FUNCTIONAL EQUIVALENCE OF NATIVE LIGHT-SENSITIVE CHANNELS IN THE DROSOPHILA TRP(301) MUTANT AND TRPL CATION CHANNELS EXPRESSED IN A STABLY TRANSFECTED DROSOPHILA CELL-LINE

Drosophila photoreceptors express two putative cation channels encoded by the transient receptor potential (trp) and frp-like (trpl) genes, which represent prototypical members of a novel family of phosphoinosi tide-regulated calcium influx channels. Mutations of both frp and trpl selectively abolish components of the light-sensitive current and, wh en heterologously expressed, both generate cation permeable conductanc es; however, a detailed comparison of recombinant and native channel p roperties is lacking. To more rigorously test the hypothesis that TRPL channels mediate one component of the light-sensitive current we have generated cell lines (Drosophila S2 cells) stably transfected with tr pl cDNA and compared the recombinant channel properties with those of the light-sensitive conductance in situ in a Drosophila trp mutant und er identical conditions. We found close correspondence in respect of a number of quantifiable biophysical parameters including: current volt age relationships, ionic selectivity, voltage independent block by ext ernal Mg2+ ions and effective single channel conductance and gating ki netics derived by noise analysis. Our estimate of 60-70 pS for channel conductance was confirmed directly in patch clamp recordings of singl e TRPL channels in S2 cells. These findings indicate that channels enc oded by the trpl gene can completely account for the component of the light-sensitive conductance remaining in the trp mutant.