The TRPM1 channel in ON-bipolar cells is gated by both the α and the βγ subunits of the G-protein Go

Transmission from photoreceptors to ON bipolar cells in mammalian retina is mediated by a sign-inverting cascade. Upon binding glutamate, the metabotropic glutamate receptor mGluR6 activates the heterotrimeric G-protein Ga-o beta 3 gamma 13, and this leads to closure of the TRPM1 channel (melastatin). TRPM1 is thought to be constitutively open, but the mechanism that leads to its closure is unclear. We investigated this question in mouse rod bipolar cells by dialyzing reagents that modify the activity of either G alpha(o) or G beta gamma and then observing their effects on the basal holding current. After opening the TRPM1 channels with light, a constitutively active mutant of G alpha(o) closed the channel, but wild-type G alpha(o) did not. After closing the channels by dark adaptation, phosducin or inactive G alpha(o) (both sequester G beta gamma) opened the channel while the active mutant of G alpha(o) did not. Co-immunoprecipitation showed that TRPM1 interacts with G beta 3 and with the active and inactive forms of G alpha(o). Furthermore, bioluminescent energy transfer assays indicated that while G alpha(o) interacts with both the N- and the C-termini of TRPM1, G beta gamma interacts only with the N-terminus. Our physiological and biochemical results suggest that both Gao and G beta gamma bind TRPM1 channels and cooperate to close them.