The ligand binding subunit of the insulin-like growth factor 1 receptor has properties of a peripheral membrane protein

125I-insulin-like growth factor 1 was cross-linked to its receptor in human placenta microsomal membranes. The microsomes were treated with urea, with dithiothreitol or with both reagents prior to centrifugation at 100,000 X g. We found that greater than 80% of the label was membrane-associated following separate treatment with urea or dithiothreitol, but greater than 80% of the radioactivity remained in the supernatant after simultaneous exposure to both reagents. In identical experiments employing 125I-epidermal growth factor, no condition led to the release of greater than 10% of label from the membrane. We conclude that the ligand binding subunit of the insulin-like growth factor 1 receptor, like peripheral membrane proteins, lacks a membrane anchoring domain.