Spatial relationships of the human chorionic gonadotropin (hCG) subunits in the assembly of the hCG-receptor complex in the luteinized rat ovary

In an attempt to examine the spatial relationships of the human chorionic gonadotropin (hCG) subunits in the assembly of the hCG-receptor complex, the recombined ¹²⁵I-labeled hCG, with label in either the α subunit or the β subunit, was cross-linked to the luteinizing hormone (LH)/hCG receptor. The efficacy of the cross-linking of the ¹²⁵I-α subunit or the ¹²⁵I-β subunit of hCG to the LH/hCG receptor was then examined. The autoradiographic profile of ¹²⁵I-hCG-receptor complex containing the label in the α subunit of hCG showed that the α subunit can cross-link with all four subunits of the LH/hCG receptor. However, only one faint labeled band, corresponding to M(r) = 68,000, was detected when the ¹²⁵I-hCG-receptor complex with label in the β subunit was subjected to sodium dodecyl sulfate/polyacrylamide gel electrophoresis under reducing conditions. When the electrophoresis was performed under nonreducing conditions, the M(r) 68,000 band disappeared concomitantly with the accumulation of radioactivity in the high molecular weight region. These results indicated that the β subunit of hCG, unlike the α subunit, can cross-link only weakly with the smallest subunit of the LH/hCG receptor. A comparison of the differential effectiveness of the cross-linking of ¹²⁵I-α subunit with ¹²⁵I-β subunit of hCG to the LH/hCG receptor suggests that both α and β subunits of hCG are intimately associated with the receptor, but the bulk of the β subunit of hCG is buried in between the receptor and the α subunit of hCG. On the basis of our data, a model for the spatial arrangement of hCG subunits in the hCG-receptor complex is proposed.