TRPM7 is a divalent cation-permeable channel which is ubiquitously expressed. Here, we demonstrated that human TRPM7 expressed either heterologously or endogenously exhibits proton conductivity. The gene silencing of TRPM7 by siRNA suppressed H+ currents in human cervical epithelial HeLa cells. In HEK293T cells transfected with human TRPM7, the inward proton conductance was suppressed by extracellular Mg2+ and Ca2+ with the IC50 values of 0.5 and 1.9 mM, respectively. Anomalous mole fraction behavior of H+ currents in the presence of Mg2+ or Ca2+ indicated that these divalent cations compete with protons for binding sites. Systematic mutation of negatively charged amino acid residues within the putative pore-forming region of human TRPM7 into the neutral amino acid alanine was then tested. E1047A resulted in non-functional channels and D1054A abolished proton conductance, whereas E1052A and D1059A only partially reduced proton conductivity. Thus, it is concluded that D1054 is an essential determinant of the proton conductivity, whereas E1047 might be required for channel formation; and the remaining negatively charged amino acids in the pore region (E1052 and D1059) may play a facilitating role in the proton conductivity of human TRPM7. It is suggested that proton conductivity of endogenous human TRPM7 plays a role in physiologically/pathologically acidic situations.
