ACTOMYOSIN-LIKE PROTEIN IN BRAIN: SUBCELLULAR DISTRIBUTION

Abstract— An actomyosin-like protein (neurostenin) has been isolated from the synaptosomal fraction of bovine and rat brains. A similar protein could not be obtained from the mitochondrial, microsomal, myelin or supernatant fractions. The synaptosomal protein is a Mg²⁺ -Ca²⁺ -stimulated ATPase which exhibited the phenomenon of superprecipitation and viscosimetric sensitivity to ATP characteristic of actomyosins. It constituted 8–10 per cent of the total synaptosomal proteins. The protein could be dissociated into actin-like (neurin) and myosin-like (stenin) proteins by ultracentrifugation in sucrose gradients containing KI and ATP. The neurin, as well as muscle actin, stimulated the Mg²⁺ -ATPase activity of stenin and muscle myosin; the relative viscosities of the mixtures were increased and became sensitive to added ATP. The neurin contained 0.9 mol of 3-methylhistidine per 50,000 g of protein. The presence of these proteins in the synaptosomes suggests the possibility that they participate in nerve-ending functions, e.g. release of transmitter materials.