Cap methyltransferase selective binding and methylation of GpppG-RNA are stimulated by importin-α

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Abstract
We screened a human cDNA library for proteins that bind mRNA cap methyltransferase (MT) and isolated nuclear transporter importin-α (Impα). This direct association was confirmed by glutathione S-transferase (GST) pulldown, coimmunoprecipitation, and nuclear colocalization. In gel shift assays, MT selectively bound RNA containing 5′-terminal GpppG, and binding was inhibited by GpppG and not by m7GpppC. Impα markedly enhanced MT binding to GpppG-RNA and stimulated MT activity. MT/RNA/Impα complexes were dissociated by importin-β, which also blocked the stimulation of cap methylation by Impα. The presence of RanGTP but not RanGDP prevented these effects of importin-β. These findings indicate that importins play a novel role in mRNA biogenesis at the level of cap methylation.