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

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Abstract
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 snRNP proteins are all encoded by separate messages. One of the proteins (the A protein, MW 32,000) was seen to lose antigenicity upon RNase treatment either when extracted from cells or when generated in vitro. RNase treatment of immunoprecipitated snRNP released the A protein in an electrophoretically pure form. Analysis of snRNP translated in vitro revealed the presence of unassembled and assembled particles as determined by sucrose density gradient sedimentation. Post-translational assembly of snRNP 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. The presence of unassembled snRNP proteins permitted the determination of the precise antigen peptides recognized by Sm and RNP autoimmune sera. Sm sera are capable of recognizing each of the 8 snRNP proteins, whereas RNP sera recognize only 2 of the 8.