Leucine zipper-EF-hand containing transmembrane protein 1 (LETM1) forms a Ca²⁺/H⁺ antiporter

Leucine zipper-EF-hand-containing transmembrane protein1 (LETM1) is located in the mitochondrial inner membrane and is defective in Wolf-Hirschhorn syndrome. LETM1 contains only one transmembrane helix, but it behaves as a putative transporter. Our data shows that LETM1 knockdown or overexpression robustly increases or decreases mitochondrial Ca²⁺ level in HeLa cells, respectively. Also the residue Glu221 of mouse LETM1 is identified to be necessary for Ca²⁺ flux. The mutation of Glu221 to glutamine abolishes the Ca²⁺-transport activity of LETM1 in cells. Furthermore, the purified LETM1 exhibits Ca²⁺/H⁺ anti-transport activity, and the activity is enhanced as the proton gradient is increased. More importantly, electron microscopy studies reveal a hexameric LETM1 with a central cavity, and also, observe two different conformational states under alkaline and acidic conditions, respectively. Our results indicate that LETM1 is a Ca²⁺/H⁺ antiporter and most likely responsible for mitochondrial Ca²⁺ output.