Membrane domains are regions of the plasma membrane that have a specific functional specialization. They arise from movement restriction of the components of the membrane. If we examine the detail of the structure of many transmembrane proteins, we see that they often possess three different domains, two hydrophilic and one hydrophobic.
Hydrophilic domain (formed by hydrophilic amino acids) at the N-terminal end projects to the external medium. The hydrophobic domain in the middle of the peptide chain (often only about 20-30 amino acids) is across the plasma membrane and a hydrophilic domain at the C-terminal end protrudes into the cytoplasm.
The transmembrane domain, because its amino acids have hydrophobic side chains, is "comfortable" in the hydrophobic layer of the plasma membrane.
<u>Since these transmembrane domains "anchor" the protein to the lipid bilayer, these proteins do not float freely and cannot be isolated without first dissolving the lipid bilayer with detergents.
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