Trypanosome Letm1 Protein Is Essential for Mitochondrial Potassium Homeostasis

Abstract

Letm1 is a conserved protein in eukaryotes bearing energized mitochondria. Hemizygous deletion of its gene has been implicated in symptoms of the human disease Wolf-Hirschhorn syndrome. Studies almost exclusively performed in opisthokonts have attributed several roles to Letm1, including maintaining mitochondrial morphology, mediating either calcium or potassium/proton antiport, and facilitating mitochondrial translation. We address the ancestral function of Letm1 in the highly diverged protist and significant pathogen, Trypanosoma brucei . We demonstrate that Letm1 is involved in maintaining mitochondrial volume via potassium/proton exchange across the inner membrane. This role is essential in the vector-dwelling procyclic and mammal-infecting bloodstream stages as well as in Trypanosoma brucei evansi , a form of the latter stage lacking an organellar genome. In the pathogenic bloodstream stage, the mitochondrion consumes ATP to maintain an energized state, whereas that of T. brucei evansi also lacks a conventional proton-driven membrane potential. Thus, Letm1 performs its function in different physiological states, suggesting that ion homeostasis is among the few characterized essential pathways of the mitochondrion at this T. brucei life stage. Interestingly, Letm1 depletion in the procyclic stage can be complemented by exogenous expression of its human counterpart, highlighting the conservation of protein function between highly divergent species. Furthermore, although mitochondrial translation is affected upon Letm1 ablation, it is an indirect consequence of K + accumulation in the matrix. Background: Letm1 is a mitochondrial protein attributed disparate roles, including cation/proton antiport and translation. Results: Letm1 RNAi silencing in Trypanosoma brucei triggers swelling mitochondria and translation arrest that is ameliorated by chemical potassium/proton exchangers. Conclusion: The ancestral function of Letm1, to maintain mitochondrial potassium homeostasis, shows remarkable conservation. Significance: Results from diverged T. brucei provide a better understanding of Letm1 function throughout eukaryotes.