Studies on the structure of rat liver messenger ribonucleoprotein. I. Isolation and characterization.

Messenger ribonucleoprotein (mRNP) was released from 0.5 M KCl washed rat liver polyribosomes after mild pancreatic ribonuclease (EC 3.1.27.5) and EDTA treatment and separated by sucrose gradient centrifugation from ribosomal subunits. The method yielded partially fragmented mRNP, which, however, was free from ribosomal contaminants. In CsCl gradient the mRNP banded at 1.46 g/cm3, indicating a protein content of about 65%. Treatment of mRNP with 0.25 M or 0.5 M KCl resulted in loss of the proteins. Urea/sodium dodecyl sulfate polyacrylamide gel electrophoresis of mRNA bound proteins showed that the most prominent polypeptides found in the mRNP fractions exhibited molecular weights of 29 000 (P29), 31 000 (P31), 38 000 (P38), 44 000 (P44), 50 000 (P50), 54 000 (P54), 63 000 (P63) 76 000 (P76) and 105 000 (P105). Three polypeptides, P38, P44 and P63 were most sensitive to high salt treatment.