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G-protein deactivation is rate-limiting for shut-off of the phototransduction cascade

Nature volume 389, pages 392–395 (1997)Cite this article

Abstract

Photoreceptors detect light through a seven-helix receptor (rhodopsin) and heterotrimeric G protein (transducin) coupled to a cyclic GMP phosphodiesterase1,2. Similar pathways are used to amplify responses to hormones, taste and smell3,4,5. The amplification of phototransduction is reduced by a fall in cytoplasmic Ca2+ (refs 6, 7, 8, 9, 10), but it is not known how the deactivation of rhodopsin and transducin influence this response and hence the extent and duration of phosphodiesterase activity11,12,13,14. Here we investigate this by recording the electrical response to flashes of light in truncated rod photoreceptors10. By removing ATP to block the deactivation of rhodopsin by phosphorylation15, we show that this reaction limits the amplitude of the response and begins within 3.2 s of a flash in a solution containing 1 μM Ca2+, falling to 0.9 s in a zero-Ca2+ solution. In contrast, the activation and amplitude of the response were unaffected when transducin deactivation by GTP hydrolysis was blocked by replacing GTP with its non-hydrolysable analogue GTP-γS11, demonstrating that there is little GTP hydrolysis occurring over the period in which photoexcited rhodopsin is quenched. The rapid deactivation of rhodopsin is therefore a Ca2+-sensitive step controlling the amplitude of the light response, whereas transducin deactivation is slower and controls recovery.

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Figure 1: Quenching of photoexcited rhodopsin is apparent after a short delay.
Figure 2: Quenching of photoexcited rhodopsin is effectively complete at the peak of the flash response.
Figure 3: Removing Ca2+ accelerates the quenching of photoexcited rhodopsin.

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Acknowledgements

We thank C. Raeburn of the LMB Instrumentation Workshop for assistance. This work was supported by the HFSP. M.S.S. held a scholarship of the University of Cambridge M.B, Ph.D. Programme.

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  1. MRC Laboratory of Molecular Biology, Hills Road, CB2 2QH, Cambridge, UK

    Mandeep S. Sagoo

  2. MRC Laboratory of Molecular Biology, Hills Road, CB2 2QH, Cambridge, UK

    Leon Lagnado

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Correspondence to Leon Lagnado.

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Sagoo, M., Lagnado, L. G-protein deactivation is rate-limiting for shut-off of the phototransduction cascade. Nature 389, 392–395 (1997). https://doi.org/10.1038/38750

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