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

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Abstract
Rhodopsin (P, .lambda.max 480 nm) of blowfly photoreceptors R1-6 is converted by light into a thermally stable metarhodopsin (M, .lambda.max 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 .fwdarw. M conversion. The 41-kDa protein might represent the .alpha.-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 .fwdarw. 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 [.gamma.-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 primairly due to a light-induced binding of the protein to the photoreceptor membrane. The binding of the 48-kDa phosphoprotein is reversible: after M .fwdarw. 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.