Mutational analysis of functional domains in Mrs2p, the mitochondrial Mg2+ channel protein of Saccharomyces cerevisiae

The nuclear gene MRS2 in Saccharomyces cerevisiae encodes an integral protein (Mrs2p) of the inner mitochondrial membrane. It forms an ion channel mediating influx of Mg²⁺ into mitochondria. Orthologues of Mrs2p have been shown to exist in other lower eukaryotes, in vertebrates and in plants. Characteristic features of the Mrs2 protein family and the distantly related CorA proteins of bacteria are the presence of two adjacent transmembrane domains near the C terminus of Mrs2p one of which ends with a F/Y-G-M-N motif. Two coiled-coil domains and several conserved primary sequence blocks in the central part of Mrs2p are identified here as additional characteristics of the Mrs2p family. Gain-of-function mutations obtained upon random mutagenesis map to these conserved sequence blocks. They lead to moderate increases in mitochondrial Mg²⁺ concentrations and concomitant positive effects on splicing of mutant group II intron RNA. Site-directed mutations in several conserved sequences reduce Mrs2p-mediated Mg²⁺ uptake. Mutants with strong effects on mitochondrial Mg²⁺ concentrations also have decreased group II intron splicing. Deletion of a nonconserved basic region, previously invoked for interaction with mitochondrial introns, lowers intramitochondrial Mg²⁺ levels as well as group II intron splicing. Data presented support the notion that effects of mutations in Mrs2p on group II intron splicing are a consequence of changes in steady-state mitochondrial Mg ²⁺ concentrations.