Transmembrane domain

Transmembrane domains are a protein's functional regions that are reaching over a phospholipid bilayer. 

Integral transmembrane proteins generally possess two or more transmembrane domains. These alternate between being intracellular and extracellular domains, arranged on either the inside or outside, respectively, of the membrane.

Amino acids on the transmembrane domains allow interaction with fatty acyl groups of membrane phospholipids. This allows anchoring of the protein in the membrane .