Most heterodimeric proteins are stabilized by intersubunit contacts or disulfide bonds. In contrast, human chorionic gonadotropin (hCG) and other glycoprotein hormones are secured by a strand of their β-subunits that is wrapped around α-subunit loop 2 “like a seatbelt.” During studies of hCG synthesis in COS-7 cells, we found that, when the seatbelt was prevented from forming the disulfide that normally “latches” it to the β-subunit, its carboxyl-terminal end can “scan” the surface of the heterodimer and become latched by a disulfide to cysteines substituted for residues in the α-subunit. Analogs in which the seatbelt was latched to residues 35, 37, 41–43, and 56 of α-subunit loop 2 had similar lutropin activities to those of hCG; that in which it was latched to residue 92 at the carboxyl terminus had 10–20% the activity of hCG. Attachment of the seatbelt to α-subunit residues 45–51, 86, 88, 90, and 91 reduced lutropin activity substantially. These findings show that the heterodimer can form before the β-subunit has folded completely and support the notions that the carboxyl-terminal end of the seatbelt, portions of α-subunit loop 2, and the end of the α-subunit carboxyl terminus do not participate in lutropin receptor interactions. They suggest also that several different architectures could have been sampled without disrupting hormone activity as the glycoprotein hormones diverged from other cysteine knot proteins.
Disulfide bonds within the α-subunit of insulin receptors in rat liver and brain membranes