Glycosyltransferase MDR1 assembles a dividing ring for mitochondrial proliferation comprising polyglucan nanofilaments

Significance The mitochondrion-dividing (MD) ring mediates binary division of mitochondria. However, the molecular identity of the MD ring is currently unknown. We show that the glycosyltransferase MITOCHONDRION-DIVIDING RING1 (MDR1) regulates the synthesis of the polyglucan nanofilament bundle that assembles the MD ring. MDR1 is essential for mitochondrial division and forms a single ring at the mitochondrial division site in the unicellular red alga Cyanidioschyzon merolae . Nanoscale imaging and componential analysis demonstrated that MDR1 is involved in MD ring formation and that the MD ring filaments are composed of polymeric-glucose nanofilaments. An MDR1 homologue performs a similar function in chloroplast division, suggesting that the establishment of the MDR1 family was crucial for the emergence of endosymbiotic organelles.