Ins(1,4,5)P3 activates Drosophila cation channel Trpl in recombinant baculovirus-infected Sf9 insect cells

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Abstract
The trp-like (trpl) gene product (Trpl) is thought to form a nonselective cation channel important for signal transduction in Drosophila photoreceptor cells. This channel may be the insect homologue of mammalian channels involved in Ca2+ signal transduction. To determine the mechanism of receptor-mediated activation of Trpl, whole cell membrane currents were examined in Sf9 insect cells after infection with recombinant baculovirus. Stimulation by bradykinin increased whole cell Trpl currents three- to fivefold. Similar activation of Trpl was observed by inclusion of D-myo-inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in the pipette solution during whole cell recordings. These currents were 1) not seen in noninfected cells or in cells expressing only the B2 receptor, 2) mimicked by D-myo-inositol 2,4,5-trisphosphate, and 3-deoxy-3-fluoro-D-myo-inositol 1,4,5-trisphosphate, 3) not seen with D-myo-inositol 1,4-bisphosphate or D-myo-inositol 1,3,4,5-tetrakisphosphate, and 4) blocked by heparin, but not by de-N-sulfated heparin. In contrast, Trpl currents were unaffected by thapsigargin. These results demonstrate that the Trpl cation channel is activated by Ins(1,4,5)P3 in a heparin-sensitive fashion. Regulation of channel activity by Ins(1,4,5)P3 may occur by a number of mechanisms, including direct binding of Ins(1,4,5)P3 to the Trpl channel or direct physical interaction between the Ins(1,4,5)P3 receptor/Ca(2+)-release channel of the endoplasmic reticulum and the Trpl protein.