Insulin and insulin-like-growth-factor-I (IGF-I) receptors in Xenopus laevis oocytes. Comparison with insulin receptors from liver and muscle.
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 (Kd = 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 10(11) versus 1.5 x 10(11)/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 alpha-subunits of approximately 130 kDa and a doublet of beta-subunits of 95 and 105 kDa, which both had ligand-induced phosphorylation patterns compatible with IGF-I receptor beta-subunits. Accordingly, the receptor tyrosine kinase was stimulated at low IGF-I concentrations [half-maximally effective concentration (EC50) approximately 0.5-1 nM], and at higher insulin concentrations (EC50 approximately 20-50 nM). Partially purified glycoproteins from Xenopus liver and muscle contained mainly receptors of the insulin-receptor type, with alpha-subunits of 140 kDa in liver and 125 kDa in muscle, and doublets of beta-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 beta-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 beta-subunit C-terminal region.
Hainaut
P
P
Institut National de la Santé et de la Recherche Médicale, U 145, Faculté de Médecine, Nice, France.
Kowalski
A
A
Giorgetti
S
S
Baron
V
V
Van Obberghen
E
E
D000818:Animals; D005260:Female; D007334:Insulin-Like Growth Factor I / Q000378:metabolism*; D007700:Kinetics; D008099:Liver / Q000378:metabolism*; D008970:Molecular Weight; D009132:Muscles / Q000378:metabolism*; D009865:Oocytes / Q000378:metabolism*; D009928:Organ Specificity; D010766:Phosphorylation; D011972:Receptor, Insulin / Q000302:isolation & purification / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000302:isolation & purification / Q000378:metabolism*; D017451:Receptors, Somatomedin; D014982:Xenopus laevis
D011956:Receptors, Cell Surface; D017451:Receptors, Somatomedin; D007334:Insulin-Like Growth Factor I; D011972:Receptor, Insulin