An ATPase with properties expected for the organelle motor of the giant amoeba, Reticulomyxa

Abstract

The rapid, vectorial, microtubule-associated transport of organelles is believed to be mediated by specific mechanochemical transducers. Recent studies of various metazoan cells have allowed the identification of novel microtubule-dependent translocator molecules capable of promoting microtubule gliding across glass surfaces and translocation of inert beads along microtubules. These translocators could be involved in force generation for directional organelle movements in vivo. Here we report the identification of a microtubule-binding protein with characteristics expected for an organelle translocator in the giant freshwater amoeba Reticulomyxa. This factor has an apparent relative molecular mass (Mr) of 440,000 (440K) and sediments at 20-22S in sucrose-density gradients. It binds to microtubules under conditions of ATP depletion, possesses an ATPase activity and is sensitive to ultraviolet-induced, vanadate-dependent cleavage. Although its pharmacological properties differ from those of axonemal dynein, it can be considered to be a variant of cytoplasmic dynein. The Reticulomyxa high-molecular-weight protein (HMWP) promotes rapid, bidirectional movement of latex beads along Reticulomyxa microtubules in vitro at an average speed of 3.6 micron s-1. This protein, therefore, is a likely candidate for a microtubule-dependent motor.