Identification of two discrete ribonucleoprotein particles within the monomer population of rat liver nuclear RNPs.

30-50 S RNP particles (monoparticles) isolated from rat liver nuclei were submitted to electrophoresis in native 0.5% agarose gels. Two RNP fractions were thus separated, a minor one remaining closer to the top of the gel (MI) and a more abundant one migrating further into the gel (MII). SDS-polyacrylamide gel electrophoresis revealed that MII contains the major monoparticle (Mr 30 000-40 000 or 'core') polypeptides and higher molecular weight proteins, whereas MI contains several minor proteins of Mr greater than 40 000. Some proteins are common to both particle classes. Urea-acrylamide gel electrophoresis revealed that HnRNA is mainly present in MII, whereas snRNA is confined to the MI particle class.