Conformational equilibria in the activation of cystine-knot hormone receptors.

Abstract

Human glycoprotein hormones such as thyroid-stimulating hormone (TSH) and follicle-stimulating hormone (FSH) belong to a broader family of cystine-knot hormones (CKHs), all of which act through leucine-rich-repeat (LRR)-containing G protein-coupled receptors (LGRs) with which they have coevolved from evolutionary predecessors in metazoan animals. There is substantial evidence for LGR dimers being required in the transmission of G-protein signals elicited by mammalian CKHs acting on their cognate LGRs. Yet, human LGRs are monomeric as extracted from cell membranes and also in cryo-EM structures, both when in apo, inactive state and when hormone bound in an LRR-elevated active state. Fortunately, the LGR from the nematode <i><i>Caenorhabditis elegans</i></i> (<i>Ce</i>LGR) remains dimeric as detergent extracted for structure determination. In this study, we synthesize structural information from <i>Ce</i>LGR, <i>Hs</i>TSHR, and the other human LGRs together with biophysical evidence about physiological dimers to produce a theoretical description of conformational equilibria involved in CKH activation of LGRs. We develop a theory for the equilibria among conformations that govern signal transmission from hormone to G protein, we define the transitions of receptor activation in quantifiable terms, and we build and validate energetically feasible models for <i>Ce</i>LGR and <i>Hs</i>TSHR in their relevant 0:2, 1:2, and 2:2 hormone:receptor complexes. These developments provide a framework for understanding of signaling through CKH receptors and for devising structure-based hypotheses to test such conceptions.