<Tr> <Td> </Td> <Td> This section needs expansion . You can help by adding to it . (November 2007) </Td> </Tr> <P> In some cases, receptor activation caused by ligand binding to a receptor is directly coupled to the cell's response to the ligand . For example, the neurotransmitter GABA can activate a cell surface receptor that is part of an ion channel . GABA binding to a GABA receptor on a neuron opens a chloride - selective ion channel that is part of the receptor . GABA receptor activation allows negatively charged chloride ions to move into the neuron, which inhibits the ability of the neuron to produce action potentials . However, for many cell surface receptors, ligand - receptor interactions are not directly linked to the cell's response . The activated receptor must first interact with other proteins inside the cell before the ultimate physiological effect of the ligand on the cell's behavior is produced . Often, the behavior of a chain of several interacting cell proteins is altered following receptor activation . The entire set of cell changes induced by receptor activation is called a signal transduction mechanism or pathway . </P> <P> In the case of Notch - mediated signaling, the signal transduction mechanism can be relatively simple . As shown in Figure 2, activation of Notch can cause the Notch protein to be altered by a protease . Part of the Notch protein is released from the cell surface membrane and takes part in gene regulation . Cell signaling research involves studying the spatial and temporal dynamics of both receptors and the components of signaling pathways that are activated by receptors in various cell types . </P> <P> A more complex signal transduction pathway is shown in Figure 3 . This pathway involves changes of protein--protein interactions inside the cell, induced by an external signal . Many growth factors bind to receptors at the cell surface and stimulate cells to progress through the cell cycle and divide . Several of these receptors are kinases that start to phosphorylate themselves and other proteins when binding to a ligand . This phosphorylation can generate a binding site for a different protein and thus induce protein--protein interaction . In Figure 3, the ligand (called epidermal growth factor (EGF)) binds to the receptor (called EGFR). This activates the receptor to phosphorylate itself . The phosphorylated receptor binds to an adaptor protein (GRB2), which couples the signal to further downstream signaling processes . For example, one of the signal transduction pathways that are activated is called the mitogen - activated protein kinase (MAPK) pathway . The signal transduction component labeled as "MAPK" in the pathway was originally called "ERK," so the pathway is called the MAPK / ERK pathway . The MAPK protein is an enzyme, a protein kinase that can attach phosphate to target proteins such as the transcription factor MYC and, thus, alter gene transcription and, ultimately, cell cycle progression . Many cellular proteins are activated downstream of the growth factor receptors (such as EGFR) that initiate this signal transduction pathway . </P>

Examples of plant and animal cell signaling pathways