STRUCTURE OF HUMAN CHORIONIC-GONADOTROPIN AT 2.6-ANGSTROM RESOLUTION FROM MAD ANALYSIS OF THE SELENOMETHIONYL PROTEIN

Background: Human chorionic gonadotropin (hCG) is a placental hormone that stimulates secretion of the pregnancy-sustaining steroid progeste rone. It is a member of a family of glycoprotein hormones that are dis ulfide-rich heterodimers, with a common alpha-chain and distinctive be ta-chains specific to their particular G-protein linked receptors. Res ults: We have produced recombinant hCG in mammalian cells as the selen omethionyl protein, and have determined its structure (after partial d eglycosylation) at 2.6 Angstrom resolution from multiwavelength anomal ous diffraction (MAD) measurements. Despite only limited sequence simi larity (10 % identity), the alpha- and beta-subunits of hCG have simil ar tertiary folds. Each subunit has a cystine-knot motif at its core o f extended hairpin loops. There is a very extensive subunit interface featuring two inter-chain beta-sheets and a unique, disulfide-tethered 'arm' from the beta-subunit which 'embraces' the alpha-subunit. The c arboxy-terminal peptide of the beta-subunit, which is rich in O-linked sugars, is disordered. Conclusions: Structural and sequence compariso ns indicate an evolutionary homology, albeit remote, between the glyco protein hormone chains and other cystine-knot proteins, notably platel et-derived growth factor. Segments of the alpha- and beta-chains that have been convincingly implicated in receptor binding by hCG are juxta posed on one side of the molecule. A glycosylation site implicated in signal transduction but not in binding is also close to the presumed b inding site suggesting a possible coupling between ligand binding and signaling. This study with selenomethionyl protein produced in mammali an cells extends the realm of MAD phasing.