To study the intranuclear localization of the U1-specific snRNP C protein and its assembly into U1 snRNPs, we injected transcripts encoding a myc-tagged C protein into amphibian oocytes. The distribution of protein translated from the injected RNA was essentially the same in continuous and pulse-label experiments. In both cases the C protein localized within the germinal vesicle in those structures known to contain U1 snRNPs, namely the lampbrush chromosome loops and hundreds of extrachromosomal granules called snurposomes. Oocytes were also injected with an antisense oligodeoxynucleotide that caused truncation of U1 snRNA at the 5' end. In these oocytes, myc-tagged C protein localized normally in the germinal vesicle and could be immunoprecipitated together with truncated U1 snRNA. These experiments suggest that the C protein can enter the germinal vesicle on its own and there associate with previously assembled U1 snRNPs. In transfected tissue culture cells, the myc-tagged C protein localized within the nucleus in a speckled pattern similar to that of endogenous U1 snRNPs.

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