Alternative leader sequences in insulin-like growth factor I mRNAs modulate translational efficiency and encode multiple signal peptides.
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.
Yang
H
H
Section on Molecular and Cellular Physiology, National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, Bethesda, Maryland 20892, USA.
Adamo
M L
ML
Koval
A P
AP
McGuinness
M C
MC
Ben-Hur
H
H
Yang
Y
Y
LeRoith
D
D
Roberts
C T
CT
D000818:Animals; D001483:Base Sequence; D004285:Dogs; D007334:Insulin-Like Growth Factor I / Q000235:genetics* / Q000378:metabolism; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D014176:Protein Biosynthesis; D021382:Protein Sorting Signals / Q000235:genetics* / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism*; D051381:Rats; D017421:Sequence Analysis
D021382:Protein Sorting Signals; D012333:RNA, Messenger; D007334:Insulin-Like Growth Factor I