Extensive purification from Acanthamoeba castellanii of a microtubule-dependent translocator with microtubule-activated Mg2+-ATPase activity.

A protein which supported MgATP-dependent movement of latex beads from the minus to the plus end of microtubules and which had microtubule-activated Mg2+-ATPase was purified from Acanthamoeba castellanii. At concentrations as low as 0.6 micrograms ml-1, the translocator supported movement of beads at a rate of 3 to 4 micron s-1. The translocator protein had a Ca2+-ATPase activity of 1.7 mumol min-1 mg-1 and a Mg2+-ATPase activity of about 0.03 mumol min-1 mg-1 in the absence of microtubules. The Mg2+-ATPase in the presence of microtubules had a Vmax of 3.4 mumol min-1 mg-1; half-maximal Mg2+-ATPase activity required only 0.45 microM microtubules (concentration of dimer subunits). The highly purified native protein had a Stokes radius of 8.5 nm, and three polypeptides of Mr 134,000, 139,000, and 147,000 were associated with the fractions that had maximum translocator and ATPase activities.