A cytosolic residue mediates Mg2+ block and regulates inward current amplitude of a transient receptor potential channel

Members of the transient receptor potential (TRP) cation channel family control a wide variety of cellular functions by regulating calcium influx. In neurons, TRP channels may also modulate cell excitability. TRPC5 is a neuronal TRP channel that plays a role in controlling neurite extension in the hippocampus. Transiently expressed TRPC5 exhibits a doubly rectifying current - voltage relationship characterized by relatively large inward currents and a unique outwardly rectifying current with a "flat" segment between + 10 and + 40 mV that may be attributable to Mg2+ block. We find that intracellular Mg2+ blocks the outward current through TRPC5 with an IC50 of 457 muM. The block is mediated by a cytosolic aspartate residue, D633, situated between the termination of the sixth transmembrane domain and the "TRP box." The substitution of noncharged or positively charged residues for the negatively charged D633 resulted in a channel with markedly reduced inward currents, in addition to decreased Mg2+ block. This suggests that electrostatic attraction of cations by D633 may contribute to inward current amplitude in TRPC5. We propose that cytosolic negatively charged residues can modulate the conductivity of TRP cation channels.