Components of the extracellular matrix such as fibronectin and hyaluronan can also bind to such receptors ( integrins and CD44, respectively). These include growth factors, cytokines and neurotransmitters. Most ligands are soluble molecules from the extracellular medium which bind to cell surface receptors. The binding of a signaling molecule with a receptor causes a change in the conformation of the receptor, known as receptor activation. The majority of signal transduction pathways involve the binding of signaling molecules, known as ligands, to receptors that trigger events inside the cell. With the advent of computational biology, the analysis of signaling pathways and networks has become an essential tool to understand cellular functions and disease, including signaling rewiring mechanisms underlying responses to acquired drug resistance. As with other signals, the transduction of biological signals is characterised by delay, noise, signal feedback and feedforward and interference, which can range from negligible to pathological. Depending on the efficiency of the nodes, a signal can be amplified (a concept known as signal gain), so that one signaling molecule can generate a response involving hundreds to millions of molecules. Such effectors are typically proteins and are often linked to second messengers, which can activate secondary effectors, and so on. Ligands are termed first messengers, while receptors are the signal transducers, which then activate primary effectors. In multicellular organisms, signal transduction pathways regulate cell communication in a wide variety of ways.Įach component (or node) of a signaling pathway is classified according to the role it plays with respect to the initial stimulus. These molecular events are the basic mechanisms controlling cell growth, proliferation, metabolism and many other processes. At the molecular level, such responses include changes in the transcription or translation of genes, and post-translational and conformational changes in proteins, as well as changes in their location. When signaling pathways interact with one another they form networks, which allow cellular responses to be coordinated, often by combinatorial signaling events. The changes elicited by ligand binding (or signal sensing) in a receptor give rise to a biochemical cascade, which is a chain of biochemical events known as a signaling pathway. Proteins responsible for detecting stimuli are generally termed receptors, although in some cases the term sensor is used. Most commonly, protein phosphorylation is catalyzed by protein kinases, ultimately resulting in a cellular response. Signal transduction is the process by which a chemical or physical signal is transmitted through a cell as a series of molecular events. For systemic signal transduction, see Transduction (physiology). This article is about signaling at the cellular level.
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