The melastatin-related transient receptor potential channel TRPM2 is a plasma membrane Ca2+-permeable cation channel that is activated by intracellular adenosine diphosphoribose (ADPR) binding to the channel's enzymatic Nudix domain. Channel activity is also seen with nicotinamide dinucleotide (NAD+) and hydrogen peroxide (H2O2), but their mechanisms of action remain unknown. Here, we identify cyclic adenosine diphosphoribose (cADPR) as an agonist of TRPM2 with dual activity: at concentrations above 100 microM, cADPR can gate the channel by itself, whereas lower concentrations of 10 microM have a potentiating effect that enables ADPR to gate the channel at nanomolar concentrations. ADPR's breakdown product adenosine monophosphate (AMP) specifically inhibits ADPR, but not cADPR-mediated gating of TRPM2, whereas the cADPR antagonist 8-Br-cADPR exhibits the reverse block specificity. Our results establish TRPM2 as a coincidence detector for ADPR and cADPR signaling and provide a functional context for cADPR as a second messenger for Ca2+ influx.

Respiring mitochondria maintain a membrane potential (⌬⌿) 1 of Ϫ150 to Ϫ180 mV (⌬⌿, inside negative). This high ⌬⌿ constitutes a large driving force for the electrophoretic influx of cations, either through specific channels or by diffusion through the membrane. Several cation channels have been characterized physiologically (reviewed in Refs. 1 and 2), and recently, a single one has been identified molecularly (3). These transport systems seem to have intrinsic control mechanisms which ensure that the matrix cation concentrations stay within physiological ranges, far below chemical equilibrium.Diffusive permeability of the inner mitochondrial membrane to ions is generally low but physiologically significant, as it lowers the pH gradient and membrane potential. Moreover, if not counteracted by extrusion, steadily increasing concentrations of matrix cations (and of compensating anions) will lead to an imbalance of osmotic pressure across the inner mitochondrial membrane. As a consequence, water will pass through the membrane, causing excessive swelling and eventual rupture of the organelle (1, 2, 4).As first proposed by P. Mitchell (5), mitochondria have carrier systems allowing the electroneutral exchange of cations against H ϩ (and anions against OH Ϫ ). These exchangers counteract the ⌬⌿-driven cation leakage of the membrane and also cation imbalances due to changes in mitochondrial physiology. Mitochondrial cation distribution is, therefore, a steady state, in which the accumulation ratio is modulated by the relative rates of cation influx and efflux by means of separate pathways.Many physiological studies have been devoted to cation/H ϩ exchange systems (reviewed in Ref.

Steady-state concentrations of mitochondrial Mg 2+ previously have been shown to vary with the expression of Mrs2p, a component of the inner mitochondrial membrane with two transmembrane domains. While its structural and functional similarity to the bacterial Mg 2+ transport protein CorA suggested a role for Mrs2p in Mg 2+ in¯ux into the organelle, other functions in cation homeostasis could not be excluded. Making use of the¯uorescent dye mag-fura 2 to measure free Mg 2+ concentrations continuously, we describe here a high capacity, rapid Mg 2+ in¯ux system in isolated yeast mitochondria, driven by the mitochondrial membrane potential Dy and inhibited by cobalt(III)hexaammine. Overexpression of Mrs2p increases in¯ux rates 5-fold, while the deletion of the MRS2 gene abolishes this high capacity Mg 2+ in¯ux. Mg 2+ ef¯ux from isolated mitochondria, observed with low Dy only, also requires the presence of Mrs2p. Cross-linking experiments revealed the presence of Mrs2p-containing complexes in the mitochondrial membrane, probably constituting Mrs2p homooligomers. Taken together, these ®ndings characterize Mrs2p as the ®rst molecularly identi®ed metal ion channel protein in the inner mitochondrial membrane.