Synthetic E-peptide sequences.
A. Rodent Igf1 3′ splicing leads to two mRNA isoforms. While mature IGF-I is encoded by exons 3 and 4, the E-peptides are encoded by exons 4, 5, and/or 6. EA isoforms exclude exon 5, while EB isoforms retain exon 5, leading to an altered reading frame and earlier stop codon in exon 6. Exons not drawn to scale B. Synthetic E-peptide amino acid sequences. EA and EB are less than 50% identical. Scr = Scrambled peptide. * = potential glycosylation sites in EA. The portion of EB that corresponds to MGF is underlined.