Effects of Sodium Chloride and Pancreatic Ribonuclease on the Rat‐Brain Nuclear Particles: The Fate of the Protein Moiety

Previous results on RNA‐labelled nuclear particles were compatible with a folded ribonucleo‐protein strand structure. In the present experiments, particles were labelled with tritiated amino acids or with tritiated amino acids and [ 32 P]orthophosphate in vivo ; they were prepared by sonication of nuclei. 1. Proteins were released progressively from the particles (ϱ= 1.39 g/ml) as shown by the accumulation of material of ϱ= 1.31–1.35 g/ml after treatment with 0.25 M and 0.4 M NaCl. Concomitantly, high‐density ribonucleoproteins were formed. 2. Proteins released from the particles at various NaCl concentrations (up to 2 M) sedimented between 0 and 15 S. 30‐S aggregates corresponding to “informofers” were never detected. Some proteins remained bound to the RNA even after treatment with 2 M NaCl. 3. Large protein aggregates contaminated the smallest particles (up to 50–60 S) as shown by the sedimentation properties of the products of extensive ribonuclease digestion and the density of protein‐labelled fractions. 4. About half of the protein released from the particles by pancreatic ribonuclease treatment had a low sedimentation coefficient (0–15 S) after formaldehyde fixation. The other half had higher polydisperse sedimentation coefficient, and possibly resulted from artificial cross‐linking. A 30‐S protein peak was not detected. 5. The material released from the particles by ribonuclease banded at ϱ= 1.35 g/ml, a higher density than that of free proteins (ϱ= 1.31 g/ml). Treatment with 1 M NaCl after ribonuclease digestion released free proteins, some of which were phosphoproteins (ϱ= 1.31 g/ml) and ribonucleoproteins of higher density. It is suggested that these complexes contain ribonuclease‐resistant RNA. These results confirm and complete our previous ones. They are not compatible with the informofer model of the monoparticle since 30‐S protein aggregates could not be isolated after salt or ribonuclease treatment. They are consistent with the folded ribonucleoprotein strand model which predicts the sequential release of proteins.