Significance of Ectodomain Cysteine Boxes 2 and 3 for the Activation Mechanism of the Thyroid-stimulating

Recently, we identified constitutively activating mutations atpositions Asp-403, Glu-404, and Asn-406 in the third extracel-lular cysteine box (C-b3) of the thyroid-stimulating hormonereceptor.Wehypothesizedthatthisregioncouldactasamolec-ularinterfacebetweentheextracellularandserpentinedomain.In this study we present a model for properties of potentialinteraction partners for this region. Moreover, we show thatPro-400andPro-407adjacenttothisepitopearealsoimportantfor stabilizing the partially active, basal conformation of thewild-type (WT) thyroid-stimulating hormone receptor. Fur-thermore, the mutation K291A in the second extracellular cys-teinebox(C-b2)wasidentifiedasanewconstitutivelyactivatingmutationthatreleasesthebasalconformationoftheWTrecep-tor like the known tryptic cleavage in its close vicinity. Takentogether, we provide an activation scenario at the C-b2/C-b3unit. Three anchor fragments (anchors I-III) most likely con-strain the basal conformation. The three anchor fragments aretightlypacked.AdisulfidebridgeholdstheC-b2/C-b3portionsin close positions. Independent of the type of conformationalinterference such as side chain modifications, tryptic cleavage,or hormone stimulation that act on the constrained C-b2/C-b3WTconformation,itwillalwaysreleaseoneoftheanchorfrag-ments. Subsequently, this results in a conformational displace-mentoftheC-b2/C-b3portionsrelativetoeachother,inducingreceptor activation.