cis-Acting signals and trans-acting proteins are involved in tau mRNA targeting into neurites of differentiating neuronal cells

Tau microtubule-associated protein is a neuron specific protein found primarily in axons and is developmentally regulated. The function of tau is in stabilization of microtubules, which is important in establishing and maintaining neuronal morphology. Axonal localization of tau involves a multistep process which is studied in differentiating primary neuronal culture. The initial step involves sorting and subcellular localization of its encoding mRNA into the proximal portion of the axon. Using the transfection assay into neuronal cells, we have demonstrated that sequences located in the 3′-untranslated region include a cis-acting signal which is involved in tau mRNA targeting. In addition, using ultraviolet cross-linking assay, two RNA-binding proteins of 43 and 38 kDa were identified, that exhibit specific binding to a minimal sequence of 91 nucleotides located within the same functional region, which is involved in targeting. The 43 and 38-kDa RNA-binding proteins are present in cytoplasmic extracts, prepared from neuronal cells, and in isolated microtubule preparations. Our results support a novel model in which cis-acting signals, together with RNA-binding proteins, are involved in the targeting of tau mRNA, that may ultimately lead to its axonal localization.