Phylogeny and a structural model of plant MHX transporters

Abstract

BackgroundThe Arabidopsis thaliana MHX gene (AtMHX) encodes a Mg2+/H+ exchanger. Among non-plant proteins, AtMHX showed the highest similarity to mammalian Na+/Ca2+ exchanger (NCX) transporters, which are part of the Ca2+/cation (CaCA) exchanger superfamily.ResultsSequences showing similarity to AtMHX were searched in the databases or sequenced from cDNA clones. Phylogenetic analysis showed that the MHX family is limited to plants, and constitutes a sixth family within the CaCA superfamily. Some plants include, besides a full MHX gene, partial MHX-related sequences. More than one full MHX gene was currently identified only in Oryza sativa and Mimulus guttatus, but an EST for more than one MHX was identified only in M. guttatus. MHX genes are not present in the currently available chlorophyte genomes. The prevalence of upstream ORFs in MHX genes is much higher than in most plant genes, and can limit their expression. A structural model of the MHXs, based on the resolved structure of NCX1, implies that the MHXs include nine transmembrane segments. The MHXs and NCXs share 32 conserved residues, including a GXG motif implicated in the formation of a tight-turn in a reentrant-loop. Three residues differ between all MHX and NCX proteins. Altered mobility under reducing and non-reducing conditions suggests the presence of an intramolecular disulfide-bond in AtMHX.ConclusionsThe absence of MHX genes in non-plant genomes and in the currently available chlorophyte genomes, and the presence of an NCX in Chlamydomonas, are consistent with the suggestion that the MHXs evolved from the NCXs after the split of the chlorophyte and streptophyte lineages of the plant kingdom. The MHXs underwent functional diploidization in most plant species. De novo duplication of MHX occurred in O. sativa before the split between the Indica and Japonica subspecies, and was apparently followed by translocation of one MHX paralog from chromosome 2 to chromosome 11 in Japonica. The structural analysis presented and the identification of elements that differ between the MHXs and the NCXs, or between the MHXs of specific plant groups, can contribute to clarification of the structural basis of the function and ion selectivity of MHX transporters.

Magnesium stress signaling in plant: Just a beginning

“…uORF significantly inhibits AtMHX expression by degradation of AtMHX mRNA, 11,23 and these processes may be regulated by ABA and auxins. 11 The novel literatures presented that overexpression of AtMHX increased expression and activity of the vacuolar H C -ATPase, moreover, transgenic plants were sensitive to high Mg 2C level, and got severe necrotic lesions, 24,25 consistent with our results that the expression of AtMHX was inhibited by MgT. 1 These possibly indicate that AtMHX mainly maintains metal and proton homeostasis in cells, the homeostasis is essential for photosynthesis and numerous enzymatic reactions.…”

Ca2+/Cation Antiporters (CaCA): Identification, Characterization and Expression Profiling in Bread Wheat (Triticum aestivum L.)

“…To identify the CaCA superfamily proteins (TaCaCA) in the genome of T. aestivum , BLASTp search ( e -value 0.00001) of known CaCA proteins sequences from rice and Arabidopsis (Emery et al, 2012; Gaash et al, 2013; Singh et al, 2015; Pittman and Hirschi, 2016a) was performed against the protein model sequences of T. aestivum obtained from IWGSC server (Nussbaumer et al, 2013; IWGSC, 2014) (http://www.wheatgenome.org/, http://wheat-urgi.versailles.inra.fr/Seq-Repository/Genes-annotations). A total of 6 CAX, 5 CCX and 1 each MHX and NCL sequences from Arabidopsis, and 6 CAX, 5 CCX, 2 MHX, and 2 NCL sequences from rice were used in BLAST search as described earlier (Pittman and Hirschi, 2016a).…”

“…However, land plants have evolved two additional groups of CaCA proteins- Mg 2+ /H + exchanger (MHX) proteins and EF-hand domain containing CAX (EF-CAX) proteins (Emery et al, 2012; Gaash et al, 2013). The EF-CAX was also found in algae (Emery et al, 2012) and later named as NCX like protein (NCL) due to their functional divergence from CAX proteins (Wang et al, 2012; Li et al, 2016).…”

“…Recently, the MHX proteins were identified from numerous land plants, which formed a distinct group with tight phylogenetic clustering (Emery et al, 2012; Gaash et al, 2013; Pittman and Hirschi, 2016a). The plant MHX proteins also differ from NCX proteins in certain key amino acid residues in α1 and α2 repeat regions responsible for Na + /Ca 2+ exchange activity (Nicoll et al, 1996a; Iwamoto et al, 2000; Philipson and Nicoll, 2000; Ottolia et al, 2005; Emery et al, 2012).…”

“…The plant MHX proteins also differ from NCX proteins in certain key amino acid residues in α1 and α2 repeat regions responsible for Na + /Ca 2+ exchange activity (Nicoll et al, 1996a; Iwamoto et al, 2000; Philipson and Nicoll, 2000; Ottolia et al, 2005; Emery et al, 2012). Furthermore, the MHX proteins consist of smaller central loop without Ca 2+ binding domain (Lytton, 2007; Gaash et al, 2013). …”