The Rhodopsin-Transducin-cGMP-Phosphodiesterase Cascade of Visual Transduction
The light activated cascade of vision shares many features with transduction pathways in hormonal systems involving G-proteins (L. Stryer, 1986, M. Chabre et al., 1984, L. Stryer and H.R. Bourne, 1986). Indeed rhodopsin can be considered as a particular hormone receptor where the retinal, the equivalent of an hormone, is already bound but will become fully active only after illumination. Apart from this original aspect, rhodopsin clearly appears as a member of the “seven α-helix” receptors family which includes the M1 and M2 muscarinic, the α2, β1 and β2 adrenergic and the substance K receptors (H. G. Dohlman et al., 1987). In all cases, activation of the receptors triggers an enzymatic cascade involving interactions with G-proteins which are thus allowed to exchange GDP for GTP, become active and interact with effector proteins that control an internal messenger. Many analogies (in the subunit structure of the G-proteins, in the mechanisms of activation at the different steps of the cascades) have been documented: evolution seems to have conserved typical key structures. The visual system is however special in many respects: (i) rhodopsin is exceptionally abundant in the photoreceptor cell, at least thousand times more abundant than receptors in hormone responsive cells. This extremely dense population of receptors is here necessary in order to capture efficiently photons. The total number of rhodopsins is ten times higher than the total number of G proteins, transducins. However, at a physiological level of illumination, the number of transducins involved in transduction will be much higher than that of photoexcited rhodopsins, like in hormone responsive systems. (ii) Transducin (T) and the cGMP phosphodiesterase (PDE, the effector protein of the visual system) are soluble proteins that can be detached from the disc membrane in the absence of detergent. (iii) The system is conveniently excited by light flashes. (iv) Spectral properties of the retinal buried in the receptor allow time-resolved studies of the conformations of the receptor. A simplified sketch of the complete transduction process, from the photon to the closure of the cGMP dependent cationic channels in the rod cell plasma membrane is shown on fig. 1. Here we shall only discuss the amplifying cascade from rhodopsin to the cGMP phosphodiesterase.
KeywordsNucleotide Site Disc Membrane Inhibitory Subunit Lateral Diffusion Coefficient cGMP Phosphodiesterase
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