The filopodial myosin DdMyo7 is a slow, calcium-regulated motor

MyTH4-FERM (MF) myosins are a family of molecular motors with critical roles in the formation and organization of thin membrane protrusions supported by parallel bundles of actin - filopodia, microvilli, and stereocilia. The amoeboid MF myosin DdMyo7 is essential for filopodia formation but its mechanism of action is unknown. The motor properties of a forced-dimer of the DdMyo7 motor were characterized using an in vitro motility assay to address this question. The DdMyo7 motor associates with two different light chains, the Dictyostelium calmodulins CalA and CalB, whose binding is shown to be sensitive to the presence of calcium. Total internal reflection fluorescence motility assays of the dimerized DdMyo7 motor reveal that it is a slow, processive motor that moves along actin at ∼ 40 nm/sec, and the activity of the motor is significantly reduced in the presence of Ca²⁺. The speed of DdMyo7 is similar to that of other Myo7 family members such as human Myo7A and fly DmMyo7A, but is at least 10-fold slower than the mammalian filopodial MF myosin, Myo10. The results show that evolutionarily distant native filopodial myosins can promote filopodia elongation using motors with distinct properties, revealing diverse mechanisms of myosin-based filopodia formation.