We report the isolation of a protein from mammalian nerve which shows ATP-sensitive binding to microtubules and ATPase activity. This protein, which we have designated HMW4, was prepared from bovine spinal nerve roots by microtubule affinity and ATP-induced release, and was further purified by sucrose density gradient centrifugation. It is a high molecular weight protein with a denatured Mr of 315,000, a Stokes radius of 90 A, and a sedimentation value of approximately 19S. It can be resolved electrophoretically from the well-characterized bovine brain microtubule-associated proteins (MAPs) and also appears to be distinct from MAP 1C. HMW4 has a vanadate-sensitive and azide-insensitive ATPase activity which averages 20 nmol Pi/min per mg protein and is different from dynein and myosin ATPases. HMW4 prepared on sucrose gradients exhibits binding to MAP-free microtubules in the absence of ATP which is reduced by ATP addition. Assayed by darkfield microscopy, HMW4 causes bundling of MAP-free microtubules which is reversed by ATP addition.

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