The G-M-N motif determines ion selectivity in the yeast magnesium channel Mrs2p

Abstract

The highly conserved G-M-N motif of the CorA-Mrs2-Alr1 family of Mg²⁺ channels has been shown to be essential for Mg²⁺ transport. We performed random mutagenesis of the G-M-N sequence of Saccharomyces cerevisiae Mrs2p in an unbiased genetic screen. A large number of mutants still capable of Mg²⁺ influx, albeit below the wild-type level, were generated. Growth complementation assays, performed in media supplemented with Ca²⁺ or Co²⁺ or Mn²⁺ or Zn²⁺ at varying concentrations, lead to identification of mutants with reduced growth in the presence of Mn²⁺ and Zn²⁺. We hereby conclude that (1) at least two, but predominantly all three amino acids of the G-M-N motif must be replaced by certain combinations of other amino acids to remain functional, (2) replacement of any single amino acid within the G-M-N motif always impairs the function of Mrs2p, and (3) we show that the G-M-N motif determines ion selectivity, likely in concurrence with the negatively charged loop at the entrance of the channel thereby forming the Mrs2p selectivity filter.