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.
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.
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.