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