Insulin and IGF-1 receptors contain covalently bound palmitic acid

We have studied the biosynthesis of the insulin receptor in a human hepatoma cell line, HepG2. As previously reported, these cells synthesize a disulphide-bonded alpha 2 beta 2 tetrameric insulin receptor. Labelling of HepG2 cells with [3H]palmitate or [3H]myristate followed by immunoprecipitation with a polyclonal antireceptor antibody revealed the incorporation of palmitate, but not myristate, into the beta-subunit and alpha beta-precursor of the receptor in a hydroxylamine-sensitive linkage. The extracellular alpha-subunit was not labelled, demonstrating the specificity of incorporation. Acylation of the insulin receptor was an early event as judged by fatty acid incorporation into the alpha beta-precursor and prevention by protein synthesis inhibitors. Pulse-chase studies demonstrated the expected processing of the alpha beta-precursor to mature alpha- and beta-subunits, but no evidence for preferential turnover of the fatty acid moiety was found. The site of acylation appears to be in the transmembrane or cytoplasmic domain since proteolytic treatment of intact cells produced a truncated beta-subunit still containing label. Binding studies showed that HepG2 cells contain approximately half as many insulin-like growth factor-1 receptors as insulin receptors, raising the possibility that this receptor may also be acylated. Indeed, immunoprecipitation with the antiinsulin receptor serum of MDCK cells expressing IGF-1 receptors, but not insulin receptors, revealed bands corresponding to the alpha beta-precursor, alpha- and beta-subunits, of which the alpha beta-precursor and beta-subunits incorporated [3H]palmitate but the alpha-subunit did not.