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Membrane protein

Last reviewed dd mmm yyyy. Last edited dd mmm yyyy

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Membrane proteins constitutes about half of the mass of the plasma membrane. They are often very complex polypeptides which have two essential positions:

  • loosely attached to one or other side of the plasma membrane
  • traversing the membrane as alpha-helical structures:
    • termed integral proteins
    • may pass once across membrane or loop back several times e.g. neurotransmitter receptors
    • hydrophobic amino acids interact with the hydrophobic centre of the membrane
    • tend to be very mobile and this may contribute to function

The functions of plasma membrane proteins include:

  • receptors for extracellular signals e.g. hormones, antibodies, endocytotic vesicles; carbohydrate residues aid this function
  • transduction of extracellular signal: binding of a specific molecule results in a change of configuration of the protein such that its internal structure is changed; thus, secondary messenger formation is triggered internally
  • active pumps: use energy to remove certain ions or molecules e.g. Na-K ATPase
  • passive exchangers: swap molecules between intracellular and extracellular environments
  • channels, e.g. gap junctions
  • site for enzymes
  • structural proteins: tight junction, desmosomes; permit the specialization of individual membrane regions e.g. the difference between apical and basal sides of a small intestinal epithelial cell

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