Two-dimensional gel fractionation has revealed the existence of a number (:8) of additional species of HeLa cell small RNAs that have 5' trimethylguanosine cap structures and are bound by proteins containing Sm epitopes. Therefore, these low-abundance (103-104 per cell) RNAs belong to the Sm class of small nuclear ribonucleoproteins (snRNPs), whose best-known members are the four highly abundant (-10 per cell) particles required for pre-mRNA splicing. The complexity of Sm snRNPs in mammalian cells is thus not greatly different from that previously established for lower eukaryotes. Two of the new RNAs, designated Ull (131 nucleotides) and U12 (150 nucleotides), have been sequenced. The Ull and U12 snRNPs have been characterized further by examining their nuclease sensitivity and their possible interactions with other snRNPs. Potential roles for the low-abundance snRNPs in aspects of pre-mRNA processing are discussed.In mammalian cells there exists a set ofhighly abundant small nuclear ribonucleoprotein particles (snRNPs) containing U1, U2, U4, U5, and U6 RNAs (reviewed in ref. 1). Each of these particles is present in the nucleoplasm at 105-106 copies per cell and contains at least one small nuclear RNA (snRNA) and a set of specific proteins. The snRNAs are very stable, and all except U6 have at their 5' ends a unique 2,2,7-trimethylguanosine (m3G)-cap structure (see Fig. 2). Anti-Sm antibodies from patients with autoimmune disorders recognize a common epitope present in one or more proteins of the U1, U2, U4/U6, and U5 snRNPs (2). Therefore, snRNPs of the Sm family are defined by their ability to be immunoprecipitated with both anti-Sm antibodies and antibodies directed against the m3G-cap structure.The Sm snRNPs whose functions are known have been assigned roles in RNA processing. The highly abundant U1, U2, U4/U6, and U5 particles are essential components of the pre-mRNA splicing machinery (reviewed in refs. 3 and 4). The U7 particle, the first low-abundance Sm snRNP to be identified, is required for correct 3' end formation of histone pre-mRNAs (reviewed in ref. Sm snRNPs precipitable by anti-m3G and anti-Sm antibodies are also present in fungi (7-9), plant (10)(11)(12), and parasitic (13) organisms but in much lower abundance than their mammalian counterparts. Furthermore, the number of distinct snRNA species identified in yeast and plant cells [about 20-30 (7, 9)] is much greater than in HeLa cells. This larger number may represent the actual complexity of snRNAs in all eukaryotic cell nuclei. Indeed, several low-abundance HeLa snRNAs, U7-U11, have already been identified (14-18).Here, we have used two-dimensional gel fractionation techniques to assess the true complexity of HeLa cell Sm snRNPs. We have uncovered the existence of a number of additional low-abundance particles. We present the sequences of the RNA components of two of these and the partial characterization of the snRNPs in which they reside.*
MATERIALS AND METHODSPreparation of Labeled RNA. HeLa whole-cell extracts were immunoprecipi...