A peroxisome protein (green) spreads (left to right) from the ER of one yeast cell to the peroxisomes (red) of a mating partner.

The question of whether cells manufacture new peroxisomes from scratch or cleave existing organelles has divided researchers. On page 399, Motley and Hettema show that yeast opt for the second mechanism, only turning to synthesis if they run out of the organelles.

Packed with enzymes, peroxisomes are essential for defusing cellular toxins—a shortage of the organelles triggers the lethal disorder Zellweger syndrome. Before a cell divides, it duplicates its stock of peroxisomes, half of which it then parcels out to its daughter cell. But research on how cells fashion more peroxisomes has produced contradictory results. Some work suggests that new structures bud from the endoplasmic reticulum. Other findings suggest that new peroxisomes form when existing ones split. A third explanation is that cells rely on both mechanisms.

To sort through these possibilities in yeast cells, Motley and Hettema attached GFP to a protein segment that directs molecules to the peroxisomes. They briefly switched on production of the combination, then shut it off and followed what happened through several rounds of cell division.

If yeast relied on ER budding to produce new peroxisomes, the researchers reasoned, the number of labeled peroxisomes per cell should decline as new, nonlabeled structures emerged from the ER. If cells relied on the fission of existing peroxisomes, however, the number of labeled structures should remain the same, but their brightness should decline as peroxisomes repeatedly halved their store of GFP. The second possibility was correct, Motley and Hettema discovered.

The researchers then devised a new technique to temporarily trap a peroxisome membrane protein in the ER until yeast cells mate. The team found that the ER did furnish some peroxisome membrane components. However, yeast cells only used ER budding to produce new peroxisomes when they had none of their own. Motley and Hettema conclude that the ER is continually releasing preperoxisomes, but they usually merge with existing organelles. Only if the cell lacks peroxisomes will these structures mature. The researchers suspect that the cells prefer fission to make peroxisomes because it's faster than ER budding.