Evidence for a New Member of the Myosin I Family from Mammalian Brain

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Abstract
Myosin I is an actin‐based motor responsible for powering a wide variety of motile activities in amebae and slime molds and has been found previously in vertebrates as the lateral bridges within intestinal epithelial cell microvilli. Although neurons exhibit extensive cellular and intracellular motility, including the production of ameboid‐like growth cones during development, the proteins responsible for the motor in these processes are unknown. Here, we report the isolation of a partially purified protein fraction from bovine brain that is enriched for a 150‐kDa protein; immunochemical and biochemical analyses suggest that this protein possesses a number of functional properties that have been ascribed to myosin I from various sources. These properties include an elevated K+‐EDTA ATPase, a modestactin‐activated Mg2+‐ATPase, the ability to bind calmodulin, and a ready association with phospholipid vesicles made from phosphatidylserine, but not from phosphatidylcholine. The combination of these properties, together with a molecular mass of 150 kDa (most myosin I molecules found to date have molecular masses in the range 110–130 kDa) yet recognition by an anti‐myosin I antibody, suggests the presence of a new member of the myosin I family within mammalian brain.