Thyroid Cytoplasmic Ribonucleoprotein Particles of Very High Size Containing Messenger‐Like and Ribosomal Ribonucleic Acids

Cytoplasmic giant‐size ribonucleoprotein particles has been purified from the deoxycholate treated 15000 ×g supernatant of sheep thyroid slices incubated in the presence of [3H]uridine for 0.5 to 8 hours. After treatment of unclarified polyribosomes with 10 mM EDTA, 400 to 1000 S ribonucleoprotein particles have been isolated free of ribosomal subparticles by differential sedimentation. They are heterogeneous in size and buoyant density in CaCl gradients (from 1.38 to 1.52 g/ml, with maxima at 1.52, 1.46, and 1.42). Polyribosome‐associated 400 to 1000 S ribonucleoprotein particles deprived of free ribosomal subparticles contain messenger‐like and ribosomal RNA as shown by the rate of labelling and sucrose gradient centrifugation, polyacrylamide gel electrophoresis and base composition. Particle proteins obtained by 3 M LiCl‐6 M urea treatment separate in polyacrylamide gel electrophoresis into 3 to 4 basic components which disclose the same electrophoretic mobility as some ribosomal proteins. These 3 to 4 protein bands are distinct from total thyroid soluble proteins and from histones. Similar protein species are present in ribonucleoprotein particles diffusing spontaneously from thyroid nuclei in vitro. The artifactual formation of these structures during EDTA exposure or slice incubation has been rendered unlikely by appropriate controls. Although tentative, the possible physiological significance of these particles is briefly discussed.