Identification of an alternate type I insulin-like growth factor receptor beta subunit mRNA transcript

Several studies have suggested that heterogeneity exists in the type I insulin-like growth factor (IGF) receptor beta subunit. We have examined type I IGF receptor mRNA transcripts by ribonuclease (RNase) protection assay to determine if the heterogeneity could result from alternative splicing of the gene. An area that corresponded to the nucleotide sequence just upstream of the region encoding the transmembrane domain of the beta subunit was identified as being a potential site of alteration in the transcript. Since the 5' and 3' ends were known, polymerase chain reaction was used to clone a cDNA that included this region. Analysis revealed that an alternate type I IGF receptor mRNA transcript with a 3-base pair deletion could account for the results of the RNase protection assay. The deletion changes the amino acid sequence at position 899 substituting Arg for a Thr-Gly. Furthermore, this alternate transcript was ubiquitously found in tissue and cell line RNAs. Although the identified transcript cannot fully account for the documented heterogeneity in type I IGF receptor beta subunit sizes, the results suggest that a form of the beta subunit with an alternate primary sequence may exist.