Receptors in Cellular Signalling
Receptors in Cellular Signalling
Intracellular signaling is triggered by a cell surface event such as a receptor-ligand interaction, cell-cell contact, or cell-ECM contact. The contact with other cells or the environment can stimulate many cellular reactions, including proliferation, motility, differentiation, or even programmed cell death, termed apoptosis. The receptor or cell surface binding proteins are classified by the protein structure and ligand characteristics. The basic protein domains for a receptor are an extracellular ligand binding domain, membrane spanning domain, and a cytoplasmic domain. Most modern cell biology textbooks list at least four types of cell surface receptors, including the G-protein-coupled receptors, ion-channel receptors, tyrosine kinase-linked receptors, and receptors with intrinsic enzymatic activity. The G-protein-coupled receptors (GPCR) are characterized by multiple transmembrane domains (usually seven) that wind the protein in and out of the membrane in a serpentine conformation. The ion-channel receptors are closely related and actually open a membrane channel when the ligand binds. Many cytokine receptors are in the tyrosine kinase-linked class, as they lack intrinsic activity, but when the ligand binds, intracellular tyrosine kinases become activated to generate cellular changes. The classic growth factor receptors do have intrinsic kinase activity and, therefore, make up the fourth class of receptors, the receptor tyrosine kinases, or receptor serine/threonine kinases. These receptors usually have one transmembrane domain and at least two molecules must dimerize to activate the signal.
In addition to these classic receptor classes, cells can respond to their extracellular environment through integrin receptors or proteoglycan receptors such as the syndecan family. Single transmembrane domains with large extracellular and much smaller cytoplasmic domains characterize these receptors. The syndecan molecules have long glycosaminoglycan chains that assist in sequestering the fibroblast growth factors close to the cell membrane. The integrin receptors are heterodimers composed of α and β subunits. The family is very large with at least 25 integrin heterodimers, including 19 α subunits and 8 β subunits identified. The integrins do not have kinase activity, but upon binding to ECM molecules, some associated proteins become activated by autophosphorylation and then phosphorylate surrounding proteins to generate signals for specific cellular activities that usually change the actin cytoskeleton. One of the first proteins to become phosphorylated is focal adhesion kinase (FAK).
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