Myosin IXb is a single-headed minus-end-directed processive motor

Myosin is an actin-based molecular motor that constitutes a diverse superfamily(1,2). In contrast to conventional myosin, which binds to actin for only a short time during cross-bridge cycling, recent studies have demonstrated that class V myosin moves along actin filaments for a long distance without dissociating(3,4). This would make it suitable for supporting cargo movement in cells. Because myosin V has a two-headed structure with an expanded neck domain, it has been postulated to 'walk' along the 36-nm helical repeat of the actin filament, with one head attached to the actin and leading the other head to the neighbouring helical pitch(5). Here, we report that myosin IXb, a single-headed myosin, moves processively on actin filaments. Furthermore, we found that myosin IXb is a minus-end-directed motor. In addition to class VI myosin, this is the first myosin superfamily member identified that moves in the reverse directions. The processive movement of the single-headed myosin IXb cannot be explained by a 'hand-over-hand' mechanism. This suggests that an alternative mechanism must be operating for the processive movement of single-headed myosin IXb.