U1 small nuclear ribonucleoprotein studied by in vitro assembly.

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Abstract
The small nuclear RNA (snRNA) are known to be complexed with proteins in the cell (snRNP). To learn more about these proteins, an in vitro system was developed for studying their interactions with individual snRNA species. Translation of HeLa cell poly(A)+ mRNA in an exogenous message-dependent reticulocyte lysate results in the synthesis of snRNP proteins. Addition of human snRNA U1 to the translation products leads to the formation of a U1 RNA-protein complex that is recognized by a human autoimmune antibody specific for U1 snRNP. This antibody does not react with free U1 RNA. Moreover, addition of a 10- to 20-fold molar excess of tRNA instead of U1 RNA does not lead to the formation of an antibody-recognized RNP. The proteins forming the specific complex with U1 RNA correspond to the A, B1, and B2 species (32,000, 27,000 and 26,000 MW, respectively) observed in previous studies with U1 snRNP obtained by antibody-precipitation of nuclear extracts. The availability of this in vitro system now permits, for the first time, direct analysis of snRNA-protein binding interactions and, in addition, provides useful information on the mRNA for snRNP proteins.