A member of a novel Arabidopsis thaliana gene family of candidate Mg2+ ion transporters complements a yeast mitochondrial group II intron-splicing mutant

Abstract

Autocatalytic activity of some group II introns has been demonstrated in vitro, but helper functions such as the yeast MRS2 protein are essential for splicing in vivo. In our search for such helper factors in plants, we pursued the cloning of two Arabidopsis thaliana homologues, atmrs2-1 and atmrs2-2. Atmrs2-1, but not atmrs2-2, complements the yeast deletion mutant of mrs2, and this is congruent with the prediction of two adjacent transmembrane stretches in AtMRS2-1 and yeast MRS2 but not in AtMRS2-2. This complementation depends on fusion of the native yeast mitochondrial import sequence to atmrs2-1. A differing, non-mitochondrial, cellular targeting in Arabidopsis is supported by the analysis of green fluorescent protein fusion constructs after transient transformation into plant protoplasts. Further members of what now appears to be a family of 10 mrs2 homologues are identified in the Arabidopsis genome. Similarity searches with the PSI-BLAST algorithm in the protein database fail to identify homologues of this novel gene family in any eukaryotes other than yeasts, but do identify its distant relatedness to the corA group of bacterial magnesium transporters. In line with this observation, intramitochondrial magnesium concentrations are indeed restored to wild-type levels in the yeast mutant on complementation with atmrs2-1.

Mrs2p is an essential component of the major electrophoretic Mg2+ influx system in mitochondria

“…Support for a role for Mrs2p in forming a Mg 2+ channel, and not of a regulatory factor of a channel, comes from previous findings that distantly related homologs such as araMrs2p, hsMrs2p or CorA can substitute for yeast Mrs2p (Bui et al ., 1999; Schock et al ., 2000; Zsurka et al ., 2001). Having 30% (araMrs2p, hsMrs2p) or much less (CorA) sequence identity with yeast Mrs2p, or with each other, these homologs are unlikely to interact as regulatory factors with the same Mg 2+ influx system when heterologously expressed in yeast.…”

“…Two protein families in eukaryotes, Mrs2p in mitochondria and Alr1p in the plasma membrane, have been recognized as distant relatives of the bacterial CorA Mg 2+ transport protein family (MacDiarmid and Gardner, 1998; Bui et al ., 1999; Schock et al ., 2000; Graschopf et al ., 2001; Zsurka et al ., 2001). The only apparent feature in common of all of these proteins is the presence of two predicted transmembrane domains near the C‐terminus, with a conserved motif Y/F‐G‐M‐N in the short sequence connecting them.…”

Phylogeny and a structural model of plant MHX transporters

“…The first group comprises transporters that are part of the CorA superfamily [3,5,6]. In Arabidopsis thaliana , this gene family has ten members, and the family was named MRS2 [5], or alternatively MGT [6]. The slow-vacuolar (SV) channel is a non-selective cation channel that can also carry Mg 2+ ions (reviewed in [7]).…”

An exchanger-like protein underlies the large Mg 2+ current in Paramecium

“…transporters encoded by corA, mgtA, mgtCB, and mgtE loci have been studied in detail (1), including their distant homologs in eukaryotes, some of which may be restricted to mitochondrial membranes (2)(3)(4)(5). In eukaryotes, the intracellular Mg 2ϩ concentration is maintained at 0.3-1 mM by the activity of a plausible Na ϩ ͞Mg 2ϩ exchanger (6)(7)(8)(9)(10) and other perhaps less specific pathways (8,(11)(12)(13)(14), but they have yet to be identified in molecular terms.…”