THE BIOLOGIC ACTION OF SINGLE-CHAIN CHORIOGONADOTROPIN IS NOT DEPENDENT ON THE INDIVIDUAL DISULFIDE BONDS OF THE BETA-SUBUNIT

Disrupting disulfide loops in the human chorionic gonadotropin beta su bunit (CG beta) inhibits combination with the alpha subunit. Because t he bioactivity requires a heterodimer, studies on the role of disulfid e bonds on receptor binding/signal transduction have previously been p recluded. To address this problem, we bypassed the assembly step and g enetically fused CGP subunits bearing paired cysteine mutations to a w ild-type a (WT alpha) subunit. The changes altered secretion of the si ngle-chain mutants which parallel that seen for the CGP monomeric subu nit. Despite conformational changes in CG disulfide bond mutants (assa yed by gel electrophoresis and conformationally sensitive monoclonal a ntibodies), the variants bind to the lutropin/CG receptor and activate d adenylate cyclase in vitro. The data show that the structural requir ements for secretion and bioactivity are not the same. The results als o suggest that the extensive native subunit interactions determined by the cystine bonds are not required for signal transduction. Moreover, these studies demonstrate that the single-chain model is an effective approach to structure-activity relationships of residues and structur al domains associated with assembly of multisubunit ligands.