In addition to the lipid bilayer, the cell membrane also contains a number of proteins. While the lipid bilayer provides the structure for the cell membrane, membrane proteins allow for many of the interactions that occur between cells. Though membrane proteins are free to move within the lipid bilayer as a result of its fluidity, they can also be confined to certain areas of the bilayer with enzymes. Membrane proteins perform various functions, and this diversity is reflected in the significantly different types of proteins associated with the lipid bilayer.
Classifications of Membrane Proteins
Proteins are generally broken down into the smaller classifications of integral proteins, peripheral proteins, and lipid-anchored proteins.
- Integral proteins
Integral membrane proteins, also called intrinsic proteins, are permanently embedded within the lipid bilayer. Such proteins can be separated from the biological membranes only using detergents, nonpolar solvents, or sometimes denaturing agents.
Integral proteins float rather freely within the bilayer. In addition, integral proteins are usually transmembrane proteins, extending through the lipid bilayer so that one end contacts the interior of the cell and the other touches the exterior. Many transmembrane proteins function as gateways to permit the transport of specific substances across the biological membrane. They frequently undergo significant conformational changes to move a substance through the membrane. The stretch of the integral protein within the hydrophobic interior of the bilayer is also hydrophobic, made up of non-polar amino acids. Like the lipid bilayer, the exposed ends of the integral protein are hydrophilic. In all the transmembrane proteins examined to date, the membrane-spanning domains are α helices or multiple β strands.
- Peripheral proteins
Peripheral membrane proteins or extrinsic proteins, do not interact with the hydrophobic core of the phospholipid bilayer. They are temporarily attached either to the lipid bilayer or to integral proteins by a combination of hydrophobic, electrostatic, and other non-covalent interactions. Peripheral proteins dissociate following treatment with a polar reagent, such as a solution with an elevated pH or high salt concentrations. Peripheral proteins are less mobile within the lipid bilayer.
Peripheral proteins localized to the cytosolic face of the plasma membrane include the cytoskeletal proteins spectrin and actin in erythrocytes and the enzyme protein kinase C. This enzyme shuttles between the cytosol and the cytosolic face of the plasma membrane and plays a role in signal transduction. Other peripheral proteins, including certain proteins of the extracellular matrix, are localized to the outer (exoplasmic) surface of the plasma membrane.
- Lipid-anchored proteins
Lipid-anchored proteins are located entirely within the boundaries of the lipid bilayer. These proteins located on the surface of the cell membrane are covalently attached to lipids embedded within the cell membrane. The proteins insert and assume a place in the bilayer structure of the membrane alongside the similar fatty acid tails. The lipid-anchored protein can be located on either side of the cell membrane.Thus, the lipid serves to anchor the protein to the cell membrane.
The lipid serves as a mediator of membrane associations or as a determinant for specific protein-protein interactions. For example, lipid groups can play an important role in increasing molecular hydrophobicity. This allows for the interaction of proteins with cellular membranes and protein domains.
Overall, there are three main types of lipid-anchored proteins which include prenylated proteins, fatty acylated proteins and glycosylphosphatidylinositol-linked proteins (GPI).