Eukaryotic Small Ribonucleoproteins

Anti-La sera from patients with autoimmune disorders precipitate a set of nuclear and cytoplasmic small RNA-protein complexes. Up to now, it has been thought that the La antigen is associated only with RNAs transcribed by RNA polymerase 111, including precursors of tRNA and 5 S ribosomal RNA. Here we report that anti-La sera also react with ribonucleoprotein particles containing small nuclear RNA U1, which is transcribed by RNA polymerase 11. Anti-La sera from 12 out of 12 patients tested were found to precipitate U l RNA-protein complexes from HeLa cell nuclear extracts, under conditions where nonimmune sera do not. Ribonucleoprotein particles containing a second small nuclear RNA, U2, do not react appreciably with anti-La sera although they are present in HeLa cell nuclei at the same concentration as U 1 RNA. Anti-La sera also react with U 1 RNA-protein complexes in mouse and frog cells, but not in Drosophila or Chironomus, two organisms which lack the La antigen. Hybridization of cloned U 1 DNA with anti-Lareactive RNA from HeLa cell nuclear extracts reveals mature U 1 RNA, whereas anti-La-reactive cytoplasmic RNA contains a series of hybridizing bands that represent molecules 1-7 nucleotides longer than U 1 and which may include precursors of nuclear U 1 RNA (Madore, S. J., Wieben, E. D., and Pederson, T. (1984) J. Cell Biol., 188-192). Pulse-chase experiments suggest that the association of La antigenicity with these cytoplasmic U 1 RNA molecules is transient. These results are discussed in relation to the presence of uridylate-rich sequences in the 3’ termini of U 1 RNA precursors and mature U 1 RNA, which are similar to La antigen binding sites in several RNAs transcribed by RNA polymerase 111.