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Key concepts: Principles of signaling
Each cell in a multicellular animal is programmed to respond to a specific set of extracellular
signal molecules produced by other cells. The signal molecules act by binding to a
complementary set of receptor proteins expressed by the target cells.
Most extracellular signal molecules activate cell-surface receptor proteins, which act as signal
transducers, converting the extracellular signal into intracellular ones that alter the behavior of
the target cell.
Activated receptors relay the signal into the cell interior by activating intracellular signaling
proteins. Some of these signaling proteins transduce, amplify, or spread the signal as they relay
it, while others integrate signals from different signaling pathways. Some function as switches
that are transiently activated by phosphorylation or GTP binding.
Large signaling complexes form by means of modular interaction domains in the signaling
proteins, which allow the proteins to form functional signaling networks.
Target cells use various mechanisms, including feedback loops, to adjust the ways in which
they respond to extracellular signals. Positive feedback loops can help cells to respond in an allor-none fashion to a gradually increasing concentration of an extracellular signal and to convert
a short-lasting signal into a long-lasting, or even irreversible, response.
Negative feedback allows cells to adapt to a signal molecule, which enables them to respond to
small changes in the concentration of the signal molecule over a large concentration range.