesicles • Types separated using cell fractionation • ER and Golgi – form microsomes • Fractioned into rough and smooth Topic 4

  11. Protein Synthesis and Transport within the Cell • The Endomembrane System • The Endoplasmic Reticulum (ER) • Two categories • Rough (RER) • Smooth (SER) • Composition of luminal or cisternal space unique • RER – ribosomes bound to cytosolic surface • SER – no ribosomes • RER & SER – many complex differences Topic 4

  12. Protein Synthesis and Transport within the Cell • The Endomembrane System • RER vs SER • RER • Extensive • Interconnected sacs – cisternae • Continuous with nuclear envelope • SER • Tubular • Interconnecting pipelines Topic 4

  13. Protein Synthesis and Transport within the Cell • The Endomembrane System • SER functions • Skeletal muscle • Kidney tubules • Steriod producing cells • Liver • Detoxification • Glucose-6-phosphatase Topic 4

  14. Protein Synthesis and Transport within the Cell • The Endomembrane System • RER functions – Protein Synthesis • Polypeptides synthesized at two locales • 1. On ribosomes attached to cytosolic surface of RER • Proteins secreted from the cell • Integral membrane proteins • Soluble proteins within ER, Golgi, lysosomes, endosomes Topic 4

  15. Protein Synthesis and Transport within the Cell • The Endomembrane System • RER functions – Protein Synthesis • Polypeptides synthesized at two locales • 2. Other polypeptides – on free ribosomes • Released into the cytosol • Enzymes • Peripheral proteins – plasma membranes • Proteins nucleus • Protein mitochondria, peroxisomes Topic 4

  16. Protein Synthesis and Transport within the Cell • RER functions – Protein Synthesis • How are proteins synthesized at different sites? • Secretory proteins – signal sequence • Polypeptide moves to cisternal space of ER • cotranslationally Topic 4

  17. Protein Synthesis and Transport within the Cell • RER functions – Protein Synthesis • Begins after a messenger RNA binds to a free ribosome Topic 4

  18. Protein Synthesis and Transport within the Cell • RER functions – Protein Synthesis • Other points to note • Interaction between signal sequence and SRP – a key step • SRP is the tag – enables specific binding of the complex to the ER • Binding through at least two interactions • SRP – SRP receptor • Ribosome and translocon • Following binding • Nascent peptide released from SRP • Inserted into translocon • On termination • Ribsome released • Translocon channel reverts to narrow conformation Topic 4

  19. Protein Synthesis and Transport within the Cell • RER functions – Protein Synthesis Topic 4

  20. Protein Synthesis and Transport within the Cell • RER functions – Processing of proteins • Polypeptide is moved from the translocon to the cisterna • Enzymes in the membrane • Signal peptide removed – signal peptidase • Carbohydrates added – oligosaccharyltransferase Topic 4

  21. Protein Synthesis and Transport within the Cell • RER functions – Processing of proteins • In the lumen • Molecular chaperones – BiP (binding protein) • Recognize and bind unfolded proteins • Aid in attaining native conformation • Movement of the protein into the ER lumen • Protein disulfide isomerase (PDI) Topic 4

  22. Protein Synthesis and Transport within the Cell • RER functions – Processing of proteins • Misfolded proteins • Transported to cytosol • Destroyed in proteasomes • Quality control Topic 4

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