Rh1

Rh1, or rhodopsin-1, is a light-sensitive G protein-coupled receptor expressed in the R1–R6 photoreceptor cells of the adult Drosophila melanogaster compound eye. It is encoded by the ninaE gene and uses 11-cis-retinal as its chromophore. When photons are absorbed, 11-cis-retinal isomers to all-trans-retinal, triggering a conformational change in Rh1 that activates the G protein Gq. This in turn stimulates phospholipase C (norpA), leading to the opening of TRP-type cation channels and depolarization of the photoreceptor cell.

Rh1 has peak spectral sensitivity in the blue-green portion of the spectrum, around 480–500 nanometers, and it is the predominant rhodopsin in the R1–R6 photoreceptors. These cells contribute to luminance detection and motion vision in Drosophila, rather than fine color discrimination. The Rh1 signaling pathway is a canonical model for insect phototransduction, illustrating GPCR activation, Gq signaling, and phosphoinositide turnover.

Biologically, Rh1 is integral to normal vision in flies; disruptions in ninaE or Rh1 function impair phototransduction and light responses. The protein is seven-transmembrane in structure and undergoes continual cycling and regeneration of its chromophore via a visual cycle involving neighboring cells. Due to its well-characterized signaling cascade and robust genetics, Rh1 has served as a foundational system for studying GPCR biology, sensory transduction, and circadian regulation in insects.