Regulatory Mechanism Of Gαq Splice Variants In Drosophila Phototransduction

The G-protein coupled receptors（GPCRs）are one of the most widely known membrane receptors,which mediate signal transmission processes such as vision,olfactory,gustation,hormones and neurotransmitters.The binding of GPCR and the ligand makes the conformation change,then activates the heterotrimeric G proteins and opens the downstream signaling pathways.The Drosophila visual signal transmission system is a good model for studying GPCR signaling.The Gαq gene encoding the alpha subunit of Gq generates seven splicing variants and three different proteins,Gaq1,Gaq3 and Gaq4.Gaq1 plays a major role in fly phototransduction but its null-mutants(Gaq⁹⁶¹)still exhibit a residual light response,indicating that other Gαq isoform proteins or additional Gqαsubunits are involved in phototransduction.Meanwhile,Gαq1-null mutants exhibit only slight retinal degeneration and the role of Gαq protein in retinal degeneration is therefore unclear.Here,using the Drosophila visual signal transmission system as a model,with the combination of the approaches in genetics,molecular biology,biochemistry and electrophysiological recordings,we investigate the mechanisms of Gαq splice variants in phototransduction cascade.First,we isolate a mutant fly based on electroretinogram screen,which shows no detectable ERG responses to saturated light stimulations,and refer to the mutant as nlr（no-light-response）.We reveal that the defective light responses in nlr mutants is due to the absence of Gαq protein through intracellular recording and Western blot.Through DNA sequencing and rescue experiments,we demonstrate that Gαq1 and Gαq3 mediate phototransduction,and further clarify that Gαq3 but not Gαq homologs CG30054 or CG17760 is responsible for the residual light response in Gαq1-null mutants.EM analysis shows that depletion of all Gαq splice variants results in rapid light-dependent retinal degeneration,which is due to formation of stable Rh1/Arr2 complexes.Finally,we reveal that Gαq3 but not Gαq1 mediates Rh1 synthesis.Our study clarifies that Gαq1 and Gαq3 splice variants encoded by Gαq gene mediate phototransduction and the absence of Gαq protein also leads to retinal degeneration in Drosophila and reveal that Gαq3 rather than Gαq1 mediates Rh1 synthesis.Our work improves our understandings of phototransduction and the mechanism of retinal degeneration in Drosophila,and offers insights into the study of GPCR signal transduction pathways.