Insulin and insulin-like-growth-factor-I (IGF-I) receptors in Xenopus laevis oocytes. Comparison with insulin receptors from liver and muscle

Abstract

Insulin and insulin-like-growth-factor-I (IGF-I) receptors were partially purified from full-grown (stages V-VI) Xenopus laevis oocytes by affinity chromatography on wheat-germ agglutinin-agarose. Competitive-binding assays revealed high-affinity binding sites for both insulin and IGF-I (K(d) = 2.5 x 10-10 M and 8 x 10-10 M respectively). However, IGF-I receptors were about 15 times more abundant than insulin receptors (22.5 x 1011 versus 1.5 x 1011/mg of protein). Moreover, comparison of intact and collagenase-treated oocytes showed that most of the insulin receptors were in the oocyte envelopes, whereas IGF-I receptors were essentially at the oocyte surface. Oocyte receptors were composed of α-subunits of ~130 kDa and a doublet of β-subunits of 95 and 105 kDa, which both had ligand-induced phosphorylation patterns compatible with IGF-I receptor β-subunits. Accordingly, the receptor tyrosine kinase was stimulated at low IGF-I concentrations [half-maximally effective concentration (EC50) ~0.5-1 nM], and at higher insulin concentrations (EC50 ~20-50 nM). Partially purified glycoproteins from Xenopus liver and muscle contained mainly receptors of the insulin-receptor type, with α-subunits of 140 kDa in liver and 125 kDa in muscle, and doublets of β-subunits of 92-98 kDa in liver and 85-94 kDa in muscle. Immunoprecipitation of receptors from oocytes, liver and muscle by receptor-specific anti-peptide antibodies suggested that the β-subunit heterogeneity resulted from the existence of two distinct IGF-I receptors in oocytes and of two distinct insulin receptors in both liver and muscle. In the different tissues, the two receptor subtypes differed at least by their β-subunit C-terminal region.