4'StructureFunction of the Organelles Synthesis

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4. Structure/Function of the Organelles -
Synthesis
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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

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Protein Synthesis and Transport within the Cell

• The Endomembrane System

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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

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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

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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

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Protein Synthesis and Transport within the Cell

• The Endomembrane System
• Autoradiography
• Example Acinar cells synthesis of digestive
  enzymes

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Protein Synthesis and Transport within the Cell

• The Endomembrane System
• Autoradiography
• Cells overlayed with photographic film
• Exposed to radioisotopes in the sample
• Radiolabeled aa

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Protein Synthesis and Transport within the Cell

• The Endomembrane System
• Autoradiography
• Tissues washed and transferred to unlabelled aa
• pulse-chase

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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

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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

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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

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Protein Synthesis and Transport within the Cell

• The Endomembrane System
• SER functions
• Skeletal muscle
• Kidney tubules
• Steriod producing cells
• Liver
• Detoxification
• Glucose-6-phosphatase

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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

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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

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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

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Protein Synthesis and Transport within the Cell

• RER functions Protein Synthesis
• Begins after a messenger RNA binds to a free
  ribosome

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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

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Protein Synthesis and Transport within the Cell

• RER functions Protein Synthesis

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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

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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)

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Protein Synthesis and Transport within the Cell

• RER functions Processing of proteins
• Misfolded proteins
• Transported to cytosol
• Destroyed in proteasomes
• Quality control