FUNCTIONAL HOMODIMERIC GLYCOPROTEIN HORMONES - IMPLICATIONS FOR HORMONE ACTION AND EVOLUTION

Background: Human chorionic gonadotropin (hCG), lutropin, follitropin, and thyrotropin act as alpha beta heterodimers to control reproductio n and thyroid function. The alpha and beta subunits of these proteins are divided into three loops (alpha 1,alpha 2,alpha 3; beta 1,beta 2,b eta 3) by cysteine knots and the heterodimer is stabilized by 20 beta- subunit residues wrapped around alpha 2 like a seatbelt. Understanding how these hormones interact with their receptors, a matter of conside rable dispute, would facilitate design of pro-and anti-fertility agent s. Results: By swapping alpha 2 for beta 2 and vice versa and, in some cases, adding an amino-terminal coiled-coil dimerization domain, we p repared homodimeric analogs that have the conformation found in each ' half' of hCG. Homodimers containing loops beta 1,alpha 2,beta 3 and no ne, part, or all of the seatbelt stimulated signal transduction to the same extent as hCG, albeit with lower potency. Those containing alpha 1,beta 2,alpha 3 were inactive. Conclusions: The activities of homodi mers containing the beta 1,alpha 2,beta 3 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 glyc oprotein hormone heterodimers can be converted to functional homodimer s supports the proposal that this protein family evolved from an activ e homodimeric ancestor by gene duplication and acquisition of mutation s to loop 2 that prevent homodimerization, This approach to protein mi nimization should be applicable to other proteins composed of architec turally related subunits, including those that might have arisen by ge ne duplication.