Isolation and characterization of nuclear hnRNP complexes from Drosophila melanogaster tissue culture cells

Fractionation on sucrose gradients of nuclear described extracts prepared from cultured Drosophila melanogaster cells by sonication of the nuclei in the presence of rat liver cytosol RNAase inhibitor revealed a complex polysome-like pattern of nuclear ribonucleoprotein complexes. The bulk of these heterogeneous ribonucleoprotein (hnRNP) complexes sedimented in the 30S to 80S zone of the sucrose gradient. According to biochemical and morphological data, the monomer particle proved to be the 45S hnRNP and its average diameter was found by electron microscopy to be 24-26 nm. The buoyant density of both the mono and polyparticles was about 1.4 g/cm3, with a slight degree of heterogeneity. The proteins from different zones of the sucrose gradient were composed primarily of similar polypeptides of 47 000, 56 000, 64 000, 96 000 and 130 000 daltons. Complete dissociation of nuclear hnRNP complexes was observed by resedimentation of the particles in the presence of 0.7 M NaCl or 4 M urea. RNAase A digestion (0.1 microgram/ml at 0 degree C for 10 min) resulted in the solubilization of part of the hnRNP and aggregation of some particles. The bulk of the RNA isolated from the different sized hnRNP complexes sedimented in the 7 to 11S region in the sucrose gradient. The large hnRNP complexes contained hnRNA strands larger than 15S, up to 28S. The base composition of the RNA from the 45S monoparticles proved to be AU type: A + U/G + C = 1.7. The RNAs from the 60-75S and 90- 100S polyparticles were also AU type, with an A + U/G + C ratio of 1.46 and 1.21, respectively. The hnRNP complexes exhibited marked heterogeneity in the electron microscope. Our biochemical and morphological observation point to a nonrandom organization of hnRNP particles in Drosophila melanogaster nuclei.