12.14:
Membrane Domains
The lipid bilayer of the cell membrane contains distinct clusters of lipids and proteins, called domains.
These domains often concentrate proteins and specific lipid molecules to increase their interactions and provide different environments in distinct portions of the membrane.
For example, an intestinal epithelial cell has apical and basolateral domains that differ in their protein and lipid composition. The apical domain lines the intestinal cavity and contains proteins involved in ion uptake and sugar metabolism.
In contrast, the basolateral domain contains proteins that interact with neighboring cells and the basement membrane and help in cell-cell communication and adhesion.
Proteins are inhibited from entering into another domain by cell-cell junctions, also known as tight junctions.
Specific membrane lipids including sphingolipids and cholesterol form microdomains known as lipid rafts. The saturated chains of sphingolipids and cholesterol are more closely packed than the phospholipids with unsaturated fatty acid tails, leading to the formation of denser and distinct lipid rafts in the membrane.
12.14:
Membrane Domains
The membrane domains concentrate specific lipids and proteins at one place within the membrane, which helps in cell signaling, adhesion, and other critical cellular processes. These domains can differ in size, composition, function, and lifespan.
Protein Domains
The membrane comprises a group of distinct proteins responsible for carrying out a cell's specific function. For example, the plasma membrane of the human sperm, or a single germ cell, contains a unique set of proteins in the anterior head, posterior head, and tail of the sperm. The exact mechanisms that allow the sperm cells to confine proteins to the specific parts without intermixing them are yet unknown. However, this example shows the level of sophistication that can be achieved by membrane proteins when it comes to forming distinct domains.
Lipid Domains
Sphingolipids, along with cholesterol, can form distinct membrane microdomains, known as lipid rafts. Caveolae are a form of lipid raft characterized by the formation of invaginations or small cave-like structures in the membrane. These lipid rafts contain protein caveolin on the cytoplasmic side of the membrane. Caveolae are present in some vertebrate cells, specifically abundant in adipocytes and endothelial cells, and play a role in cell signaling and endocytosis. Due to their small size and transient nature, lipid rafts are difficult to study in cell membranes. However, different experimental data support their presence in the biological membranes. For example, treatment of nonionic solvents such as Tween 20 gives non-soluble membrane fractions rich in sphingolipids, cholesterol, and associated proteins.