During mitochondrial fission, the mitochondrial outer membrane proteins Mff, MiD49 and MiD51 recruit Drp1 to execute organelle constriction. Intriguingly, recent studies indicate that endoplasmic reticulum (ER) tubules can wrap around mitochondria and mediate membrane constriction in a Drp1-independent manner, but it is still unclear whether other proteins are involved. To investigate whether MiD49 and MiD51 are linked to the role of the ER in mitochondrial fission, Kirstin Elgass and colleagues (p. ) used live-cell confocal imaging, correlative cryogenic fluorescence microscopy (CRM) and soft X-ray tomography (SXT) to render a 3D reconstruction of ER–mitochondria contact sites. They observed that MiD49 colocalised with Mff, and MiD51 colocalised with Mff and Drp1. Both MiD proteins formed dynamic foci that were found both within and outside of constriction sites, and within sites that underwent repeated constriction–expansion cycles. The authors then established that mitochondria–ER contact sites colocalise with MiD foci, but that only 40% of these contacts were at constriction sites. Using CRM-SXT they next reconstructed the 3D ER–mitochondria landscape, and found that the ER forms short extensions that contact the mitochondria at MiD foci, the length of which was under the limit of resolution for confocal microscopy. Therefore, besides showing that ER tubules contact mitochondria at MiD foci, this study also employs a novel imaging approach that could be extremely useful for high-resolution studies of intracellular structures.
