Receptor binding and signal transduction are dissociable functions requiring different sites on follicle-stimulating hormone.
Separate sites for glycoprotein hormone receptor binding and signal transduction have yet to be elucidated. In general, certain peptide regions are thought to be critical for receptor binding, whereas the oligosacharides are thought to be important for signal transduction. Using site-directed mutagenesis of FSH, we made selective amino acid substitutions and oligosaccharide alterations to try and identify specific sites mediating receptor binding distinct from signal transduction and vice versa. We substituted Lys or Asp for beta Arg35 in the purported receptor binding loop between cysteine-32 and -51, and we substituted Gln for alpha Asn52, alpha Asn78, beta Asn7, or beta Asn24, the attachment sites for each of the oligosaccharide side-chains. We determined the binding and signal-transducing activity of wild-type and mutant human FSH at the human FSH receptor, as recent data suggest that glycoprotein hormone-receptor interactions are species specific. The binding activities of FSH with Lys or Asp substituted for beta Arg35 were reduced 71% and 98%, respectively, but their signal transduction, at equivalent levels of binding activity, was unaffected. The binding activity of FSH lacking the oligosaccharide at alpha Asn52 was enhanced 2- to 3-fold, but its signal-transducing activity, at equivalent levels of receptor binding, was decreased 72%. In contrast, the binding and signal-transducing activities of FSH lacking the alpha Asn78, or alpha Asn7, or beta Asn24 oligosaccharide were unaffected. Thus, a specific amino acid (beta Arg35) is important for high affinity binding, but is not involved in signal transduction, whereas a specific oligosaccharide (alpha Asn52) is important for signal transduction, but is not required for high affinity binding. Therefore, receptor binding and signal transduction are dissociable functions involving different sites on the FSH glycoprotein.