IDENTIFICATION OF A MITOCHONDRIAL NA+ H+ EXCHANGER/

The electroneutral exchange of protons for Na+ and K+ across the mitoc hondrial inner membrane contributes to organellar volume and Ca2+ home ostasis. The molecular nature of these transporters remains unknown. I n this report, we characterize a novel gene (YDR456w; renamed NHA2) in Saccharomyces cerevisiae whose deduced protein sequence is homologous to members of the mammalian Na+/H+ exchanger gene family. Fluorescenc e microscopy showed that a Nha2-green fluorescent protein chimera colo calizes with 4',6-diamidino-2-phenylindole staining of mitochondrial D NA. To assess the function of Nha2, we deleted the NHA2 gene by homolo gous disruption and found that benzamil-inhibitable, acid-activated Na -22(+) uptake into mitochondria was abolished in the mutant strain. It also showed retarded growth on nonfermentable carbon sources and seve rely reduced survival during the stationary phase of the cell cycle co mpared with the parental strain, consistent with a defect in aerobic m etabolism. Sequence comparisons revealed that Nha2 has highest identit y to a putative Na+/K+ exchanger homologue (KIAA0267; renamed NHE6) in humans, Northern blot analysis demonstrated that NHE6 is ubiquitously expressed but is most abundant in mitochondrion-rich tissues such as brain, skeletal muscle, and heart. Fluorescence microscopy showed that a NHE6-green fluorescent protein chimera also accumulates in mitochon dria of transfected HeLa cells. These data indicate that NHA2 and NHE6 encode homologous Na+/M+ exchangers and suggest they may be important for mitochondrial function in lower and higher eukaryotes, respective ly.