Mitochondria (red) are stickier in cells that make faulty MARCH5 (green).

Breaking up isn't hard to do for mitochondria, which are continually separating and merging. On page 71, Karbowski et al. pinpoint a protein that prods the organelles to go their own way.

Cells carefully control mitochondrial fusion and fission, and an imbalance between the processes can be disastrous. For example, a faulty fusion-promoting protein triggers dominant optic atrophy, the leading cause of inherited blindness. Researchers know more about the regulation of fusion and fission in yeast than in mammals. But they do know that one of the mammalian proteins essential for mitochondrial breakup is Drp1.

Karbowski et al. pinpointed another, called MARCH5, which colocalizes with Drp1. Although two studies published last year suggested that MARCH5 promoted mitochondrial fusion, the scientists now find the opposite. When they altered cells to produce a defective version of the protein, mitochondria stuck together to form extra-long networks instead of breaking apart. The researchers also observed this abnormal elongation when they added RNAi against MARCH5.

The protein normally spreads out around the mitochondrial membrane, but the mutant MARCH5 clumped. These clusters trapped Drp1. These findings suggest that MARCH5 spurs mitochondrial splitting by helping to direct Drp1 to the future separation site. MARCH5 is a ubiquitin ligase that works by attaching a ubiquitin molecule to its target. The researchers' next move is to track down that target.