Structural Principles of Transmembrane Receptors

Transmembrane receptors have the following roles in signal transduction  

Transmembrane receptors are integral membrane proteins, i.e., they possess a structural portion that spans the membrane. An extracellular domain, a transmembrane domain and an intracellular or cytosolic domain can be differentiated within the structure (Fig. 5.2a). 

In many receptors, the extracellular domain contains the hgand binding site. Glycosy-lation 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 

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Fig. 5.2. Structural principles of transmembrane receptors, a) Representation of the most important functional domains of transmembrane receptors, b) Examples of subunit structures. Transmembrane receptors can exist in a monomeric form (1), dimeric form (2) and as higher oligomers (3,4). Further subunits may associate at the extracellular and cytosohc domains, via disulfide bridges (3) or via non-covalent interactions (4). c) Examples of structures of the transmembrane domains of receptors. The transmembrane domain may be composed of an a-hehx (1) or several a-helices linked by loops at the cytosolic and extracellular side (2). The 7-helix transmembrane receptors are a frequently occurring receptor type (see 5.3). Several subunits of a transmembrane protein may associate into an ohgomeric structure (3), as is the case for voltage-controUed ion channels (e.g., K channel) or for receptors with intrinsic ion channel function (see Chapter 17).

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