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.

Filipino MD

An independent organization catered to bringing enriching opportunities to doctors, researchers and medical institutions.

TERMS OF SERVICE
ADVERTISE
administrator@filipinomd.com

Like Us On Facebook

Facebook Pagelike Widget

You cannot copy the contents of this page.