Mitochondrial RNA granules are fluid condensates, positioned by membrane dynamics

Mitochondria contain the genetic information and expression machinery to produce proteins essential for cellular respiration. Within the mitochondrial matrix, newly synthesized RNA, RNA processing proteins, and mitoribosome assembly factors are known to form punctate subcompartments referred to as mitochondrial RNA granules (MRGs) 1-3. Despite their proposed role in regulating gene expression, little is known about the structural and dynamic properties of MRGs. We investigated the organization of MRGs using fluorescence super-resolution localization microscopy and correlative electron microscopy techniques, obtaining ultrastructural details of their internal architecture. We find that MRGs are organized into nanoscale RNA cores surrounded by a protein shell. Using live-cell super-resolution structured illumination microscopy and photobleaching perturbations, we reveal that MRGs undergo fusion and rapidly exchange components, consistent with liquid-liquid phase separation (LLPS). Furthermore, MRGs associate with the inner mitochondrial membrane and their fusion coincides with membrane remodeling. Inhibition of mitochondrial fission leads to an aberrant distribution of MRGs into concentrated pockets, where they remain as distinct individual units despite their close apposition. Together, our results reveal a role for LLPS in concentrating RNA and its processing proteins into MRGs, which are positioned along mitochondria by membrane dynamics.