| The phototransduction cascade in Drosophila is a prototypical G-protein signaling pathway and many of its components have been well-studied. Early genetic screens identified many of the major proteins involved in this pathway and study over the past several decades has not only elucidated their function, but also contributed to our overall understanding of the visual signaling cascade. However, there are still several unanswered questions concerning the phototransduction process and many proteins that remain unidentified.; In order to isolate previously unidentified proteins involved in Drosophila phototransduction, a genetic screen aimed at isolating suppressors of retinal degeneration was conducted. Two of the mutant lines from this screen were found to have interesting phenotypes that suggested that the defective proteins were affecting phototransduction.; Characterization of the mutant phenotypes followed by identification of the defective genes revealed that the aberrant proteins are involved in photoreceptor cell maintenance and structure. The first of these mutants, pawn14S7, exhibits an oscillatory electrophysiological response to light as well as rapid and severe light-dependent retinal degeneration. pawn14S7 is a gain-of-function allele of pawn, a putative cell adhesion molecule that is a single-pass transmembrane protein with a large extracellular region and short cytoplasmic tail. In pawn14S7 a deletion followed by a small insertion, results in the production of a soluble, unanchored protein that likely disrupts photoreceptor cell structure and therefore the phototransduction process.; The second mutant, 28S2 displays an oscillatory electrophysiological response to light, slow light-dependent retinal degeneration and aggregation of the photopigment, Rhodopsin. Genetic data revealed that 28S2 is an allele of the Na+/Cl--dependent neurotransmitter transporter family member, Ine. Members of this protein family have been implicated in the reuptake of neurotransmitters as well as the transport of osmolytes across membranes. For these reasons, we speculate that Ine functions in photoreceptor cells to maintain the appropriate osmotic or ionic conditions necessary for cell survival. The identification of two mutants that affect aspects of photoreceptor cell structure and/or ion homeostasis points to the importance of photoreceptor integrity to phototransduction. |
