Molecular Endocrinology, Vol 9, 1380-1395, Copyright © 1995 by Endocrine Society

ARTICLES

Alternative leader sequences in insulin-like growth factor I mRNAs modulate translational efficiency and encode multiple signal peptides

H Yang, ML Adamo, AP Koval, MC McGuinness, H Ben-Hur, Y Yang, D LeRoith and CT Roberts Jr
Section on Molecular and Cellular Physiology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.

Rat insulin-like growth factor I (IGF-I) mRNAs contain multiple 5'- untranslated regions due to the use of leader exons transcribed from several transcription initiation sites and to alternative splicing within leader exon 1. Synthetic RNAs with 5'-ends corresponding to the use of exon 1 transcription initiation sites were translated in vitro into prepro-IGF-I peptides initiated at a Met-48 codon in exon 1 or a Met-22 codon in exon 3, and RNAs with a 5'-end corresponding to the major exon 2 transcription start site were translated into a prepro-IGF- I peptide initiated at a Met-32 codon in exon 2. All forms of prepro- IGF-I were processed by canine pancreatic microsomes, suggesting that all these prepeptides function as signal peptides. The translational efficiency of IGF-I RNAs was inversely proportional to the length of the 5'-untranslated region. Mutation of the first of three upstream AUG codons in exon 1, which potentially initiates a 14-amino acid open reading frame, did not affect prepro-IGF-I translation. The other two AUG codons are immediately followed by stop codons. The absence of both upstream AUG codons in a completely spliced exon 1-derived RNA enhanced the in vitro and in vivo translatability of this RNA as compared with the full-length RNA. Mutation of the downstream initiation codon in particular increased translational efficiency in vitro and in intact cells, suggesting that an inefficient reinitiation event at the Met-48 codon contributes to the poorer translation of IGF-I mRNAs in which these upstream AUGUGA motifs occur. We conclude that IGF-I mRNAs potentially encode multiple forms of preproIGF and that specific differences in their 5'-untranslated regions provide a molecular basis for translational control of IGF-I biosynthesis.

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