Structure of human chorionic gonadotropin at 2.6 A 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 progesterone. It is a member of a family of glycoprotein hormones that are disulfide-rich heterodimers, with a common alpha-chain and distinctive beta-chains specific to their particular G-protein linked receptors.


RESULTS
We have produced recombinant hCG in mammalian cells as the selenomethionyl protein, and have determined its structure (after partial deglycosylation) at 2.6 A resolution from multiwavelength anomalous diffraction (MAD) measurements. Despite only limited sequence similarity (10% identity), the alpha- and beta-subunits of hCG have similar tertiary folds. Each subunit has a cystine-knot motif at its core of 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 carboxy-terminal peptide of the beta-subunit, which is rich in O-linked sugars, is disordered.


CONCLUSIONS
Structural and sequence comparisons indicate an evolutionary homology, albeit remote, between the glycoprotein hormone chains and other cystine-knot proteins, notably platelet-derived growth factor. Segments of the alpha- and beta-chains that have been convincingly implicated in receptor binding by hCG are juxtaposed on one side of the molecule. A glycosylation site implicated in signal transduction but not in binding is also close to the presumed binding site suggesting a possible coupling between ligand binding and signaling. This study with selenomethionyl protein produced in mammalian cells extends the realm of MAD phasing.
Wu
H
H
Department of Biochemistry and Molecular Biophysics, Columbia University, New York, NY 10032.
Lustbader
J W
JW
Liu
Y
Y
Canfield
R E
RE
Hendrickson
W A
WA
D000595:Amino Acid Sequence; D002478:Cells, Cultured; D006063:Chorionic Gonadotropin / Q000737:chemistry*; D006031:Glycosylation; D006801:Humans; D006860:Hydrogen Bonding; D008958:Models, Molecular; D008969:Molecular Sequence Data; D011487:Protein Conformation; D011974:Receptors, LH / Q000737:chemistry; D011994:Recombinant Proteins / Q000737:chemistry; D012645:Selenomethionine / Q000737:chemistry; D016415:Sequence Alignment; D014961:X-Ray Diffraction
D006063:Chorionic Gonadotropin; D011974:Receptors, LH; D011994:Recombinant Proteins; D012645:Selenomethionine