The GET pathway can increase the risk of mitochondrial outer membrane proteins to be mistargeted to the ER

Tail-anchored (TA) proteins are anchored to their corresponding membrane via a single transmembrane segment (TMS) at their C-terminus. In yeast, the targeting of TA proteins to the endoplasmic reticulum (ER) can be mediated by the guided entry of TA proteins (GET) pathway, whereas it is not yet clear how mitochondrial TA proteins are targeted to their destination. It is widely observed that some mitochondrial outer membrane (OM) proteins are mistargeted to the ER when overexpressed or when their targeting signal is masked. However, the mechanism of this erroneous sorting is currently unknown. In this study, we demonstrate the involvement of the GET machinery in mistargeting of non-optimal mitochondrial OM proteins to the ER. These findings suggest that the GET machinery can, in principle, recognize and guide mitochondrial and non-canonical TA proteins. Hence, under normal conditions, an active mitochondrial targeting pathway must exist that dominates the kinetic competition against other pathways

The GET pathway can increase the risk of mitochondrial outer membrane proteins to be mistargeted to the ER

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Stress-dependent macromolecular crowding in the mitochondrial matrix

Macromolecules of various sizes induce crowding of the cellular environment. This crowding impacts on biochemical reactions by increasing solvent viscosity, decreasing the water-accessible volume and altering protein shape, function, and interactions. Although mitochondria represent highly protein-rich organelles, most of these proteins are somehow immobilized. Therefore, whether the mitochondrial matrix solvent exhibits macromolecular crowding is still unclear. Here, we demonstrate that fluorescent protein fusion peptides (AcGFP1 concatemers) in the mitochondrial matrix of HeLa cells display an elongated molecular structure and that their diffusion constant decreases with increasing molecular weight in a manner typical of macromolecular crowding. Chloramphenicol (CAP) treatment impaired mitochondrial function and reduced the number of cristae without triggering mitochondrial orthodox-to-condensed transition or a mitochondrial unfolded protein response. CAP-treated cells displayed progressive concatemer immobilization with increasing molecular weight and an eightfold matrix viscosity increase, compatible with increased macromolecular crowding. These results establish that the matrix solvent exhibits macromolecular crowding in functional and dysfunctional mitochondria. Therefore, changes in matrix crowding likely affect matrix biochemical reactions in a manner depending on the molecular weight of the involved crowders and reactants

SMDT1 variants impair EMRE-mediated mitochondrial calcium uptake in patients with muscle involvement

Ionic calcium (Ca2+) is a key messenger in signal transduction and its mitochondrial uptake plays an important role in cell physiology. This uptake is mediated by the mitochondrial Ca2+ uniporter (MCU), which is regulated by EMRE (essential MCU regulator) encoded by the SMDT1 (single-pass membrane protein with aspartate rich tail 1) gene. This work presents the genetic, clinical and cellular characterization of two patients harbouring SMDT1 variants and presenting with muscle problems. Analysis of patient fibroblasts and complementation experiments demonstrated that these variants lead to absence of EMRE protein, induce MCU subcomplex formation and impair mitochondrial Ca2+ uptake. However, the activity of oxidative phosphorylation enzymes, mitochondrial morphology and membrane potential, as well as routine/ATP-linked respiration were not affected. We hypothesize that the muscle-related symptoms in the SMDT1 patients result from aberrant mitochondrial Ca2+ uptake

The Set7 Lysine Methyltransferase Regulates Plasticity in Oxidative Phosphorylation Necessary for Trained Immunity Induced by β-Glucan.

Trained immunity confers a sustained augmented response of innate immune cells to a secondary challenge, via a process dependent on metabolic and transcriptional reprogramming. Because of its previous associations with metabolic and transcriptional memory, as well as the importance of H3 histone lysine 4 monomethylation (H3K4me1) to innate immune memory, we hypothesize that the Set7 methyltransferase has an important role in trained immunity induced by β-glucan. Using pharmacological studies of human primary monocytes, we identify trained immunity-specific immunometabolic pathways regulated by Set7, including a previously unreported H3K4me1-dependent plasticity in the induction of oxidative phosphorylation. Recapitulation of β-glucan training in vivo additionally identifies Set7-dependent changes in gene expression previously associated with the modulation of myelopoiesis progenitors in trained immunity. By revealing Set7 as a key regulator of trained immunity, these findings provide mechanistic insight into sustained metabolic changes and underscore the importance of characterizing regulatory circuits of innate immune memory