membrane protein

Last reviewed 01/2018

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