Competition for the motor protein Myo2 helps coordinate the inheritance of budding yeast organelles, Eves et al. reveal.
Myo2 transports numerous organelles from the mother cell into the bud. The different organelles associate with Myo2 via specific cargo adaptors that bind to a cargo-binding domain in the motor protein's C terminus. Eves et al. found that Mmr1 and Vac17, the adaptors for mitochondria and vacuoles, respectively, bound to overlapping sites on Myo2 and competed for access to the motor protein in vitro.
Mitochondria and vacuoles might take turns binding Myo2, but Eves et al. found that both organelles enter the bud at the same stage of the cell cycle. The researchers selectively inhibited the inheritance of mitochondria or vacuoles by expressing Myo2 mutants unable to bind either Mmr1 or Vac17. When Myo2’s interaction with Mmr1 was abolished, fewer mitochondria and more vacuoles were delivered into the bud. Inhibiting Myo2’s association with Vac17 had the opposite effect. On the other hand, overexpressing either of the cargo adaptors boosted the inheritance of its associated organelle at the expense of the other.
Changes in the quantities of organelles persisted in the daughter cells, indicating that inheritance—and competition for Myo2—is a key regulator of organelle volume. Moreover, Eves et al. found that six other cargo adaptors, including the adaptors for peroxisomes and secretory vesicles, all bound to a second site on Myo2, suggesting that competition may be a common mechanism of coordinating inheritance. Senior author Lois Weisman now wants to investigate whether the two adaptor-binding sites on Myo2 can modulate each other's function.