Adjustment of sensitivity following repeated stimulation; allows a cell or organism to register small changes in a signal despite a high background level of stimulation.
Enzyme that catalyzes the formation of cyclic AMP from ATP; an important component in some intracellular signaling pathways.
Enzyme that phosphorylates target proteins in response to an increase in Ca2+ ion concentration through its interaction with the Ca2+-binding protein calmodulin.
Ca2+/calmodulin-dependent protein kinase (CaM kinase)
Small Ca2+-binding protein that modifies the activity of many target proteins in response to changes in Ca2+ concentration.
The molecular mechanisms by which cells detect and respond to external stimuli and send messages to other cells.
Small intracellular signaling molecule generated from ATP in response to hormonal stimulation of cell-surface receptors.
cyclic AMP (cAMP)
Enzyme that phosphorylates target proteins in response to a rise in intracellular cyclic AMP concentration.
cyclic-AMP-dependent protein kinase (protein kinase A, PKA)
Small messenger molecule produced by the cleavage of membrane inositol phospholipids in response to extracellular signals. Helps activate protein kinase C.
Transmembrane protein that, when stimulated by the binding of a ligand, activates an intracellular enzyme (either a separate enzyme or part of the receptor itself).
A membrane-bound GTP-binding protein involved in intracellular signaling; composed of three subunits, this intermediary is usually activated by the binding of a hormone or other ligand to a transmembrane receptor.
Cell-surface receptor that associates with an intracellular trimeric GTP-binding protein (G protein) after activation by an extracellular ligand. These receptors are embedded in the membrane by seven transmembrane α helices.
G-protein-coupled receptor (GPCR)
Intracellular signaling protein whose activity is determined by its association with either GTP or GDP. Includes both trimeric G proteins and monomeric GTPases, such as Ras.
Extracellular signal molecule that is secreted and transported via the bloodstream (in animals) or the sap (in plants) to target tissues on which it exerts a specific effect.
Small intracellular signaling molecule that triggers the release of Ca 2+ from the endoplasmic reticulum into the cytosol; produced when a signal molecule activates a membrane-bound protein called phospholipase C.
inositol 1,4,5-trisphosphate (IP3)
Minor lipid component of plasma membranes that plays a part in signal transduction in eukaryotic cells; cleavage yields two small messenger molecules, IP3 and diacylglycerol.
A set of proteins and small-molecule second messengers that interact with each other to relay a signal from the cell membrane to its final destination in the cytoplasm or nucleus.
intracellular signaling pathway
Transmembrane receptor protein or protein complex that opens in response to the binding of a ligand to its external face, allowing the passage of a specific inorganic ion.
Secreted signal molecule that acts at a short range on adjacent cells.
Mitogen-activated protein kinase. Signaling molecule that is the final kinase in a three-kinase sequence called the MAP-kinase signaling module.
Set of three functionally interlinked protein kinases that allows cells to respond to extracellular signal molecules that stimulate proliferation; includes a mitogen-activated protein kinase (MAP kinase), a MAP kinase kinase, and a MAP kinase kinase kinase.
MAP-kinase signaling module
Intracellular signaling protein that toggles between an active and inactive state in response to receiving a signal.
Small, single-subunit GTP-binding protein. Proteins of this family, such as Ras and Rho, are part of many different signaling pathways.
Locally acting gaseous signal molecule that diffuses across cell membranes to affect the activity of intracellular proteins.
nitric oxide (NO)
Protein inside a eukaryotic cell that, on binding to a signal molecule, enters the nucleus and regulates transcription.
Enzyme that phosphorylates inositol phospholipids in the plasma membrane, which generates docking sites for intracellular signaling proteins that promote cell growth and survival.
phosphoinositide 3-kinase (PI 3-kinase)
Enzyme associated with the plasma membrane that generates two small messenger molecules in response to activation.
Enzyme that phosphorylates target proteins in response to a rise in diacylglycerol and Ca2+ ions.
protein kinase C (PKC)
One of a large family of small GTP-binding proteins (the monomeric GTPases) that helps relay signals from cell-surface receptors to the nucleus. Many human cancers contain an overactive mutant form of the protein.
Protein that recognizes and responds to a specific signal molecule.
Enzyme-coupled receptor that phosphorylates target proteins on serine or threonine.
receptor serine/threonine kinase
Enzyme-coupled receptor in which the intracellular domain has a tyrosine kinase activity, which is activated by ligand binding to the receptor’s extracellular domain.
receptor tyrosine kinase (RTK)
Small intracellular signaling molecule generated or released in response to an extracellular signal. Examples include cAMP, IP3, and Ca2+.
Enzyme that phosphorylates target proteins on serines or threonines.
Conversion of an impulse or stimulus from one physical or chemical form to another. In cell biology, the process by which a cell responds to an extracellular signal.
Nucleotide, lipid, ion, or other small molecule generated or released in response to an extracellular signal. Examples include cAMP, IP3, and Ca2+. Also called second messengers.
small intracellular signaling molecule
Hydrophobic signal molecule related to cholesterol; can pass through the plasma membrane to interact with intracellular receptors that affect gene expression in the target cell. Examples include estrogen and testosterone.
Small signaling molecule, made and secreted by cells, that acts on neighboring cells to alter their behavior. Usually a protein, polypeptide, or glycoprotein.
Sugar molecule with six hydroxyl groups that forms the structural basis for inositol phospholipids, which can act as membrane-bound signaling molecules.
Sequence of linked reactions, often including phosphorylation and dephosphorylation, that carries information within a cell, often amplifying an initial signal.
High-energy bond formed by a condensation reaction between an acid (acyl) group and a thiol group (–SH); seen, for example, in acetyl CoA and in many enzyme–substrate complexes.