Functional properties of an isolated .alpha..beta. heterodimeric human placenta insulin-like growth factor 1 receptor complex

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Abstract
Treatment of human placenta membranes at pH 8.5 in the presence of 2.0 mM dithiothreitol (DTT) for 5 min, followed by the simultaneous removal of the DTT and pH adjustment to pH 7.6, resulted in the formation of a functional .alpha..beta. heterodimeric insulin-like growth factor 1 (IGF-1) receptor complex from the native .alpha.2.beta.2 heterotetrameric disulfide-linked state. The membrane-bound .alpha..beta. heterodimeric complex displayed similar curvilinear 125I-IGF-1 equilibrium binding compared to the .alpha.2.beta.2 heterotetrameric complex. Triton X-100 solubilization of the alkaline pH and DTT-pretreated placenta membranes, followed by Bio-Gel A-1.5m gel filtration chromatography, was found to effectively separate the .alpha.2.beta.2 heterotetrameric and .alpha..beta. heterodimeric IGF-1 receptor species. 125I-IGF-1 binding to both the isolated .alpha.2.beta.2 heterotetrameric and .alpha..beta. heterodimeric complexes demonstrated a marked straightening of the Scatchard plots, compared to the placenta membrane-bound IGF-1 receptors, with a 2-fold increase in the high-affinity binding component. Similar to the membrane-bound IFG-1 receptor species, the 125I-IGF-1 binding properties between the .alpha.2.beta.2 heterotetrameric and .alpha..beta. heterodimeric complexes were not significantly different. IGF-1 stimulation of IGF-1 receptor autophosphorylation indicated that the ligand-dependent activation of .alpha..beta. heterodimeric protein kinase activity occurred concomitant with the reassociation into a covalent .alpha.2.beta.2 heterotetrameric state. These data demonstrate that (i) a combination of alkaline pH and DTT treatment of human placenta membranes results in the formation of an .alpha..beta. heterodimeric IGF-1 receptor complex, (ii) unlike the insulin receptor, high-affinity homogeneous IGF-1 binding occurs in both the .alpha.2.beta.2 heterotetrameric and .alpha..beta. heterodimeric complexes, and (iii) IGF-1-dependent autophosphorylation of the .alpha..beta. heterodimeric IGF-1 receptor complex correlates with an IGF-1-dependent covalent reassociation into an .alpha.2.beta.2 heterotetrameric disulfide-linked state.

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