The yeast class V myosins, Myo2p and Myo4p, are nonprocessive actin-based motors

The motor properties of the two yeast class V myosins, Myo2p and Myo4p, wer e examined using in vitro motility assays. Both myosins are active motors w ith maximum velocities of 4.5 mum/s for Myo2p and 1.1 mum/s for Myo4p. Myo2 p motility is Ca2+ insensitive. Both myosins have properties of a nonproces sive motor, unlike chick myosin-Va (M5a), which behaves as a processive mot or when assayed under identical conditions. Additional support for the idea that Myo2p is a nonprocessive motor comes from actin cosedimentation assay s, which show that Myo2p has a low affinity for F-actin in the presence of ATP and Ca2+, unlike chick brain M5a. These studies suggest that if Myo2p f unctions in organelle transport, at least five molecules of Myo2p must be p resent per organelle to promote directed movement.