B, Scatter plot of dark bump rate as a function of G qα protein levels in the membrane of different mutants and rearing conditions as in A. A, Histogram plotting the mean bump rate of dark bumps of WT and G qα 1 heterozygote ( G qα 1/+) and homozygote mutant raised either in the dark or in the light ( n = 5, mean ± SEM). We thus demonstrate how a G-protein-mediated transduction system, with PLC as its target, selectively suppresses its intrinsic noise while preserving reliable signaling.ĭark bumps are the result of spontaneous GDP-GTP exchange on the G qα subunit. This minimal PLC activity level is reliably obtained by photon-induced synchronized activation of several neighboring G(q)α molecules activating several PLC molecules, but not by random activation of single G(q)α molecules. The required minimal level of PLC activity selectively suppressed random production of single G(q)α-activated dark bumps despite a high rate of spontaneous G(q)α activation. Manipulations of PLC activity using PLC mutant flies and Ca(2+) modulations revealed that a critical level of PLC activity is required to induce bump production. In this study we show that reduced PLC catalytic activity selectively suppressed production of dark bumps but not light-induced bumps. Therefore, it is unclear how phototransduction suppresses dark bump production arising from spontaneous G(q)α activation, while still maintaining high-fidelity representation of single photons. Nevertheless, this high rate is not manifested in a substantially high rate of dark bumps. We found that in wild-type flies the in vivo rate of spontaneous G(q)α activation is very high. A high rate of spontaneous G(q)α activation and dark bump production potentially hampers single-photon detection. In photoreceptor cells, dark activation of G(q)α molecules occurs spontaneously and produces unitary dark events (dark bumps). This is manifested in the single-photon responses (quantum bumps). Drosophila photoreceptor cells use the ubiquitous G-protein-mediated phospholipase C (PLC) cascade to achieve ultimate single-photon sensitivity.

Intrinsic dark noise