Nuclear Ribonucleoprotein Particles Contain Specific Proteins and Unspecific Non-Histone Nuclear Proteins

The protein composition of nuclear particles was compared to those of purified deoxyribonucleoproteins and nucleosol in rat brain and liver. In the two tissues, the major proteins of particles could be divided into two classes. The first class comprised proteins found in particles but not in chromatin. It was primarily composed of nine proteins clustered between 28 000 and 38 000 molecular weight and of 1–2 proteins which were also tissue-specific. The proteins specifically associated to the heterogeneous nuclear RNA amounted to 35–50% in mass of the particle proteins. They were either absent from or detected in very small amounts in the nuclear soluble pool. They may play a general role in the processing or packaging of premessenger RNA. The second class included more than 20 proteins which had the same electrophoretic mobilities as certain non-histone proteins from chromatin. Their molecular weight was above 40 000. They represented 50–65% of the mass of the particle proteins. Most of them were found in relatively large amounts in the nucleosol. Taking into account the presence of these proteins in particles, chromatin and nucleosol, it was estimated that they amounted to approximately 60% by mass of the nuclear non-histone proteins. Assuming that the identity of electrophoretic mobilities corresponds to an identity of primary structure, it is proposed that these proteins able to shuttle between DNA and RNA may play an important role in the architecture of the nucleus and in the interconnection of transcriptional and post-transcriptional events.