Ancient dynamin segments capture early stages of host–mitochondrial integration

Significance Mitochondria were originally free-living bacteria with their own division machinery, which took up residence within another cell 2 billion y ago. The host cell subsequently tamed mitochondrial division using dynamin, a membrane-pinching protein. We have found that a single ancient dynamin at the root of extant eukaryotes, capable of pinching both mitochondria and vesicles, duplicated independently in plants and animals into specialized mitochondrial and vesicle variants. A “living fossil” of this ancient bifunctional dynamin still survives in scattered eukaryotic species, along with the original bacterial FtsZ division protein. The mitochondria of these organisms, preserved as if in amber, might teach us how the fateful partnership between host and endosymbiont was first established.