The Selectivity Filter of the Cation Channel TRPM4

Transient receptor potential channel melastatin subfamily (TRPM) 4 and its close homologue, TRPM5, are the only two members of the large transient receptor potential superfamily of cation channels that are impermeable to Ca 2+ . In this study, we located the TRPM4 selectivity filter and investigated possible structural elements that render it Ca 2+ -impermeable. Based on homology with known cation channel pores, we identified an acidic stretch of six amino acids in the loop between transmembrane helices TM5 and TM6 ( 981 EDMDVA 986 ) as a potential selectivity filter. Substitution of this six-amino acid stretch with the selectivity filter of TRPV6 (TIIDGP) resulted in a functional channel that combined the gating hallmarks of TRPM4 (activation by Ca 2+ , voltage dependence) with TRPV6-like sensitivity to block by extracellular Ca 2+ and Mg 2+ as well as Ca 2+ permeation. Neutralization of Glu 981 resulted in a channel with normal permeability properties but a strongly reduced sensitivity to block by intracellular spermine. Neutralization of Asp 982 yielded a functional channel that exhibited extremely fast desensitization (τ < 5 s), possibly indicating destabilization of the pore. Neutralization of Asp 984 resulted in a non-functional channel with a dominant negative phenotype when coexpressed with wild type TRPM4. Combined neutralization of all three acidic residues resulted in a functional channel whose voltage dependence was shifted toward very positive potentials. Substitution of Gln 977 by a glutamate, the corresponding residue in divalent cation-permeable TRPM channels, altered the monovalent cation permeability sequence and resulted in a pore with moderate Ca 2+ permeability. Our findings delineate the selectivity filter of TRPM channels and provide the first insight into the molecular basis of monovalent cation selectivity.