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The Journal of Neuroscience, May 23, 2007, 27(21):5571-5583; doi:10.1523/JNEUROSCI.0310-07.2007

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Cellular/Molecular
Translocation of G_q Mediates Long-Term Adaptation in Drosophila Photoreceptors

Shahar Frechter,¹ Natalie Elia,² Vered Tzarfaty,¹ Zvi Selinger,² and Baruch Minke¹

Departments of ¹Physiology and ²Biological Chemistry and Kühne Minerva Center for Studies of Visual Transduction, The Hebrew University of Jerusalem, Jerusalem 91120, Israel

Correspondence should be addressed to Baruch Minke, Department of Physiology, The Hebrew University-Hadassah Medical School, P.O. Box 12272, Jerusalem 91120, Israel. Email: minke{at}md.huji.ac.il

Light adaptation is a process that enables photoreceptor cells to operate over a wide range of light intensities without saturation. In invertebrate photoreceptors, fast adaptation is mediated by a Ca²⁺-dependent negative-feedback mechanism, which mainly affects the terminal steps of the cascade. Therefore, the response to each photon is smaller as light intensity increases, accommodating both high sensitivity and a vast dynamic range. Here, we describe a novel type of adaptation, which is mediated by one of the first steps in the phototransduction cascade affecting the sensitivity to absorbed photons. Long exposure to light resulted in dramatic reduction in the probability of each absorbed photon to elicit a response, whereas the size and shape of each single photon response did not change. To dissect the molecular mechanism underlying this form of adaptation we used a series of Drosophila mutants. Genetic dissection showed a pivotal role for light-induced translocation of G_q between the signaling membrane and the cytosol. Biochemical studies revealed that the sensitivity to light depends on membrane G_q concentration, which was modulated either by light or by mutations that impaired its targeting to the membrane. We conclude that long-term adaptation is mediated by the movement of G_q from the signaling membrane to the cytosol, thereby reducing the probability of each photon to elicit a response. The slow time scale of this adaptation fits well with day/night light intensity changes, because there is no need to maintain single photon sensitivity during daytime.

Key words: heterotrimeric G_q protein; quantum bumps; phototransduction; light adaptation; Drosophila mutants; protein translocation

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Received Oct. 18, 2006; revised March 28, 2007; accepted April 2, 2007.

Correspondence should be addressed to Baruch Minke, Department of Physiology, The Hebrew University-Hadassah Medical School, P.O. Box 12272, Jerusalem 91120, Israel. Email: minke{at}md.huji.ac.il

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