Cytoplasmic transport and localization of mRNA has been reported for a range of oocytes and somatic cells. The heterogeneous nuclear ribonucleoprotein (hnRNP) A2 response element (A2RE) is a 21-nucleotide segment of the myelin basic protein mRNA that is necessary and sufficient for cytoplasmic transport of this message in oligodendrocytes. The predominant A2RE-binding protein in rat brain has previously been identified as hnRNP A2. Here we report that an 11-nucleotide subsegment of the A2RE (A2RE11) was as effective as the fulllength A2RE in binding hnRNP A2 and mediating transport of heterologous RNA in oligodendrocytes. Point mutations of the A2RE11 that eliminated binding to hnRNP A2 also markedly reduced the ability of these oligoribonucleotides to support RNA transport. Oligodendrocytes treated with antisense oligonucleotides directed against the translation start site of hnRNP A2 had reduced levels of this protein and disrupted transport of microinjected myelin basic protein RNA. Several A2RE-like sequences from localized neuronal RNAs also bound hnRNP A2 and promoted RNA transport in oligodendrocytes. These data demonstrate the specificity of A2RE recognition by hnRNP A2, provide direct evidence for the involvement of hnRNP A2 in cytoplasmic RNA transport, and suggest that this protein may interact with a wide variety of localized messages that possess A2RE-like sequences.Vectorial transport and localized translation of mRNA in the cytoplasm affords a mechanism for establishing an asymmetric distribution of cytosolic proteins in cells. This is particularly important in oogenesis and embryonic development (1-8). In somatic cells, some mRNAs are also transported to discrete locations within the cell. These include myelin basic protein (MBP) 1 mRNA, which is localized in the myelinating periphery of oligodendrocytes (9 -11), microtubule-associated protein 2A (MAP2A) and tau, which are localized in neurites (12-14), and -actin mRNA, which is localized in the leading edge of fibroblasts (15-17). An active, multistep, cytoplasmic transport pathway has been delineated for MBP mRNA in cultured oligodendrocytes. MBP transcripts microinjected into the soma assemble into granules that move out along the myelin-forming processes (18,19). Transport of these granules is dependent on the presence of kinesin and intact microtubules (20). Similar RNA-rich granules have been observed in other cells (21)(22)(23)(24).A cis-acting sequence sufficient and necessary for MBP mRNA transport has been identified within a 21-nucleotide stretch of the 3Ј-untranslated region. Incorporation of this segment, the hnRNP A2 response element (A2RE, formerly termed the RNA transport sequence (25)), into heterologous RNAs mediates transport into the processes and also enhances translation.2 A2RE-like sequences are present in other localized mRNAs including mouse MAP2A, mouse protamine 2, and rat glial fibrillary acidic protein (25), suggesting that the A2RE represents a general signal for RNA transport.Earlier we used biotin-labeled oligorib...