INFLUENCE OF SUBUNIT INTERACTIONS ON LUTROPIN SPECIFICITY - IMPLICATIONS FOR STUDIES OF GLYCOPROTEIN HORMONE FUNCTION

Bovine lutropin (bLH) and human chorionic gonadotropin (hCG) are heter odimeric glycoprotein hormones required for reproduction. Both bind ra t LH receptors (rLHRs), but hCG binds human LH receptors (hLHRs) 1000- 10,000 fold better than bLH. We tested the premise that this differenc e in affinity could be used to identify lutropin receptor contacts. He terodimers containing hCG/bLH alpha- or beta-subunit chimeras that bou nd hLHR like hCG (or bLH) were expected to have hCG (or bLH) residues at the receptor contact sites. Analogs containing one subunit derived from hCG bound hLHR, much more like hCG than bLH, indicating that each bLH subunit contains all the residues sufficient for high affinity hL HR binding. Indeed, the presence of bovine alpha-subunit residues incr eased the activities of some hCG analogs. The low hLHR activity of bLH was due primarily to an interaction between its alpha-subunit and bet a-subunit residue Leu(95). Leu(95) does not appear to contact the hLHR since it did not influence the hLHR activity of heterodimers containi ng human alpha-subunit. These observations show that interactions with in and between the subunits can significantly influence the activities of lutropins, thereby confounding efforts to identify ligand residues that contact these receptors.