The TRPM subfamily of mammalian TRP channels displays unusually diverse activation mechanisms and selectivities [1]. One member of this subfamily, TRPM5, functions in taste receptor cells and has been reported to be activated through G protein-coupled receptors linked to phospholipase C 2, 3. However, the specific mechanisms regulating TRPM5 have not been described. Here, we demonstrate that TRPM5 is a monovalent-specific cation channel with a 23 pS unitary conductance. TRPM5 does not display constitutive activity. Rather, it is activated by stimulation of a receptor pathway coupled to phospholipase C and by IP3-mediated Ca2+ release. Gating of TRPM5 was dependent on a rise in Ca2+ because it was fully activated by Ca2+. Unlike any previously described mammalian TRP channel, TRPM5 displayed voltage modulation and rapid activation and deactivation kinetics upon receptor stimulation. The most closely related protein, the Ca2+-activated monovalent-selective cation channel TRPM4b, also showed voltage modulation, although with slower relaxation kinetics than TRPM5. Taken together, the data demonstrate that TRPM5 and TRPM4b represent the first examples of voltage-modulated, Ca2+-activated, monovalent cation channels (VCAMs). The voltage modulation and rapid kinetics provide TRPM5 with an excellent set of properties for participating in signaling in taste receptors and other excitable cells.
