Dynamic interaction of REEP5–MFN1/2 enables mitochondrial hitchhiking on tubular ER

Mitochondrial functions can be regulated by membrane contact sites with the endoplasmic reticulum (ER). These mitochondria-ER contact sites (MERCs) are functionally heterogeneous and maintained by various tethers. Here, we found that REEP5, an ER tubule-shaping protein, interacts with Mitofusins 1/2 to mediate mitochondrial distribution throughout the cytosol by a new transport mechanism, mitochondrial "hitchhiking" with tubular ER on microtubules. REEP5 depletion led to reduced tethering and increased perinuclear localization of mitochondria. Conversely, increasing REEP5 expression facilitated mitochondrial distribution throughout the cytoplasm. Rapamycin-induced irreversible REEP5-MFN1/2 interaction led to mitochondrial hyperfusion, implying that the dynamic release of mitochondria from tethering is necessary for normal mitochondrial distribution and dynamics. Functionally, disruption of MFN2-REEP5 interaction dynamics by forced dimerization or silencing REEP5 modulated the production of mitochondrial reactive oxygen species (ROS). Overall, our results indicate that dynamic REEP5-MFN1/2 interaction mediates cytosolic distribution and connectivity of the mitochondrial network by "hitchhiking" and this process regulates mitochondrial ROS, which is vital for multiple physiological functions.