When peroxisome transport is blocked (right), Inp2p (green) localizes to every peroxisome (red) in the mother cell rather than just those that are moving into the bud (left).

When peroxisome transport is blocked (right), Inp2p (green) localizes to every peroxisome (red) in the mother cell rather than just those that are moving into the bud (left).

Every yeast cell has numerous peroxisomes—organelles that metabolize fatty acids—and roughly half are passed on to their daughter cells. Signaling from the bud to the mother cell ensures even peroxisome distribution by controlling the organelles' transport, say Fagarasanu et al.

Peroxisomes travel into the bud along actin tracks using the myosin motor Myo2p, which binds to peroxisomes via a specific receptor called Inp2p. Cells ramp up Inp2p expression during mitosis, but the receptor only appears on a subset of peroxisomes, potentially selecting them for Myo2p-mediated movement to the bud.

Fagarasanu et al. made point mutants in Myo2p that blocked peroxisome transport, and were surprised that Inp2p levels increased until every peroxisome in the mother cell carried the receptor. This suggests that once enough peroxisomes have accumulated in the bud of wild-type cells, a signal is relayed to the mother cell resulting in Inp2p degradation and a halt to further peroxisome transfer. The group also observed this feedback mechanism within the membrane of a single peroxisome: mutants with a solitary large peroxisome that positions itself halfway into the bud lost Inp2p from the half that remained in the mother cell.

Thus, both the cell cycle and the distribution of peroxisomes control Inp2p expression levels to regulate peroxisome inheritance in time and space. The next challenge, says lead author Andrei Fagarasanu, is to work out how cells sense the presence of peroxisomes in the bud and transmit a signal back to the mother cell to trigger Inp2p's degradation. BS

References

Fagarasanu
A.
et al
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2009
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J. Cell Biol.
doi: .