Abstract
Visual signal transmission by Drosophila melanogaster photoreceptors is mediated by a Gq protein that activates a phospholipase C (PLC). Mutations and deficiencies in expression of either of these proteins cause severe defects in phototransduction. Here we investigated whether these proteins are also involved in the cockroach, Periplaneta americana, phototransduction by silencing Gq α-subunit (Gqα) and phosphoinositide-specific phospholipase C (PLC) by RNA interference and observing responses to single photons (quantum bumps, QB). We found (1) non-specific decreases in membrane resistance, membrane capacitance and absolute sensitivity in the photoreceptors of both Gqα and PLC knockdowns, and (2) small changes in QB statistics. Despite significant decreases in expressions of Gq and PLC mRNA, the changes in QB properties were surprisingly modest, with mean latencies increasing by ~ 10%, and without significant decrease in their amplitudes. To better understand our results, we used a mathematical model of the phototransduction cascade. By modifying the Gq and PLC abundances, and diffusion rates for Gq, we found that QB latencies and amplitudes deteriorated noticeably only after large decreases in the protein levels, especially when Gq diffusion was slow. Also, reduction in Gq but not PLC lowered quantum efficiency. These results suggest that expression of the proteins may be redundant.
Highlights
The microvillus of an insect photoreceptor is a highly organized compartment containing molecular machinery to reliably and repeatedly generate quantum bumps (QB), the electrical responses to absorption of single photons
The actual concentrations of other molecules involved in phototransduction in the D. melanogaster microvillus are, to our knowledge, not yet determined, but they have been estimated to contain ~ 100 trimeric Gq proteins, ~ 100 phospholipase C (PLC) molecules, ~ 3000 phosphatidylinositol 4,5-bisphosphate (PIP2) molecules that fuel the phototransduction reaction, ~ 25 tetrameric cationic TRP and TRPL channels that generate the electrical responses, and ~ 100
The phosphoinositide-specific phospholipase C (PLC, GenBank accession number MN443916) found in P. americana retina codes a 1095 amino acid protein that has 65% sequence similarity with the protein encoded by norpA gene in D. melanogaster The gene that codes the 353 amino acid α-subunit of guanine nucleotide-binding protein Gq (Gqα, accession number MN443915) in P. americana retina has 86% sequence identity with the α-subunit of D. melanogaster Gq α-subunit (Gqα), isoform D
Summary
The microvillus of an insect photoreceptor is a highly organized compartment containing molecular machinery to reliably and repeatedly generate quantum bumps (QB), the electrical responses to absorption of single photons. The actual concentrations of other molecules involved in phototransduction in the D. melanogaster microvillus are, to our knowledge, not yet determined, but they have been estimated to contain ~ 100 trimeric Gq proteins, ~ 100 phospholipase C (PLC) molecules, ~ 3000 phosphatidylinositol 4,5-bisphosphate (PIP2) molecules that fuel the phototransduction reaction, ~ 25 tetrameric cationic TRP and TRPL channels that generate the electrical responses, and ~ 100. Journal of Comparative Physiology A (2020) 206:597–610 protein kinase C (PKC) molecules that are involved in the response termination (Huber et al 1996; Nikolic et al 2010). The TRP and TRPL channels, PLCs, and PKCs are tethered to the central axial actin filament via links to scaffolding protein INAD and myosin III (NINAC), which render them effectively immobile (Hardie and Raghu 2001). As rhodopsin is considered essentially immobile (Nikolic et al 2010), signal propagation from the activated receptor (metarhodopsin) to PLC depends on the diffusional sliding of Gq proteins below the plane of the membrane, whereas the catalytic activity of PLC depends on the free diffusion of PIP2 within the membrane
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