DISTINCT BETA-SUBUNITS ARE PRESENT IN HYBRID INSULIN-LIKE-GROWTH-FACTOR-1 RECEPTORS IN THE CENTRAL-NERVOUS-SYSTEM

Previous work suggests the existence of different isoforms of the insu lin-like-growth-factor-1 (IGF-1) receptor in various tissues. In the p resent study we provide support for the concept that heterogeneous IGF -1 receptors exist in the brain and that part of the heterogeneity is derived from IGF-1 receptor hybrids formed from different beta-subunit s. IGF-1 receptors were extracted from adult-rat forebrain synaptosome s and partially purified by wheat-germ agglutinin (WGA) chromatography . Hormone-binding studies in this preparation demonstrate the presence of receptors for IGF-1 and insulin. An antibody, a-RIR, specific for the rat insulin receptor was used to remove insulin receptors from the WGA extract. Studies with the immunodepleted material demonstrated tw o proteins of 92 and 99 kDa that are phosphorylated on tyrosine during incubation with low concentrations of IGF-1. Both proteins bound with high affinity and specificity to IGF-1 immobilized on agarose, and ea ch underwent phosphorylation when the agarose beads were incubated wit h [gamma-P-32]ATP and MnCl2. Two-dimensional phosphopeptide maps after exhaustive trypsin treatment of the two proteins showed significant d ifferences in their structure as well as differences from the phosphop eptide map for the beta-subunit of the insulin receptor. The relations hip of the two proteins to the IGF-1 receptor was further probed by an antibody (a-HF) raised against a specific sequence in the beta-subuni t of the human IGF-1 receptor, and a polyclonal antibody raised agains t the liver insulin receptor (L1) which cross-reacts with the IGF-1 re ceptor. Both antibodies immunoprecipitated the two phosphorylated prot eins. However, reduction of the receptors to form receptor dimers or m onomers showed that a-HF precipitated only the 99 kDa protein, whereas L1 precipitated primarily the 92 kDa protein. In conclusion, the brai n IGF-1 receptor apparently has two structurally different beta-subuni ts, one of 92 kDa and a second of 99 kDa. Interestingly, at least a po rtion of the IGF-1 receptor population has both isoforms in the same r eceptor.