Signal Transduction

Fig 6.12  Activation and inactivation of transducin.

Photoactivated rhodopsin Rh* catalyses guanine nucleotide exchange on the -subunit of transducin, which is then released into the cytosol. Here it activates phosphodiesterase, causing hydrolysis of cGMP. The consequent reduction in the concentration of cGMP allows the membrane ion channels to close, curbing the inflow of Na and Ca2 . The resulting membrane hyperpolarization acts as the signal to restrict the secretion of transmitter from the synaptic terminal. Inactivation of transducin occurs through hydrolysis of the bound GTP. The basal catalytic rate of the GTPase is enhanced 100 times through interaction with the phosphodiesterase, together with the heterodimeric complex of RGS9 and G 5.

target enzyme, the phosphodiesterase, which also initiates GTPase activation. However, under no circumstances must the activated transducin be allowed to linger. In mammalian rods, the secondary interaction of the transducin– PDE complex with the RGS9–G 5 complex ensures that inactivation occurs within 200 ms. These mechanisms are outlined in Figure 6.12. A full and detailed description of the inactivation process may be found in Arshavsky et al.30

Note on phototransduction in invertebrates

We have described the basic elements of the signal transduction process as it occurs in the rod cells of vertebrate eyes. The situation in invertebrates is very different, as summarized in Figure 6.13.

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The Regulation of Visual Transduction and Olfaction

Fig 6.13  Comparing the main features of retinal phototransduction in vertebrates and flies.

The main elements of visual phototransduction in vertebrates (a) and in Drosophila (b, c) are illustrated. For further detail, see text. The diagram in

(c) illustrates the rhodopsin cycle in Drosophila in which the chromophore remains attached to the receptor throughout the process of activation and subsequent recovery. As with vertebrates, activation is initiated by a photon that causes the conversion of 11-cis-retinal to the all-trans form. In the invertebrate however, recovery is effected by a second, longer-wavelength pulse of light following phosphorylation of the rhodopsin.

In some ways, phototransduction in flies and other spineless creatures appears to follow more familiar pathways. Here, the light-activated rhodopsin is coupled through the G protein Gq, not transducin. This regulates phospholipase C , producing IP3 and DAG, and results in the elevation of intracellular Ca2 and the activation of protein kinase C. As with vertebrates, these events initiate both electrical and chemical signals, but here the consequence is the opening, not the closure of plasma membrane ion channels. This causes a very transient depolarization (instead of hyperpolarization) and further elevation of the concentration of cytosol

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