Mg²⁺-dependent Gating and Strong Inward Rectification of the Cation Channel TRPV6

TRPV6 (CaT1/ECaC2), a highly Ca²⁺-selective member of the TRP superfamily of cation channels, becomes permeable to monovalent cations in the absence of extracellular divalent cations. The monovalent currents display characteristic voltage-dependent gating and almost absolute inward rectification. Here, we show that these two features are dependent on the voltage-dependent block/unblock of the channel by intracellular Mg²⁺. Mg²⁺ blocks the channel by binding to a site within the transmembrane electrical field where it interacts with permeant cations. The block is relieved at positive potentials, indicating that under these conditions Mg²⁺ is able to permeate the selectivity filter of the channel. Although sizeable outward monovalent currents were recorded in the absence of intracellular Mg²⁺, outward conductance is still ∼10 times lower than inward conductance under symmetric, divalent-free ionic conditions. This Mg²⁺-independent rectification was preserved in inside-out patches and not altered by high intracellular concentrations of spermine, indicating that TRPV6 displays intrinsic rectification. Neutralization of a single aspartate residue within the putative pore loop abolished the Mg²⁺ sensitivity of the channel, yielding voltage-independent, moderately inwardly rectifying monovalent currents in the presence of intracellular Mg²⁺. The effects of intracellular Mg²⁺ on TRPV6 are partially reminiscent of the gating mechanism of inwardly rectifying K⁺ channels and may represent a novel regulatory mechanism for TRPV6 function in vivo.