Outer pore architecture of a Ca2+-selective TRP channel

The TRP superfamily forms a functionally important class of cation channels related to the product of the Drosophila trp gene. TRP channels display an unusual diversity in activation mechanisms and permeation properties, but the basis of this diversity is unknown, as the structure of these channels has not been studied in detail. To obtain insight in the pore architecture of TRPV6, a Ca2+-selective member of the TRPV subfamily, we probed the dimensions of its pore and determined pore-lining segments using cysteine-scanning mutagenesis. Based on the permeability of the channel to organic cations, we estimated a pore diameter of 5.4 Angstrom. Mutating Asp(541), a residue involved in high affinity Ca2+ binding, altered the apparent pore diameter, indicating that this residue lines the narrowest part of the pore. Cysteines introduced in a region preceding Asp(541) displayed a cyclic pattern of reactivity to Ag+ and cationic methylthiosulfanate reagents, indicative of a pore helix. The anionic methanethiosulfonate ethylsulfonate showed only limited reactivity in this region, consistent with the presence of a cation-selective filter at the outer part of the pore helix. Based on these data and on homology with the bacterial KcsA channel, we present the first structural model of a TRP channel pore. We conclude that main structural features of the outer pore, namely a selectivity filter preceded by a pore helix, are conserved between K+ channels and TRPV6. However, the selectivity filter of TRPV6 is wider than that of K+ channels and lined by amino acid side chains rather than main chain carbonyls.