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adenylyl cyclase

An enzyme that converts ATP to cyclic AMP in response to a chemical signal.

cyclic AMP (cAMP)

Cyclic adenosine monophosphate, a ring-shaped molecule made from ATP that is a common intracellular signaling molecule (second messenger) in eukaryotic cells (for example, in vertebrate endocrine cells). It is also a regulator of some bacterial operons.

diacylglycerol (DAG)

A second messenger produced by the cleavage of a certain kind of phospholipid in the plasma membrane.

G protein

A GTP-binding protein that relays signals from a plasma membrane signal receptor, known as a G-protein-linked receptor, to other signal transduction proteins inside the cell. When such a receptor is activated, it in turn activates the G protein, causing it to bind a molecule of GTP in place of GDP. Hydrolysis of the bound GTP to GDP inactivates the G protein.

G-protein-linked receptor

A signal receptor protein in the plasma membrane that responds to the binding signal molecule by activating a G protein.


In multicellular organisms, one of many types of circulating chemical signals that are formed in specialized cells, travel in body fluids, and act on specific target cells to change their functioning.

inositol trisphosphate (IP3)

A second messenger that functions as an intermediate between certain nonsteroid hormones and a third messenger, a rise in cytoplasmic Ca2+ concentration.


A molecule that binds specifically to a receptor site of another molecule.

ligand-gated ion channel

A protein pore in the plasma membrane that opens or closes in response to a chemical signal, allowing or blocking the flow of specific ions.

local regulator

A chemical messenger that influences cells in the vicinity.

protein kinase

An enzyme that transfers phosphate groups from ATP to a protein.

protein phosphatase

An enzyme that removes phosphate groups from proteins, often functioning to reverse the effect of a protein kinase.


In cellular communication, the target cell's detection (by binding to a receptor protein) of a signal molecule from outside the cell.

receptor tyrosine kinase

A receptor protein in the plasma membrane that responds to the binding of a signal molecule by catalyzing the transfer of phosphate groups from ATP to tyrosines on the cytoplasmic side of the receptor. The phosphorylated tyrosines activate other signal transduction proteins within the cell.


In cellular communication, the change in a specific cellular activity brought about by a transduced signal from outside the cell.

scaffolding protein

A type of large relay protein to which several other relay proteins are simultaneously attached to increase the efficiency of signal transduction.

second messenger

A small, nonprotein, water-soluble molecule or ion, such as calcium ion or cyclic AMP, that relays a signal to a cell's interior in response to a signal received by a signal receptor protein.

signal transduction pathway

A mechanism linking a mechanical or chemical stimulus to a specific cellular response.

transduction (1)

A DNA transfer process in which phages carry bacterial genes from one host cell to another. (2) In cellular communication, the conversion of a signal from outside the cell to a form that can bring about a specific cellular response.

tyrosine kinase

An enzyme that catalyzes the transfer of phosphate groups from ATP to the amino acid tyrosine on a substrate protein.

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