TraB is a component of the ER-Mitochondrial contact site regulating ER-Mitochondrial interaction and mitophagy

ER-mitochondria contact sites (EMCSs) are important for mitochondrial function. Here, we have identified a EMCS complex, comprising a family of uncharacterised mitochondrial outer- membrane proteins, TraB1, TraB2 and the ER protein, VAP27-1. In Arabidopsis, there are three TraB isoforms and the trab1a/trab2 double mutant exhibits abnormal mitochondrial morphology, strong starch accumulation and impaired energy metabolism, indicating that these proteins are essential for normal mitochondrial function. Moreover, TraB1 and TraB2 proteins also interact with ATG8 in order to regulate mitochondrial degradation (mitophagy). The turnover of depolarised mitochondria is significantly reduced in both trab1/trab2 and VAP27 mutants (vap27-1,3,4,6) under mitochondrial stress conditions, with an increased population of dysfunctional mitochondria present in the cytoplasm. Consequently, plant recovery after stress is significantly perturbed, suggesting that TraB1 regulated mitophagy and ER-mitochondrial interaction are two closely related processes. Taken together, we ascribe a dual role to TraB1 which is a component of the EMCS complex in eukaryotes, regulating both interaction of the mitochondria to the ER and mitophagy.