G-protein linked receptors use G-proteins as an intermediate. - These receptors have many physiological roles, including light detection in the mammalian retina (photoreceptors), detection of odors (olfactory receptors), and the regulation of mood and behavior. For example, the receptors that bind the hormones oxytocin and vasopressin, which affect mating behavior in voles (see the opening story), are G protein-linked receptors.
of 12 34 • Many intracellular receptors alter gene expression directly. • Intracellular receptors are located inside the cell and respond to physical signals such as light or chemical signals that can diffuse across the plasma membrane (for example, steroid hormones in animals). • Many intracellular receptors are transcription factors. Some are located in the cytoplasm until they are activated; after binding their ligands, these transcription factors move to the nucleus where they bind to DNA and alter the expression of specific genes. • A typical example is the receptor for the steroid hormone cortisol . This receptor is normally bound to a chaperone protein that blocks it from entering the nucleus. Binding of the hormone causes the receptor to change its shape so that the chaperone is released. This release allows the receptor to enter the nucleus, where it affects DNA transcription. Another group of intracellular receptors are always located in the nucleus, and their ligands must enter the nucleus before binding. • The same signals can result in completely different responses - Some signal transduction pathways are quite simple and direct, whereas others involve multiple steps. As we mentioned in Section 7.1, signal transduction pathways can involve enzymes and transcription factors. In addition, small nonprotein molecules called second messengers (A compound, such as cAMP, that is released within a target cell after a hormone has bound to a surface receptor on a cell; the second messenger triggers further reactions within the cell) can diffuse throughout the cytoplasm and mediate further steps in pathways.
of 13 34 • Signal transduction generates signaling cascades - Protein kinase cascade - one protein kinase activates the next, and so on. Such cascades are key to the external regulation of many cellular activities. Protein kinase cascades are examples of signal transduction pathways - Protein kinase cascades are useful for amplification of the signal • Second messengers amplify signals - In contrast to the specificity of receptor binding, second messengers such as cAMP allow a cell to respond to a single event at the plasma membrane with many events inside the cell . Thus second messengers serve to rapidly amplify and distribute the signal—for example, binding of a single epinephrine molecule leads to the production of many molecules of cAMP, which then activate many enzyme targets by binding to them noncovalently. In the case of epinephrine and the liver cell , glycogen phosphorylase is just one of several enzymes that are activated.