G proteins in vision

See also Antioxidants, Reactive Oxygen, Oxygen Metabolism and Human Disease, P-Carotene, G Proteins in Vision, Lipid-Soluble Vitamins, Vitamins... 

See also G Proteins in Vision, Vitamin A, Chemistry of Photo reception... 

See also G Protein Families and Subunits, Signal Transduction Agonists and Antagonists, G proteins in vision. Figure 12.13, Metabolic Control Mechanisms (from Chapter 12)... 

See also Nucleotides, Guanine, DeNovo Biosynthesis of Purine Nucleotides, G Proteins in Vision... 

See also GMP, GTP, cAMP, Actions of Nitric Oxide, Transducin, G Proteins in Vision, Second... 

See also Kinase Cascade, G Proteins and Signal Transduction, G Proteins in Vision, Kinase Cascade in Fat Mobilization (from Chapter 18)... 

The role of visual rhodopsins is to activate transducin, a heterotrimeric G protein, in the signal transduction cascade of vision [6,7,24]. Rhodopsin, a member of the... 

The PC is actually the sum of two processes a fast component, which saturates only at extremely high irradiance levels and a slow component (69). The maximal amplitude amounts only to 10% of the fast component. The fast potential depends solely on the photoconversion rate of the photoreceptor and is the result of a localized calcium influx (67, 68). The late PC is driven by the transport of about 10 elementary charges across the membrane triggered by the absorption of approximately 10 photons. This means there is an amplification of about 10,000 (61). The amplification could be due to the activation of GTP in animal vision one excited rhodopsin can activate up to 500 G proteins, which in turn activate thousands of phosphodiesterase molecules. In Spermatozopsis evidence for the involvement of G-proteins in photoperception was presented (57, 58, 70). Light-dependent GTPase activity in isolated eyespot apparatuses was found with an action spectrum similar to that of rhodopsin absorption. 

The a subunits and the Py complex have actions independent of those on adenylyl cyclase (see Figure 43-4 and Table 43-3). Some forms of tt stimulate channels and inhibit Ca channels, and some ttj molecules have the opposite effects. Members of the G, family activate the phospholipase C group of enzymes. The py complexes have been associated with channel stimulation and phospholipase C activation. G proteins are involved in many important biologic processes in addition to hormone action. Notable examples include olfaction (oColf) <1 vision (aj. Some examples are listed in Table 43-3. GPCRs are implicated in a number of diseases and are major targets for pharmaceutical agents. 

The best investigated is the desensitization of the adrenaline receptor type P2 and of rhodopsin. Rhodopsin has the function of a light receptor in the process of vision, ft receives light signals and conducts them to the relevant G-protein, transducin. The key reaction in desensitization of both systems is the phosphorylation of the receptor at the cytoplasmic side by specific protein kinases. 

The Gt- and Gg-proteins are also classed as Gi-proteins, based on sequence homologies. The Gt- and Gg-proteins are involved in transmitting sensory signals. Signal transmission in the vision process is mediated via G-proteins known as transducins (Gt). The Gt-proteins are activated by the photoreceptor rhodopsin and are located in the rods and cones of the retina. The sequential effector molecules of the Gt-proteins are cGMP-specific phosphdiesterases (see Fig. 17.9). 

Some details about cone cells and invertebrate vision. The biochemistry of retinal cones is less well known but is similar to that of rod cells. Cone pigments are present in the plasma membrane rather than in isolated discs (Fig. 23-40C). Different a, P, and y subunits of transducin are formed in rods and cones.522 Many differences are seen among various invertebrate visual systems. Inositol triphosphate (IP3) and Ca2+ often serve as signals of photoexcitation. G proteins also play prominent roles.522... 

An experimental difficulty lies in the fact that there are only a few thousand taste buds in the tongue, with only 50-100 cells in a bud. They age rapidly, having a lifespan of only about ten days.924 There may be only 30,000-50,000 hard-to-isolate taste receptor cells on the tongue s surface.923 However, very recently published reports describe a large family of bitter and sweet receptors in mice and humans924-928 and in Drosophila.929 930 The sweet-sour receptors are thought to activate a G protein called gustducin,931/932 which plays a role similar to that of transducin in vision and... 

The sequence of events involved in the vision process are listed in Fig. 3. The overall process triggered by the 11 -cis retinyl chromophore is very specific to the protein medium. Through cis-trans isomerization of the chromophore, light energy is transduced into chemical free energy, which in turn is utilized to cause conformational changes in the protein and ultimately activate the retinal G-protein. The important role of the protein... 

G-protein-coupled receptors (GPCRs), also known as seven transmembrane receptors (7TMs), are the largest known superfamily of proteins. They are involved in all types of responses to stimuli, from intercellular communication to the senses of vision, taste, and smell. They respond to diverse ligands ranging from photons (e.g., rhodopsin. Fig. 1) to small molecules (e.g., binding of epinephrine to the f52-adrenergic receptor) and... 

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