Ringing out the Truth How Do Eukaryotes Divide their Mitochondria

Mitochondria are semi-autonomous, energy-producing organelles within eukaryotic cells. These organelles evolved from a bacterial endosymbiont that was acquired more than a billion years ago and have been faithfully inherited from one cell generation to the next during this time. New mitochondria cannot be made de novo but must be produced by the division of pre-existing organelles. Cells require new mitochondria, not only as they divide and grow, but also to replace damaged or ageing organelles. In recent years, several proteins involved in the division of mitochondria have been discovered and their functions are being characterised (1-4). These proteins form patches and/or rings around the outside and the inside of dividing organelles. Outer rings may have been developed by eukaryotes from the earliest times, when the bacterial endosymbiont that later became a mitochondrion lost its cell wall. Protein components of the inner ring are not as well characterised, and may be much more variable in composition across the eukaryotic spectrum than those of the outer ring. In animals and fungi the inner ring appears to share similar proteins with the outer ring but, in the ancient protistan eukaryotes, mitochondrial inner ring proteins are often direct descendants of the proteins that bacteria use to divide (Fig. 1).