ABSTRACT
Cell-surface receptors are involved in transmission of extracellular signals across the plasma membrane and regulation of intracellular signal-transduction pathways mediating development and multicellular communication in all living organisms. These receptors bind a large variety of watersoluble ligands, including amines, amino acids, lipids, peptides, and proteins. For convenience, they can be sorted into four major classes with different signaling mechanisms: G-protein-coupled receptors, ion-channel receptors, cytoplasmic tyrosine kinase (CTK)-linked receptors, and receptors with intrinsic enzymatic activity. In the latter class, receptor tyrosine kinases (RTKs) are predominant, whereas guanylate cyclase receptors and serine/threonine kinase receptors are minor groups. CTK-linked receptors mediate the responses to cytokines and hormones such as erythropoietin (EPO), interferon, and growth hormone (GH). RTKs bind a variety of growth factors and hormones, such as epidermal growth factor (EGF), fibroblast growth factor (FGF), and insulin. Although RTKs and CTK-linked receptors formally belong to different classes, the signaling mechanism shows similarities regarding receptor dimerization and tyrosine phosphorylation.
