2.A.97 The Mitochondrial Inner Membrane K⁺/H⁺ and Ca²⁺/H⁺ Exchanger (LetM1) Family

Mitochondria are integral components of cellular calcium (Ca²⁺) signaling. Calcium stimulates mitochondrial adenosine 5''-triphosphate production, but can also initiate apoptosis. In turn, cytoplasmic Ca²⁺ concentrations are regulated by mitochondria. Leucine zipper EF hand-containing transmembrane protein 1 (LetM1) and uncoupling proteins 2 and 3 (UCP2/3) contribute to two distinct mitochondrial Ca²⁺ uptake pathways (Waldeck-Weiermair et al., 2011).  Lin and Stathopulos 2019 presented an overview of the current understanding of LETM1 structure, mechanism and function. LETM1 is required for mitochondrial homeostasis and cellular viability (Li et al. 2019). The LetM1 family has also been designated the SLC55 family (Gyimesi and Hediger 2020).

LetM1 is a large human protein of 739 aas with 1 TMS. It is implicated in the Wolf-Hirschhorn Syndrome. Homologues are found in plants and fungi. These proteins in yeast and humans are in the inner mitochondrial membrane and are complementary. The human protein corrects the yeast defect. Deletion of the yeast protein yields defective, swollen mitochondria with increased cation (K⁺) contents and low Δψ. The primary defect may be in the mitochondrial K⁺/H⁺ exchange activity. These proteins therefore function both in K⁺ homeostasis and organelle volume control (Nowikovsky et al., 2004).

Jiang et al. (2009) searched for genes that regulate mitochondrial Ca²⁺ and H⁺ concentrations using a genome-wide Drosophila RNA interference (RNAi) screen. The mammalian homolog of one Drosophila gene identified in the screen, LetM1, was found to specifically mediate coupled Ca²⁺/H⁺ exchange. RNAi knockdown, overexpression, and liposome reconstitution of the purified LetM1 protein demonstrate that LetM1 is a mitochondrial Ca²⁺/H⁺ antiporter (Jiang et al., 2009). This system is discussed by Santo-Domingo and Demaurex (2010). 

Leucine zipper-EF-hand containing transmembrane protein 1 (LetM1), one of the genes deleted in Wolf-Hirschhorn syndrome, encodes a putative mitochondrial Ca²⁺/H⁺ antiporter. Cellular Letm1 knockdown reduced Ca²⁺mito uptake, H⁺mito extrusion and impaired mitochondrial ATP generation capacity. Homozygous deletion of LetM1 in mice resulted in embryonic lethality before day 6.5 of embryogenesis and ~50% of the heterozygotes died before day 13.5 of embryogenesis (Jiang et al. 2013). The surviving heterozygous mice exhibited altered glucose metabolism, impaired control of brain ATP levels, and increased seizure activity. Thus, loss of LetM1 contributes to the pathology of Wolf-Hirschhorn syndrome in humans and may contribute to seizure phenotypes by reducing glucose oxidation.

Purified, and reconstituted human LetM1 exhibits apparent affinities of cations in the order Ca²⁺ = Mn²⁺ > Gd³⁺ = La³⁺ > Sr²⁺ >> Ba²⁺, Mg²⁺, K⁺, Na⁺. Kinetic analyses suggested a LetM1 turnover rate of 2 Ca²⁺/s and a K_m of ~ 25 microM. LetM1 mediates electroneutral 1 Ca²⁺/2 H⁺ antiport. LetM1 is insensitive to ruthenium red, an inhibitor of the mitochondrial calcium uniporter, and CGP-37157, an inhibitor of the mitochondrial Na⁺/Ca²⁺exchanger. Functional properties of LetM1 are similar to those of the H⁺-dependent Ca²⁺ transport mechanism identified in intact mitochondria (Tsai et al. 2013).