Protein Phosphorylation/Dephosphorylation and Reversible Phosphoprotein Binding in Rhabdomeric Photoreceptors
In the rhabdomeric photoreceptors of invertebrates, the transduction machinery is located in the rhabdomere, a part of the cell which is elaborated into a stack of photosensitive microvilli. The activation of the phototransduction mechanism causes this photoreceptor cell to depolarize, i.e., cation channels open in response to light. Recent progress in the biochemistry of invertebrate photoreceptors suggests that at least the molecular events leading to the activation of the phototransduction process are similar to that verified for the light-triggered enzyme cascade of the ciliary photoreceptors of vertebrates. It is a particular property of many invertebrate visual pigment systems, that the transduction mechanism is triggered by the conversion of rhodopsin (P) into a long-lived (thermostable) metarho- dopsin (M)1,2. M in turn, is reconverted by light to P. This photoregeneration constitutes one of the main pathways of visual pigment regeneration in the living animal3. As photoregeneration is also possible in isolated membranes, invertebrate photoreceptors offer the unique opportunity to study the reversibility of reactions coupling photochemical and enzymatic transduction steps. This for example permits one to investigate the interaction of M with other proteins and to characterize the reactions leading to a reversible inactivation of the photoactivated rhodopsin state.
KeywordsVisual Pigment Photoreceptor Membrane Transduction Machinery Phosphate Binding Site Visual Transduction
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