A comparative study on the two classes of heterogeneous nuclear ribonucleoprotein particles separated in metrizamide density gradient,by electrophoresis of proteins and chase experiments. Evidence for two distinct subfractions of HnRNP in mammalian nuclei

Heterogeneous nuclear ribonucleoprotein particles (HnRNP) were separated in metrizamide density gradients, into two fractions migrating to 1.31 g ml^-1 and 1.18 g ml^-1, respectively. Proteins associated with each of these fractions were analysed by SDS-acrylamide gel electrophoresis. It is shown that the whole proteins extracted from these two metrizamide fractions exhibit clearly different electrophoretic patterns: 1.31 HnRNP particles contain as major polypeptide chains molecules with molecular weights ranging from 40 000 to 65 000, while major polypeptides of 1.18 HnRNP are banding in the 30 000–40 000 molecular weight region of the gels. Both fractions contain numerous other associated polypeptide chains whose molecular weights are above 65 000. A possible kinetic relationship between these two HnRNP classes was investigatedin vivo by performing chase experiments. No clear evidence for a precursor-product relationship was found. Implications arising from these structural and kinetic observations, and problems relating to nuclear maturation of pre-messenger RNA, are discussed.