PROPERTIES OF SINGLE DROSOPHILA TRPL CHANNELS EXPRESSED IN SF9 INSECTCELLS

The transient receptor potential (trp)-like (trpl) gene is thought to encode an ion channel important for signal transduction in Drosophila photoreceptor cells. Consistent with this hypothesis, heterologous exp ression of the trpl-encoded protein (Trpl) is associated with the appe arance of an outwardly rectifying, nonselective cation current. In the present study, single channels were recorded in cell-attached, inside -out, and outside-out membrane patches from Sf9 insect cells infected with recombinant baculovirus-containing trpl cDNA under control of the polyhedrin promoter. The single-channel current-voltage relationship was linear from -100 to +80 mV with a slope conductance of 89-110 pS. The probability of opening was voltage sensitive, increasing at positi ve potentials contributing to the outwardly rectifying properties of t he whole cell currents. The single channels 1) were never observed in Sf9 cells infected with recombinant baculovirus containing the Bz brad ykinin receptor cDNA or in noninfected Sf9 cells; 2) appear at the sam e time postinfection as the Trpl whole cell current; 3) were nonselect ive with respect to Na+, Ca2+, and Ba2+; 4) were blocked by 1-2 mM La3 + and Gd3+ (but not 10 mu M); and 5) were blocked by 4-8 mM Mg2+. The single Trpl channel activity increased spontaneously with time after p atch formation, and the activity was further increased by application of bradykinin to cells expressing both the B-2 bradykinin receptor and the Trpl protein. These results suggest that this single-channel acti vity reflects expression of the Trpl protein and provides conclusive e vidence that trpl encodes a nonselective cation channel consistent wit h its proposed role in Drosophila phototransduction.