PROTEIN ENGINEERING OF A NOVEL CONSTITUTIVELY ACTIVE HORMONE-RECEPTORCOMPLEX

Human chorionic gonadotropin (hCG) is a heterodimeric glycoprotein hor mone consisting of an alpha and a beta subunit that stimulates intrace llular levels of cAMP via a G protein-coupled receptor, Herein we repo rt the engineering and characterization of a novel molecule in which t he receptor and its heterodimeric ligand were covalently Linked in a s ingle polypeptide chain, The hormone-receptor complex was expressed in cells transfected with this construct, but the cells were unable to b ind significant amounts of exogenous hCG. However, cleavage of the hor mone with a site-specific protease rendered the receptor accessible to exogenously added hormone. Cells transfected with the hCG-receptor co nstruct contained elevated basal levels of cAMP; moreover, addition of hormone had no significant effect. These results are consistent with a strong and stable interaction between the single-chain hormone and i ts covalently linked receptor that results in a constitutively active complex.