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Greatly reduced

Gta activation

Gta activation

Activation of

Slightly reduced

Greatly reduced

▲ FIGURE 13-26 Role of opsin phosphorylation in adaptation of rod cells to changes in ambient light levels. Light-activated opsin (O*), but not dark-adapted rhodopsin, is a substrate for rhodopsin kinase. The extent of opsin phosphorylation is directly proportional to the amount of time each opsin molecule spends in the light-activated form and thus to the average ambient light level over the previous few minutes. The ability of O* to activate

Gta is inversely proportional to the number of phosphorylated residues. Thus the higher the ambient light level, the greater the extent of opsin phosphorylation and the larger the increase in light level needed to activate the same number of Gta (transducin) molecules. At very high light levels, arrestin binds to the completely phosphorylated opsin, forming a complex that cannot activate transducin at all. [See L. Lagnado and D. Baylor, 1992, Neuron 8:995, and A. Mendez et al., 2000, Neuron 28:153.]

Gta activation

Gta activation

Gta activation

■ The effector protein activated by Gta-GTP is cGMP phosphodiesterase. Reduction in the cGMP level by this enzyme leads to closing of cGMP-gated Na+/Ca2+ channels, hyperpolarization of the membrane, and decreased release of neurotransmitter (see Figure 13-24).

■ As with other Ga proteins, binding of GTP to Gta causes conformational changes in the protein that disrupt its molecular interactions with Gp7 and enable Gta-GTP to bind to its downstream effector (see Figure 13-25).

■ Phosphorylation of light-activated opsin by rhodopsin kinase and subsequent binding of arrestin to phosphory-lated opsin inhibit its ability to activate transducin (see Figure 13-26). This general mechanism of adaptation, or desensitization, is utilized by other GPCRs at high ligand levels.

▲ EXPERIMENTAL FIGURE 13-27 Movement of Gt from outer segments of rod cells contributes to visual adaptation.

As shown by immunofluorescence staining of retinas of dark-adapted rats, both the a and p subunits of transducin (Gat and Gpt) are localized to the outer segments (OS) of rod cells, where they can be activated by rhodopsin photoreceptors in the membrane disks (see Figure 13-22). After several minutes of bright light most of the transducin a and p subunits have moved to the inner segment (IS) of the rod cells, where they cannot interact with active opsin; this contributes to desensitization of rod cells at high light intensities. [From M. Sokolov et al., 2002, Neuron 33:95. Courtesy of Vadim Arshavsky, Harvard Medical School.]

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