The receptors for insulin and the insulin-like growth factor (IGF) I are two structurally homologous disulfide-linked multisubunit complexes of apparent Mr = 350,000. The similar subunit structures of these two types of receptors suggested that their genetic expression might be affected by common genetic defects. We have examined this possibility in an insulinresistant, diabetic patient who exhibits decreased insulin binding activity. The receptors for IGF-I and insulin in skin fibroblasts from this patient were affinity labeled with 125I-IGF-I and 125I-insulin, respectively, and visualized by electrophoresis and autoradiography in polyacrylamide gels. Control fibroblasts exhibited the usual affinity labeling of the disulfide-linked Mr = 350,000 insulin and IGF-I receptor structures.

The intensity of labeling of both receptor types in the patient's fibroblasts was less than in control fibroblasts. Binding data indicated that this decrease is due to a decreased receptor number with little or no decrease in affinity for the respective ligands. The highaffinity bIGF-II receptor in fibroblasts affinity labeled with 125I-IGF-II or 125I-IGF-I consists of a single polypeptide not disulfide linked to any other membrane component. The molecular size and intensity of labeling of the IGF-II receptor in the patient's fibroblasts were unaltered when compared with those of controls. These observations suggest that a common genetic defect alters the expression of the homologous receptor structures for insulin and IGF-I.

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