Light‐modulated ADP‐ribosylation, protein phosphorylation and protein binding in isolated fly photoreceptor membranes

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Abstract
Rhodopsin (P, λmax 480 nm) of blowfly photoreceptors R1–6 is converted by light into a thermally stable metarhodopsin (M, λmax 565 nm). In isolated blowfly rhabdoms photoconversion of P to M affects bacterial toxin‐catalyzed ADP‐ribosylation of a 41‐kDa protein, activates phosphorylation of opsin and induces the binding of a 48‐kDa phosphoprotein to the rhabdomeric membrane.ADP‐ribosylation of the 41‐kDa protein is catalyzed by cholera toxin and is inhibited by P → M conversion. The 41‐kDa protein might represent the α‐subunit of the G‐protein, proposed to be part of the phototransduction mechanism [Blumenfeld, A. et al. (1985) Proc. Natl Acad. Sci. USA 82, 7116–7120].P → M conversion leads to phosphorylation of opsin at multiple binding sites: up to 4 mol phosphate are bound/mol M formed. Dephosphorylation of the phosphate binding sites is induced by photoconversion of M to P. High levels of calcium (2 mM) inhibit phosphorylation of M and increase dephosphorylation of P.Protein patterns obtained by sodium dodecyl sulfate gel electrophoresis of irradiated retina membranes show an increased incorporation of label from [γ‐32P]ATP also into protein bands of 48 kDa, 68 kDa and 200 kDa. Binding studies reveal that in the case of the 48‐kDa protein this effect is primarily due to a light‐induced binding of the protein to the photoreceptor membrane. The binding of the 48‐kDa phosphoprotein is reversible: after M → P conversion the protein becomes extractable by isotonic buffers. These data suggest that in rhabdomeric photoreceptors of invertebrates light‐activation of rhodopsin is coupled to an enzyme cascade in a similar way as in the ciliary photoreceptors of vertebrates, although there may be differences, e.g. in the type of G‐protein which mediates between the activated state of metarhodopsin and a signal‐amplifying enzyme reaction.

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