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

We screened a human cDNA library for proteins that bind mRNA cap methyltran sferase (MT) and isolated nuclear transporter importin-alpha (Imp alpha). T his direct association was confirmed by glutathione S-transferase (GST) pul ldown, coimmnnoprecipitation, and nuclear colacalization. In gel shift assa ys, MT selectively bound RNA containing 5'-terminal GpppG, and binding was inhibited by GpppG and not by m(7)GpppC. Imp alpha markedly enhanced MT bin ding to GpppG-RNA and stimulated MT activity. MT/RNA/Imp alpha complexes we re dissociated by importin-beta, which also blocked the stimulation of cap methylation by Imp alpha. The presence of RanGTP but not RanGDP prevented t hese effects of importin-beta. These findings indicate that importins play a novel role in mRNA biogenesis at the level of cap methylation.