![]() Ca 2+ influx via TRP channels is essential for both amplification and rapid kinetics ( Hardie 1991 Henderson et al. Photoisomerization of 1 rhodopsin results in generation of a quantum bump ∼10 pA in amplitude representing the opening of ∼15 TRP channels in a single microvillus of the light-absorbing rhabdomere ( Henderson et al. 2010 Hardie and Postma 2008 Yau and Hardie 2009). The light response is characterized by high sensitivity, rapid kinetics, and wide dynamic range, in part achieved by the ultracompartmentalization inherent in the microvillar design of the photoreceptors ( Fain et al. Phototransduction in drosophila is mediated by a PLC cascade, culminating in activation of two distinct Ca 2+-permeable channels encoded by the transient receptor potential ( trp) and trp-like ( trpl) genes (reviews: Hardie 2012 Katz and Minke 2009 Montell 2012). By contrast, quantum bump generation is reliable but delayed until sufficient G proteins and PLC are activated to overcome threshold, thereby ensuring generation of full-size bumps with high quantum efficiency. Our results indicate that RTP, NINAC, INAD, and diacylglycerol kinase, together with a Ca 2+-dependent threshold, share common roles in suppressing dark noise and regulating quantum bump generation consequently, most spontaneous G protein activations fail to generate dark events under normal conditions. Raising cytosolic Ca 2+ in the submicromolar range increased dark noise, facilitated activation of transient receptor potential (TRP) channels by exogenous agonist, and again facilitated light responses in Gα q hypomorphs. ninaC, rtp, and rdgA/+ mutations also all facilitated residual light responses in Gα q and PLC hypomorphs. Dark noise in ninaC, rtp, and rdgA/+ mutants was greatly suppressed by mutations of the G q α-subunit ( Gα q) and the major light-sensitive channel ( trp) but not rhodopsin. ![]() ![]() Dark noise was similarly increased in heterozygote mutants of diacylglycerol kinase ( rdgA/+). This phenotype mimics that previously described in null mutants of ninaC (no inactivation no afterpotential encoding myosin III) and an associated protein, retinophilin ( rtp). These dark events were increased in rate and amplitude by a point mutation in myosin III (NINAC), which disrupts its interaction with the scaffolding protein, INAD. Absolute visual thresholds are limited by “dark noise,” which in Drosophila photoreceptors is dominated by brief (∼10 ms), small (∼2 pA) inward current events, occurring at ∼2/s, believed to reflect spontaneous G protein activations.
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