Alternative Splicing Produces Messenger RNAs Encoding Insulin-Like Growth Factor-I Prohormones that Are Differentially Glycosylatedin Vitro

Rat insulin-like growth factor-I (IGF-I) cDNA sequences predict two prohormones that differ in the carboxy-terminal extension peptide (E-peptide) as a result of the inclusion or exclusion of the 52-basepair exon 4 sequence. In the absence of exon 4, the sequence codes for the IGF-Ia prohormone, whose E region contains two potential N-glycosylation sites. With differential splicing and the inclusion of exon 4, the resultant mRNA codes for IGF-Ib, with a longer E-region sequence. In addition, as a consequence of a frame shift, both potential glycosylation sites are lost in the IGF-Ib peptide. We used an in vitro translation system supplemented with canine pancreatic microsomal membranes to analyze cotranslational processing of the IGF-I propeptides. We have demonstrated that IGF-Ia prohormone, which contains two potential N-glycosylation sites in the E region, can be N-glycosylated in vitro, and that both glycosylation sites are probably used. As expected, the IGF-Ib preprohormone is processed by microsomes, but is not glycosylated.