High affinity inhibition of [3H]-flunitrazepam binding to brain benzodiazepine receptors by CGS 9896, a novel pyrazoloquinoline

We had previously reported the extensive purification of snRNPs (ribonucleoprotein complexes containing small nuclear RNAs) from HeLa hnRNP (ribonucleoprotein complexes containing the heterogenous nuclear RNA) by taking advantage of their ability to withstand centrifugation in cesium chloride containing 0.5% sarkosyl (C. BRUNEL et al 1981, Nucleic Acids Research, 9, 815–830). Such purified snRNPs contained the five nucleoplasmic snRNAs (U₁, U₂, U₄, U₅ and U₆) and a very simple protein complement of 4–5 polypeptides between 10 and 14,000 daltons. The possible involvement of U₁ in messenger RNA splicing makes it crucial to obtain in pure form the individual U₁ snRNP, if an in vitro splicing system is to be reconstructed. The present work reports the purification of U₁ snRNP and shows that it contains exactly the same polypeptides as the mixture of all five snRNPs suggesting that the specific function, if any, depends on the RNA moiety.