5.7: Protein Associations
Proteins in the plasma membrane are critical for normal cell function, and membrane proteins fall into two major categories.
Integral proteins insert partially or fully through the membrane or, in some cases, are bound very tightly to another integral protein. Peripheral proteins do not cross the membrane but are instead linked to the membrane by weaker interactions with integral proteins.
Integral proteins are usually amphipathic molecules. This means that they contain regions that are hydrophilic—attracted to water—and regions that are hydrophobic—water-repellent. The hydrophilic regions of integral protein orient to the exterior of the plasma membrane, facing either the cytoplasm within the cell or the extracellular fluid on the outside, while the hydrophobic regions are found in proximity with the lipid tails of the phospholipid bilayer.
Transmembrane proteins—a type of integral protein that spans the entire plasma membrane—often have important roles in transporting molecules or ions across the membrane, or as receptors that trigger signaling cascades within the cell.
The portion of the protein that passes through the membrane can be a single alpha helix, multiple alpha helices, or a larger beta barrel containing a pore. Some proteins also possess a lipid chain to aid in anchoring to the membrane.
Peripheral proteins interact with the membrane by contacting integral proteins, or by contacting other structures that are found in the membrane. They often have important roles in signaling within the cell, so they need to dissociate easily to carry out their functions.