The signal requirement for the nuclear import of U1 RNA in somatic cells from different species was investigated by microinjection of both digoxygenin-labeled wild type and mutant U1 RNA molecules and in vitro reconstituted U1 snRNPs. U1 RNA was shown to be targeted to the nucleus by a temperature-dependent process that requires the prior assembly of RNPs from the common proteins and the microinjected RNA. Competition in the cell between immunoaffinity-purified U1 snRNPs and digoxygenin-labeled U1 snRNPs reconstituted in vitro showed that the transport is saturable and should therefore be a mediated process. The transport of a karyophilic protein under the same conditions was not affected, indicating the existence of a U snRNP-specific transport pathway in somatic cells, as already seen in the Xenopus laevis oocyte system. Surprisingly, the signal requirement for nuclear transport of U1 snRNP was found to differ between oocytes and somatic cells from mouse, monkey and Xenopus, in that the m3GGpppG-cap is no longer an essential signaling component in somatic cells. However, as shown by investigation of the transport kinetics of m3GpppG- and ApppG-capped U1 snRNPs, the m3GpppG-cap accelerates the rate of U1 snRNP import significantly indicating that it has retained a signaling role for nuclear targeting of U1 snRNP in somatic cells. Moreover, our data strongly suggest that cell specific rather than species specific differences account for the differential m3G-cap requirement in nuclear import of U1 snRNPs.

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