Size Discrepancy Between Somatomedin-C and Insulin Receptors*

Somatomedin-C (Sm-C) and insulin receptors have similar size and structure. Each receptor is a heterotetramer composed of two α- and two β-subunits. Sodium dodecyl sulfate (SDS)-polyacrylamide gel autoradiographic studies of affinity labeled receptor preparations demonstrated that each has a whole receptor of greater than 350,000 daltons, a half-receptor of 220,000 daltons, and binding subunit (α-subunit) ofabout 140,000 daltons. Using SDS-polyacrylamide disc gels and double labeling techniques,we demonstrated that the 125I affinity labeled α-subunit of the Sm-C receptorfrom human placenta was 8,000 daltons smaller than the 131I affinity labeled insulin receptor α-subunit. Further double label studies demonstrated that [125I]insulin and [131I]insulin cross-linked to placental insulin receptors precisely comigrated on SDS-disc gels, indicating that the difference between insulin and Sm-C α-subunits is not an artifact of the system. The difference in ligand size (Sm-C, 7,500 daltons; insulin, 5,700 daltons)...