Gene expression

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Gene expression

  • Gene expression

    • Transcription
    • Translation
  • Functional proteins: post-translational modifications

    • Phosphorylation
    • Glycosylation
  • Sorting: targeting to appropriate organelles

  • Modulation by extracellular signals

    • Covalent modification
    • Association with other molecules
  • Degradation

Sorting and Secretory pathways

  • diverged in trans-Golgi

    • Constitutive pathway
      • many soluble proteins
    • Regulated pathway
      • stored in secretory vesicles
      • active transport from cytosol to vesicles
      • complexed with macromolecules (e.g. proteglycans)

Regulated secretory pathway

  • In response to extracellular stimuli: hormone, transmitters, digestive enzymes

  • stored in secretory vesicles

  • active transport from cytosol to vesicles

  • complexed with macromolecules (e.g. proteglycans) to reach high concentration

Three pathways in Golgi

  • to lysosome

  • to secretory pathways

    • Constitutive: to apical or basolateral domain
    • Regulated

“Roadmap” for traffic

  • Transport

    • Gated transport: cytosol and nucleus via nuclear pore complexes
    • Membrane transport: via membrane-bound translocators; unfolded
    • Vesicle transport: vesicles
  • Sorting signals

Membrane transport

  • Membrane-bound translocators

  • Unfolding of protein to be transported

  • Passing through a topologically distinct space: cytosol to ER; cytosol to mitochondria

Equivalent space for transport

  • Cycles of budding and fusion permits transport of molecules

Sorting signals

  • Signal sequences

    • 15-60 aa
    • Removed by signal peptidase when reaching the target
  • Signal patch

Sorting signals

Signal sequences

  • Function: specify the direction for destination

  • for initial transfer to the ER: with a signal sequence at N-terminus; consisting of 5-10 hydrophobic aa

    • Go forward Golgi: most proteins
    • Return to ER (ER residents): with a specific sequence of 4 aa at C-terminus
    • Go to mitochondria: positively charged amino acids alternate with hydrophobic ones
    • Go to peroxisome: with a signal peptide of 3 characteristic at C terminus

Signal sequences

Experiments to demonstrate “zip code”

  • Swap the signal sequence:

  • e.g.: adding the ER signal sequences to a cytosolic protein results in the retention of this protein in ER

  • Hyprophobicilty: maybe a determing factor of signal sequences

Günter Blobel

Effect of misdirected protein

Overview of protein sorting

Protein sorting

  • ER

  • ER-Golgi intermediate

  • Glogi network

Glycosylation in ER

Lysosomal protein: phosphorylation of mannose

Sorting in Golgi

  • Export

    • Lysosome
    • Secretion
      • Constitute
      • Regulated
  • Retain in Golgi

    • transmembrane

Transport to plasma membrane

  • TGN

  • Apical domain

  • Basolateral domain


  • Phagocytosis (cell eating)

    • Ingestion of large particles (e.g. bacteria)
    • In specialized cells
  • Pinocytosis (cell drinking)

    • Up-take of fluids or macromolecules in small vesicles
    • Receptor-mediated endocytosis
    • Common among eukaryotic cells


  • Pseudopodia

  • Phagosome

  • Phagolysosome

Receptor-mediated endocytosis

  • Clathrin-coated pits

  • Clathrin-coated vesicles

  • Example: cholesterol, LDL, LDL receptor

LDL: low density lipoprotein

  • Core

    • ~1500 Cholesterol ester
  • Coat

    • ~500 cholesterol
    • ~ 800 phospholipid
    • 1 Apoprotein B100

LDL receptor

  • MS Brown, JL Goldstein

  • Familial hypercholesterolemia

  • LDL-binding domain

  • Internalization signal

LDL receptor

  • Internalization signal: Tyr

Clathrin-coated pits

Lysosomal proteins in clathrin-coated pits

Sorting in early endosome

  • Acidic ph by H+ pump for dissociation of proteins

Recycling of synaptic vesicles

Protein sorting by transcytosis

  • Membrane protein in apical domain

  • Secretory proteins from bloodstream

Lysosomal system

  • For membrane-bound proteins and proteins taken by endocytosis

  • Multiple acid proteases (cathepsins) and other hydrolases

  • Studied with weak bases to inhibit lysosomal acidification or lysosomal inhibitors (E64)

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