Functional homodimeric glycoprotein hormones: implications for hormone action and evolution

AbstractBackground: Human chorionic gonadotropin (hCG), lutropin, follitropin, and thyrotropin act as αβ heterodimers to control reproduction and thyroid function. The α and β subunits of these proteins are divided into three loops (α1,α2,α3; β1,β2,β3) by cysteine knots and the heterodimer is stabilized by 20 β-subunit residues wrapped around α2 like a seatbelt. Understanding how these hormones interact with their receptors, a matter of considerable dispute, would facilitate design of pro- and anti-fertility agents.Results: By swapping α2 for β2 and vice versa and, in some cases, adding an amino-terminal coiled-coil dimerization domain, we prepared homodimeric analogs that have the conformation found in each ‘half’ of hCG. Homodimers containing loops β1,α2,β3 and none, part, or all of the seatbelt stimulated signal transduction to the same extent as hCG, albeit with lower potency. Those containing α1,α2,α3 were inactive.Conclusions: The activities of homodimers containing the β1,α2,β3 groove exceed those of other minimized analogs more than 100–1000-fold, suggesting this portion of the hormone forms the major receptor contact. The discovery that glycoprotein hormone heterodimers can be converted to functional homodimers supports the proposal that this protein family evolved from an active homodimeric ancestor by gene duplication and acquisition of mutations to loop 2 that prevent homodimerization. This approach to protein minimization should be applicable to other proteins composed of architecturally related subunits, including those that might have arisen by gene duplication