The authors report that Letm1 regulates mitochondrial Ca2+ and H+ homeostasis and mitochondrial energetics. They state that “the MCU provides large but self-limited pulses of matrix Ca2+,” whereas “Letm1 as a Ca2+/H+ antiporter provides smaller and perhaps more localized changes in matrix Ca2+.”

The paper concludes that Letm1 mediates electroneutral 1 Ca2+/2 H+ antiport. It also states that the transport is insensitive to ruthenium red, an inhibitor of the mitochondrial calcium uniporter, and CGP-37157, an inhibitor of the mitochondrial Na+/Ca2+ exchanger, supporting a LETM1 pathway distinct from MCU and NCLX-mediated transport.

This study states that “PINK1-mediated LETM1 phosphorylation plays a critical role in mediating mitochondrial calcium transport.” It also notes that “MCU complex and mitochondrial Na+/Ca2+ exchanger (NCLX) also play a role in mitochondrial calcium uptake and release, respectively,” and examines cross-talk between LETM1 and MCU or NCLX.

Using Ca2+ fluorophore and 45Ca2+-based assays, we demonstrate directly that Letm1 is a Ca2+ transporter… Further experiments show that Letm1 mediates electroneutral 1 Ca2+/2 H+ antiport… The results demonstrate directly that Letm1 catalyzes electroneutral Ca2+/H+ antiport independently of K+… Our results establish that Letm1 is a Ca2+/H+ antiporter that moves these two cations in strict, stoichiometric exchange across the membrane, in a ratio of 1 Ca2+ to 2 H+. Letm1 is insensitive to ruthenium red, an inhibitor of the mitochondrial calcium uniporter, and CGP-37157, an inhibitor of the mitochondrial Na+/Ca2+ exchanger.

A review article states: “RNAi studies link three proteins — MICU1, NCLX and LETM1 — to the previously unknown molecular mechanism of mitochondrial calcium transport.” It also describes MCU as the mitochondrial Ca2+ uniporter for uptake, NCLX as the mitochondrial Na+/Ca2+ exchanger for efflux, and LETM1 as a proposed Ca2+/H+ transport pathway.

Originally identified as a key element of mitochondrial volume homeostasis through regulation of K+–H+ exchange (KHE), the LETM1 protein family is also involved in respiratory chain biogenesis… To add further complexity, LETM1 has been recently proposed to catalyze mitochondrial H+–Ca2+ exchange, which would imply a role in mitochondrial Ca2+ homeostasis as well.[5] Jiang et al. (2009) have suggested that LETM1 catalyzes Ca2+ accumulation in energized mitochondria only at low cytoplasmic Ca2+ concentrations, whereas MCU predominates at higher Ca2+ levels.[5] In summary, we think that the occurrence and mode of LETM1-dependent Ca2+ transport need further study to establish whether, besides its undisputable role in KHE, LETM1 can also support or modulate Ca2+ uptake in energized mitochondria.[5]

Our results indicate that LETM1 is a Ca2+/H+ antiporter and most likely responsible for mitochondrial Ca2+ output. Our data shows that LETM1 knockdown or overexpression robustly increases or decreases mitochondrial Ca2+ level in HeLa cells, respectively… Furthermore, the purified LETM1 exhibits Ca2+/H+ anti-transport activity and the activity is enhanced as the proton gradient is increased… Collectively, our results suggest that LETM1 is likely responsible for mitochondrial Ca2+ efflux.

LETM1-knockdown (LETM1-KD) decoupled the Ca2+/H+ antiport while LETM1 overexpression increased pH-induced Ca2+ transport 5-fold (Jiang et al., 2009). In subsequent studies, recombinant LETM1 reconstituted into liposomes exhibited LETM1-mediated Ca2+ transport that is bidirectional and insensitive to K+ and Na+, indicating that LETM1 is a CHE… Other studies have shown that LETM1-CHE may mediate both Ca2+-influx and efflux… LETM1-based regulation of mitochondrial Ca2+ is achieved by a LETM1-KHE activity whereby it regulates the activity of the mitochondrial Na+/H+ exchanger (mNHE), ultimately affecting NCLX-mediated Ca2+ efflux… These observations suggest that LETM1 functions as CHE and mediates mitochondrial Ca2+ efflux in these cells.

Here we expressed, purified, and reconstituted human Letm1 protein in liposomes… Using Ca2+ fluorophore and 45Ca2+-based assays, we demonstrate directly that Letm1 is a Ca2+ transporter… Further experiments show that Letm1 mediates electroneutral 1 Ca2+/2 H+ antiport.[6] Letm1 is insensitive to ruthenium red, an inhibitor of the mitochondrial calcium uniporter, and CGP-37157, an inhibitor of the mitochondrial Na+/Ca2+ exchanger.[6]

The review says LETM1 was “proposed to function as a mitochondrial Ca2+/H+ antiporter, thereby mediating mitochondrial calcium influx,” while MCU is described as the primary uptake mechanism and NCLX as a key exchanger for calcium efflux. It frames LETM1 as part of the broader network controlling mitochondrial calcium handling.

Originally identified as the mitochondrial K+/H+ exchanger, LETM1 was also considered as a candidate for the mitochondrial CHE. Defining the molecular nature of the mitochondrial Ca2+/H+ antiporter (CHE) has been challenging… Here, we identify TMBIM5 as the mitochondrial CHE that mediates Ca2+/H+ exchange across the inner mitochondrial membrane… Our data indicate that TMBIM5, and not LETM1, is responsible for the bulk Ca2+/H+ exchange activity in mammalian mitochondria.

This review describes LETM1 as a mitochondrial ion channel exchanger and presents it as part of the ongoing debate over mitochondrial calcium homeostasis. The article title itself emphasizes that LETM1 and mitochondrial calcium homeostasis remain “controversial,” indicating continued discussion of its transport role relative to other pathways.

The LETM1 gene family share a central role in regulating mitochondrial cation transport and osmotic volume.[7] LETM1 is now recognized as the mitochondrial Ca2+/H+ antiporter and has been linked to both K+ and Na+ homeostasis and mitochondrial Ca2+ efflux.[7] Studies have implicated LETM1 alongside the mitochondrial calcium uniporter (MCU) and the Na+/Ca2+/Li+ exchanger (NCLX) as part of the network controlling mitochondrial Ca2+ uptake and release.[7]

This review states that LETM1 is “essential for mitochondrial biology and cation homeostasis” and specifically discusses LETM1 in the context of mitochondrial calcium transport. It places LETM1 among the proteins implicated in mitochondrial Ca2+ handling alongside the better-established MCU and NCLX pathways.

Letm1, the mitochondrial Ca2+/H+ antiporter, is a pleiotropic protein implicated in mitochondrial morphology, metabolism, and ion homeostasis… Biophysical and reconstitution studies have demonstrated Ca2+/H+ exchange activity for LETM1; however, more recent genetic and biochemical work suggests that LETM1 is not the primary mitochondrial Ca2+/H+ antiporter in mammals, a role now attributed to TMBIM5… LETM1 likely plays a complementary and regulatory role in mitochondrial Ca2+ transport pathways that include the MCU and NCLX.

The pathway entry states that in human HeLa cells, knockdown of LETM1 causes an increase in mitochondrial calcium. It also says LETM1 causes a reduction in mitochondrial calcium uptake and proton movement, supporting its role as a proton-linked mitochondrial calcium transport pathway.

Plays an important role in maintenance of mitochondrial morphology and in mediating either calcium or potassium/proton antiport… Experimental evidence indicates LETM1 functions as a mitochondrial proton/calcium exchanger protein involved in Ca2+/H+ and K+/H+ exchange, contributing to mitochondrial cation homeostasis.

"LETM1 is a transporter protein shown to exhibit Ca2+/H+ exchange activity, acting as a crucial component in the regulation of Ca2+ homeostasis. Later it was suggested that LETM1 might also be involved in K+/H+ exchange, and its role in mitochondrial Ca2+ transport has been considered controversial…The mitochondrial calcium uniporter (MCU) complex mediates rapid Ca2+ uptake, whereas the Na+/Ca2+/Li+ exchanger (NCLX) is responsible for Ca2+ efflux. Additional pathways, including LETM1, have been proposed to modulate Ca2+ dynamics under specific conditions." This encyclopedic overview states that LETM1 exhibits Ca2+/H+ exchange and situates it as an additional pathway potentially modulating Ca2+ dynamics alongside MCU and NCLX, while noting controversy about its exact function.

LETM1 is a mitochondrial cation/proton exchanger essential for maintaining mitochondrial morphology and ion homeostasis. It mediates electroneutral potassium/proton and calcium/proton exchange across the inner mitochondrial membrane… Through its cation/proton exchange activity, LETM1 contributes to mitochondrial Ca2+ signaling together with the MCU and NCLX pathways.

The gene record identifies LETM1 as an inner mitochondrial membrane protein. While this record does not define its transport mechanism, it provides primary curated evidence that LETM1 is mitochondrial and relevant to mitochondrial ion homeostasis.

Knockdown of Letm1 in HeLa cells markedly reduced pH-dependent Ca2+ flux in mitochondria, whereas overexpression enhanced Ca2+ transport several-fold… Reconstitution of recombinant Letm1 into liposomes demonstrated Ca2+-transport activity driven by a proton gradient, consistent with Ca2+/H+ antiport… These findings suggest that Letm1 regulates mitochondrial Ca2+ homeostasis in parallel with the Ca2+ uniporter and Na+/Ca2+ exchanger.

LETM1 has been associated with regulation of fundamental cellular processes and with mitochondrial Ca2+ handling. Although initially proposed as the mitochondrial Ca2+/H+ antiporter, later work identified TMBIM5 as the main Ca2+/H+ exchanger, suggesting LETM1 may have a modulatory or context-dependent role in mitochondrial Ca2+ transport rather than being the sole CHE.