The mRNA-binding protein She2p couples two myosin motors together in order to transport mRNAs through yeast cells, Krementsova et al. report.
Myo4p is a class V myosin that carries mRNAs along actin cables into the buds of S. cerevisiae. Myo4p links to its cargo by pairing up with the adaptor protein She3p, which, in turn, binds the mRNA-binding protein She2p. In vitro, however, Myo4p and She3p fail to move continuously along actin tracks because, unlike many other class V myosins, the motor doesn't homodimerize and therefore only has one ATPase head domain, which can't take processive steps along the filament on its own.
Krementsova et al. found that Myo4p–She3p complexes gained the ability to move processively along in vitro actin filaments in the presence of She2p. The mRNA-binding protein formed tetramers in solution, each of which recruited two Myo4p–She3p dimers. The resulting complex walked along actin cables like vertebrate myosin Vs, taking similarly sized steps even though the two Myo4 motors are linked via a series of adaptor proteins instead of through a direct interaction.
Senior author Kathleen Trybus thinks that She2p's ability to couple Myo4p motors is an elegant way of regulating mRNA transport, because the myosin will only move along actin cables when linked to its cargo. Trybus now wants to investigate whether mRNAs themselves alter the activity of the Myo4p–She3p–She2p complex.