Differential polysomal localization of human insulin-like-growth-factor-2 mRNAs in cell lines and foetal liver.

Examination of the association of insulin-like-growth-factor-2 mRNAs with polyribosomes in five cell lines revealed that greater than 50% of the total mRNA population was present in the untranslated free mRNP fraction for each cell line. Of the different subtypes of insulin-like-growth-factor-2 messengers, the least abundant mRNAs, starting with exon 4 (leader 2, 5.0 kb) and exon 6 (leader 4, 4.8 kb), were found in the polysomes only, while the most abundant transcript, starting with exon 5 (leader 3, 6.0 kb and 2.1 kb) was found predominantly in the untranslated fractions. 20-30% of leader 3 mRNAs, however, were in the larger polysomes (four or more ribosomes), indicating that a subpopulation of this mRNA can be translated efficiently. The peak fraction for the leader 4 insulin-like-growth-factor-2 mRNA (4.8 kb) in the polysomes was migrating faster in the sucrose gradients than the peak fractions of leader 2 and 3 mRNAs (5.0 kb and 6.0 kb), implying that more ribosomes were associated with this type of mRNA. In foetal liver, the situation was similar, though in this case the leader 2 mRNA was most heavily loaded with polysomes. Treatment of cells with low concentrations of cycloheximide caused the polysomal RNAs to shift to even larger polysomes while the untranslated fraction of the leader 3 mRNAs stayed in the untranslated fractions. These results indicate that, both in established cell lines and in foetal liver, insulin-like-growth-factor-2 translation is influenced both by mRNP sequestration and differential translation initiation efficiency of the insulin-like-growth-factor-2 mRNAs.