title: Phospholipase C-mediated suppression of dark noise enables single-photon detection in Drosophila photoreceptors.
abstract: Drosophila photoreceptor cells use the ubiquitous G-protein-mediated phospholipase C (PLC) cascade to achieve ultimate single-photon sensitivity. This is manifested in the single-photon responses (quantum bumps). In photoreceptor cells, dark activation of G(q)α molecules occurs spontaneously and produces unitary dark events (dark bumps). A high rate of spontaneous G(q)α activation and dark bump production potentially hampers single-photon detection. We found that in wild-type flies the in vivo rate of spontaneous G(q)α activation is very high. Nevertheless, this high rate is not manifested in a substantially high rate of dark 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. In this study we show that reduced PLC catalytic activity selectively suppressed production of dark bumps but not light-induced bumps. 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. 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. 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. We thus demonstrate how a G-protein-mediated transduction system, with PLC as its target, selectively suppresses its intrinsic noise while preserving reliable signaling.
authors.affiliation: Department of Medical Neurobiology, the Institute of Medical Research, Israel.
authors.forename: Ben
authors.initials: B
authors.lastname: Katz
authors.forename: Baruch
authors.initials: B
authors.lastname: Minke
mesh_terms: D000818:Animals; D030801:Animals, Genetically Modified; D001703:Biophysics; D002118:Calcium / Q000378:metabolism; D003624:Darkness; D004330:Drosophila; D029721:Drosophila Proteins / Q000235:genetics; D004558:Electric Stimulation; D005260:Female; D043802:GTP-Binding Protein alpha Subunits, Gq-G11 / Q000235:genetics / Q000378:metabolism; D006153:Guanosine Diphosphate / Q000378:metabolism; D006160:Guanosine Triphosphate / Q000378:metabolism; D066298:In Vitro Techniques; D007477:Ions / Q000378:metabolism; D008027:Light; D055537:Light Signal Transduction / Q000502:physiology*; D008297:Male; D008564:Membrane Potentials; D009154:Mutation / Q000235:genetics; D018408:Patch-Clamp Techniques; D054799:Phospholipase C beta / Q000235:genetics; D017785:Photons; D017956:Photoreceptor Cells, Invertebrate / Q000502:physiology*; D010738:Type C Phospholipases / Q000235:genetics / Q000378:metabolism*
chemical_list: D029721:Drosophila Proteins; D007477:Ions; D006153:Guanosine Diphosphate; D006160:Guanosine Triphosphate; D010738:Type C Phospholipases; C468371:NorpA protein, Drosophila; D054799:Phospholipase C beta; D043802:GTP-Binding Protein alpha Subunits, Gq-G11; D002118:Calcium