Site-directed mutagenesis defines a domain in the gonadotropin α-subunit required for assembly with the chorionic gonadotropin β-subunit

hCG, LH, FSH, and TSH are a family of heterodimeric glycoprotein hormones that share a common α-subunit, but differ in their hormone-specific β- subunits. Using site-directed mutagenesis and gene transfer, we studied the region in the common α-subunit that has been implicated in the assembly with the β-subunits. The wild-type or mutated α-gene was cotransfected into Chinese hamster ovary cells with the wild-type hCGβ gene. Deletion of the sequence Pro³⁸-Thr³⁹-Pro⁴⁰ or a change in Tyr³⁷ or Thr³⁹ in the α- subunit eliminated or reduced combination with the β-subunit. Deletion of the sequence Leu⁴¹-Arg⁴²-Ser⁴³ had little effect on hCG dimer formation. Disruption of the disulfide bond in the carboxyl end of the subunit did not affect assembly, which suggests that the disulfide bond of Cys⁵⁹ and Cys⁸⁷ is not critical for dimer formation. Based on our data and the previously published results from several laboratories, the region encompassed by amino acids 37-40 is a key determinant in initiating and maintaining α:β assembly.