Organelles and cell trafficking

  • Rough endoplasmic reticulum
    • Site of synthesis of secretory (exported) proteins and of N-linked oligosaccharide addition to many proteins such as integral membrane proteins, proteins within the ER, Golgi network and lysosomes
    • Nissl bodies (RER in neurons)—synthesize peptide neurotransmitters for secretion.
    • Mucus-secreting goblet cells of the small intestine, pancreatic cells, antibody-secreting plasma cells are rich in RER.
    • Ribosomes attach to the RER via transolocon, a protein complex containing ribophorins that bind the large 60S subunit.


  • Free ribosomes—unattached to any membrane; site of synthesis of cytosol, nucleosol, peroxisome matrix, and nuclear-encoded mitochondrial  proteins.
  • Smooth endoplasmic reticulum
    • Site of steroid synthesis and detoxification of drugs and poisons. Lacks surface ribosomes.
    • Liver hepatocytes and steroid hormone–producing cells of the adrenal cortex and gonads are rich in SER.
  • Cell trafficking
    • Golgi is the distribution center for proteins and lipids from the ER to the vesicles and plasma membrane.
      • Modifies N-oligosaccharides on asparagine.
      • Adds O-oligosaccharides on serine and threonine.
      • Adds mannose-6-phosphate to proteins for trafficking to lysosomes.
    • Endosomes are sorting centers for material from outside the cell or from the Golgi, sending it to lysosomes for destruction or back to the membrane/Golgi for further use.
    • I-cell disease (inclusion cell disease/mucolipidosis type II)—inherited lysosomal storage disorder; defect in N-acetylglucosaminyl-1-phosphotransferase → failure of the Golgi to phosphorylate mannose residues (↓ mannose-6-phosphate) on glycoproteins → proteins are secreted extracellularly rather than delivered to lysosomes.
      • Results in coarse facial features, gingival hyperplasia, clouded corneas, restricted joint movements, claw hand deformities, kyphoscoliosis, and high plasma levels of lysosomal enzymes.
      • Often fatal in childhood.
  • Signal recognition particle (SRP)
    • Abundant, cytosolic ribonucleoprotein that traffics proteins from the ribosome to the RER. Absent or dysfunctional SRP → proteins accumulate in the cytosol.
  • Vesicular trafficking proteins
    • COPI: Golgi → Golgi (retrograde); cis-Golgi → ER.
    • COPII: ER → cis-Golgi (anterograde).

“Two (COPII) steps forward (anterograde); one (COPI) step back (retrograde).”

  • Clathrin: trans-Golgi Ž lysosomes; plasma membrane Ž endosomes (receptor-mediated endocytosis [eg, LDL receptor activity]).
  • Peroxisome
    • Membrane-enclosed organelle involved in:
      • β-oxidation of very-long-chain fatty acids (VLCFA)
      • α-oxidation (strictly peroxisomal process)
      • Catabolism of branched-chain fatty acids, amino acids, and ethanol
      • Synthesis of cholesterol, bile acids, and plasmalogens (important membrane phospholipid, especially in white matter of brain)
    • Zellweger syndrome—autosomal recessive disorder of peroxisome biogenesis due to mutated PEX genes. Hypotonia, seizures, hepatomegaly, early death.
    • Refsum disease—autosomal recessive disorder of α-oxidation → phytanic acid not metabolized to pristanic acid. Scaly skin, ataxia, cataracts/night blindness, shortening of 4th toe, epiphyseal dysplasia. Treatment: diet, plasmapheresis.
    • Adrenoleukodystrophy—X-linked recessive disorder of β-oxidation → VLCFA buildup in adrenal glands, white (leuko) matter of brain, testes. Progressive disease that can lead to adrenal gland crisis, coma, and death.
  • Proteasome
    • Barrel-shaped protein complex that degrades damaged or ubiquitin-tagged proteins. Defects in the ubiquitin-proteasome system have been implicated in some cases of Parkinson disease.

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