Synthesis and assembly of human small nuclear ribonucleoproteins generated by cell-free translation

Cell-free translation of human poly(A)+ RNA was carried out to generate and analyze the protein constituents of small nuclear ribonucleoprotein (snRNP) particles. The snRNP proteins were identified by immunoprecipitation with sera from patients with systemic lupus erythematosus. Size fractionation of mRNA prior to translation revealed that these snBNP proteins are all encoded by separate messages. One of the proteins (the A protein, molecular weight 32,000) was seen to lose antigenicity upon RNase treatment either when extracted from cells or when generated in vitro. RNase treatment of immunoprecipitated snRNPs released the A protein in an electrophoretically pure form. Analysis of snRNPs translated in vitro revealed the presence of unassembled and assembled particles as determined by sucrose density gradient sedimentation. Post-translational assembly of snRNPs involving both RNA-protein binding (as revealed by A protein antigenicity) and associations of other snRNP proteins occurred in the in vitro system employed here. In addition, the presence of unassembled snRNP proteins permitted the determination of the precise antigen peptides recognized by Sm and RNP autoimmune sera. It was observed that Sm sera are capable of recognizing each of the eight snRNP proteins, whereas RNP sera recognize only two of the eight.