Expression and characterization of recombinant beta-subunit hCG homodimer.

We have linked two human chorionic gonadotropin (hCG) beta-subunit cDNAs in tandem such that the expressed fusion protein consists of two mature beta-subunits joined through the carboxy terminal peptide of the first beta-subunit. A single glycine residue is inserted between the two subunits in the fusion protein. Chinese hamster ovary (CHO) cells transformed with a clone that contains the fused cDNAs express and secrete a protein that is consistent with it being a beta-hCG homodimer protein. These beta-homodimer molecules can recombine with two free alpha-subunits indicating that both beta-subunits within the homodimer are likely folded in their native conformation. Our data also suggest that the two beta-subunits fold upon each other as a globular protein and do not appear to exist as a simple fusion of two linear beta-subunits. Furthermore, the two beta-monomer subunits in the fusion protein form a stable homodimer that can bind and activate the hLH/CG receptor specifically. Recombination of the fusion protein with alpha-subunits appears to favor an arrangement where two alpha-subunits combine with a single molecule of the fusion protein. The recombined molecule consists of four subunits and is comparable to two tethered hCG moieties, which constitutes a hCG dimer. This hormone dimer can bind and activate the hLH/CG receptor with an activity approximating that of native hCG.