The reduced motility of mitochondria lacking Puf3p causes them to fragment (middle) or aggregate (right).

Cells with fewer mitochondria must be careful to dole the organelles out equally to their progeny. As Garcia-Rodriguez et al. show on page 197, Puf3p reduces mitochondrial numbers when demand for the organelles is low but also ensures that the few remaining mitochondria are well connected to the machinery that pushes them around during reproduction.

Researchers knew that Puf3p latches onto messenger RNAs that encode mitochondrial proteins, and then accelerates their breakdown. A related protein, Puf1p, helps sort mitochondria when a yeast cell buds to produce offspring. Puf1p links mitochondria to the Arp2/3 complex, which generates the force to propel the organelles into the new bud.

Garcia-Rodriguez et al. tested whether Puf3p does likewise. The team found that Puf3p fastens Arp2/3 to the mitochore, a complex of integral mitochondrial membrane proteins that is required for mitochondrial movement. The connection is essential for shuttling mitochondria into the bud. In cells lacking Puf3p, mitochondria bunch up, break apart, and move sluggishly.

The scientists also found evidence that Puf3p impedes the formation, or biogenesis, of new mitochondria. When yeast were switched from a sugar diet to eating ethanol, they cranked out extra mitochondria and slashed production of Puf3p, suggesting that the protein was previously standing in the way of making new organelles. Furthermore, the shifted cells were sickly if they were forced to overexpress Puf3p. Garcia-Rodriguez et al. propose that cells use Puf3p to reduce numbers when the mitochondria are not needed, but also to ensure that the reduction does not lead to mitochondrial loss and cell death.