5'-Terminal caps of snRNAs are reactive with antibodies specific for 2,2,7-trimethylguanosine in whole cells and nuclear matrices. Double-label immunofluorescent studies with anti-m3G antibodies and with anti-RNP and anti-Sm autoantibodies.

Antibodies specific for 2,2,7-trimethylguanosine (m3G), which do not cross-react with m7G-capped RNA molecules were used to study, by immunofluorescence microscopy, the reactivity of the m3G-containing cap structures of the snRNAs U1 to U5 in situ. In interphase cells, immunofluorescent sites were restricted to the nucleus, whilst nucleoli were free of fluorescence. This indicates that the 5' terminal of most of the nucleoplasmic snRNAs are not protected by an m3G cap-recognizing protein and that the snRNA caps are not necessarily required for the binding of snRNPs to subnuclear structures. The snRNAs in the nucleoplasm appeared as distinct units in the light microscope, and this allowed the comparison of the distribution of snRNP proteins by double label studies with anti-RNP or anti-Sm antibodies within the same cell. The three antibody classes produced superimposable fluorescent patterns. Taking into account that the various IgGs react with antigenic sites on snRNAs or snRNP proteins not shared by all the snRNP species, these data suggest that U1 snRNP particles are distributed in the same way as the other snRNPs in the nucleus. Qualitatively the same results were obtained with DNase-treated nuclear matrices indicating that intact snRNPs are part of the nuclear matrix. Our data are consistent with proposals that the various snRNPs may be involved in processing of hnRNA and that this may take place at the nuclear matrix.