Interplay between PIP₃ and calmodulin regulation of olfactory cyclic nucleotide-gated channels

Abstract

Phosphatidylinositol-3,4,5-trisphosphate (PIP₃) has been proposed to modulate the odorant sensitivity of olfactory sensory neurons by inhibiting activation of cyclic nucleotide-gated (CNG) channels in the cilia. When applied to the intracellular face of excised patches, PIP₃ has been shown to inhibit activation of heteromeric olfactory CNG channels, composed of CNGA2, CNGA4, and CNGB1b subunits, and homomeric CNGA2 channels. In contrast, we discovered that channels formed by CNGA3 subunits from cone photoreceptors were unaffected by PIP₃. Using chimeric channels and a deletion mutant, we determined that residues 61–90 within the N terminus of CNGA2 are necessary for PIP₃ regulation, and a biochemical “pulldown” assay suggests that PIP₃ directly binds this region. The N terminus of CNGA2 contains a previously identified calcium–calmodulin (Ca²⁺/CaM)-binding domain (residues 68–81) that mediates Ca²⁺/CaM inhibition of homomeric CNGA2 channels but is functionally silent in heteromeric channels. We discovered, however, that this region is required for PIP₃ regulation of both homomeric and heteromeric channels. Furthermore, PIP₃ occluded the action of Ca²⁺/CaM on both homomeric and heteromeric channels, in part by blocking Ca²⁺/CaM binding. Our results establish the importance of the CNGA2 N terminus for PIP₃ inhibition of olfactory CNG channels and suggest that PIP₃ inhibits channel activation by disrupting an autoexcitatory interaction between the N and C termini of adjacent subunits. By dramatically suppressing channel currents, PIP₃ may generate a shift in odorant sensitivity that does not require prior channel activity.