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mRNA localization has an essential role in localizing cytoplasmic determinants, controlling the direction of protein secretion, and allowing the local control of protein synthesis in neurons [1, 2]. In neuronal dendrites, the localization and translocation of mRNA is considered as one of the molecular bases of synaptic plasticity. Recent imaging and functional studies revealed that several RNA-binding proteins form a large messenger ribonucleoprotein (mRNP) complex that is involved in transport and translation of mRNA in dendrites [3, 4]. However, the mechanism of mRNA translocation into dendritic spines is unknown. Here, we show that an actin-based motor, myosin-Va [5, 6], plays a significant role in mRNP transport in neuronal dendrites and spines. Myosin-Va was Ca²⁺-dependently associated with TLS, an RNA-binding protein, and its target RNA Nd1-L, an actin stabilizer. A dominant-negative mutant or RNAi of …