Transmembrane receptor

Domains

Like any integral membrane protein, a transmembrane receptor may be subdivided into three parts or domains.

E=extracellular space; I=intracellular space; P=plasma membrane

The extracellular domain

The extracellular domain is the part of the receptor that sticks out of the membrane on the outside of the cell or organelle. If the polypeptide chain of the receptor crosses the bilayer several times, the external domain can comprise several "loops" sticking out of the membrane. By definition, a receptor's main function is to recognize and respond to a specific ligand, for example, a transmembrane potential), and in many receptors these ligands bind to the extracellular domain.

The transmembrane domain

In the majority of receptors for which structural evidence exists, transmembrane alpha helices make up most of the transmembrane domain. In certain receptors, such as the bacteriorhodopsin, the detailed structure of which has been determined by crystallography).

The intracellular domain

The intracellular (or cytoplasmic) domain of the receptor interacts with the interior of the cell or organelle, relaying the signal. There are two fundamentally different ways for this interaction:

• The intracellular domain communicates via specific protein-protein-interactions with effector proteins, which in turn send the signal along a signal chain to its destination.
• With enzyme-linked receptors, the intracellular domain has tyrosine kinase activity. The enzymatic activity can also be located on an enzyme associated with the intracellular domain.