Intracellular distribution of alpha-fetoprotein and albumin messenger RNAs in developing mouse liver

Intracellular distribution of active mRNAs for alpha-fetoprotein (alphaFP) and albumin in fetal and adult mouse liver was studied. Livers were fractionated into nucleus and cytoplasm. The latter was further fractionated by sucrose density-gradient centrifugation into four subfractions, the top fraction containing soluble components, the 1.0 M sucrose layer containing primarily 80S ribosomes, the 1.5 M sucrose layer containing light polysomes and the pellets containing heavy polysomes. RNA was extracted from each fraction and its ability to direct the synthesis of alphaFP and albumin was determined in a mouse sarcoma 180 cell-free system. Distribution of alphaFP and albumin mRNAs in fetal mouse liver was similar; 2% in the nucleus, 98% in the cytoplasm, of which more than 90% was found in the polysome fractions. The results suggest that alphaFP and albumin mRNAs, once formed, are quickly and efficiently utilized for protein synthesis. The major proportion of albumin mRNA in adult mouse liver was also found to be associated with polysomes. However, the amount of translatable alphaFP mRNA was low in all subcellular fractions examined, suggesting that transcription or processing of alphaFP mRNA is defective in adult mouse liver.