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Volume 272, Number 6, Issue of February 7, 1997 pp. 3309-3314
©1997 by The American Society for Biochemistry and Molecular Biology, Inc.

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Influence of Subunit Interactions on Lutropin Specificity
IMPLICATIONS FOR STUDIES OF GLYCOPROTEIN HORMONE FUNCTION

(Received for publication, June 28, 1996, and in revised form, October 2, 1996)

From the Department of Obstetrics and Gynecology, Robert Wood Johnson (Rutgers) Medical School, Piscataway, New Jersey 08854

Bovine lutropin (bLH) and human chorionic gonadotropin (hCG) are heterodimeric glycoprotein hormones required for reproduction. Both bind rat LH receptors (rLHRs), but hCG binds human LH receptors (hLHRs) 1000-10,000 fold better than bLH. We tested the premise that this difference in affinity could be used to identify lutropin receptor contacts. Heterodimers containing hCG/bLH - or -subunit chimeras that bound 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 hLHR binding. Indeed, the presence of bovine -subunit residues increased the activities of some hCG analogs. The low hLHR activity of bLH was due primarily to an interaction between its -subunit and -subunit residue Leu⁹⁵. Leu⁹⁵ does not appear to contact the hLHR since it did not influence the hLHR activity of heterodimers containing human -subunit. These observations show that interactions within and between the subunits can significantly influence the activities of lutropins, thereby confounding efforts to identify ligand residues that contact these receptors.

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