Figure 20-3. Four classes of ligand-triggered cell-surface receptors.

Figure 20-3Four classes of ligand-triggered cell-surface receptors

Common ligands for each receptor type are listed in parentheses. (a) G protein – linked receptors. Binding of ligand (maroon) triggers activation of a G protein, which then binds to and activates an enzyme that catalyzes synthesis of a specific second messenger. (b) Ion-channel receptors. A conformational change triggered by ligand binding opens the channel for ion flow. (c) Tyrosine kinase – linked receptors. Ligand binding causes formation of a homodimer or heterodimer, triggering the binding and activation of a cytosolic protein-tyrosine kinase. The activated kinase phosphorylates tyrosines in the receptor; substrate proteins then bind to these phosphotyrosine residues and are phosphorylated. (d) Receptors with intrinsic ligand-triggered enzymatic activity in the cytosolic domain. Some activated receptors are monomers with guanine cyclase activity and can generate the second messenger cGMP (left). The receptors for many growth factors have intrinsic protein-tyrosine kinase activity (right). Ligand binding to most such receptor tyrosine kinase (RTKs) causes formation of an activated homodimer, which phosphorylates several residues in its own cytosolic domain as well as certain substrate proteins. [Part (c) see J. E. Darnell et al., 1994, Science 264:1415. Part (d) see S. Schulz et al., 1989, FASEB J. 3:2026; D. Garbers, 1989, J. Biol. Chem. 264:9103; and W. J. Fantl et al., 1993, Annu. Rev. Biochem. 62: 453.]

From: Section 20.1, Overview of Extracellular Signaling

Cover of Molecular Cell Biology
Molecular Cell Biology. 4th edition.
Lodish H, Berk A, Zipursky SL, et al.
New York: W. H. Freeman; 2000.
Copyright © 2000, W. H. Freeman and Company.

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