The Extracellular Domain of Transmembrane Receptors

In many receptors, the extracellular domain contains the ligand-binding site. Glyco-sylation sites, i.e. attachment sites for carbohydrate residues, are also located nearby in the extracellular domain. 

The structure of the extracellular domain can be very diverse and is determined by the number of transmembrane sections as well as the subunit structure of the receptor. 

The extracellular localized protein portion may be formed from a continuous protein chain and may include several hundred amino acids. If the receptor crosses the membrane with several transmembrane segments, the extracellular domain is formed from several loops of the protein chain that may be linked by disulfide bridges. 

Transmembrane receptors may show homotrophic composition (identical subunits) or heterotrophic composition (different subunits, Fig. 5.2b), so that the extracellular domain may be made up of several identical or different structural elements. 

We also know of receptors in which only one subunit spans the membrane, whilst other subunits are bound to this subunit on the extracellular side via protein-protein interactions or via disulfide bridges (Fig. 5.2b and examples in Chapter 11). 

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