Asp383 in the second transmembrane domain of the lutropin receptor is important for high affinity hormone binding and cAMP production.

The lutropin (LH), follitropin, and thyrotropin receptors belong to the superfamily of G-protein coupled receptors and have some unique structural features. These glycoprotein hormone receptors comprise a C-terminal half and an N-terminal half of similar size. The C-terminal half is equivalent to the entire structure of other G-protein coupled receptors and has seven transmembrane domains, three cytoplasmic loops, three exoplasmic loops, and a C terminus. In contrast, the hydrophilic N-terminal half is exoplasmic and unique to the glycoprotein hormone receptors. This large N-terminal half of the LH receptor has recently been shown to be capable of binding the hormone. Therefore, these glycoprotein hormone receptors are structurally and functionally different from other G-protein coupled receptors. In an attempt to define the role of the membrane-associated C-terminal half of the LH receptor, we have prepared several mutant receptors in which an Asp or Glu in the seven transmembrane domains has been converted to Asn or Gln, respectively. These include Asp383----Asn in the second transmembrane domain, Glu410----Gln in the third transmembrane domain, and Asp556----Asn in the sixth transmembrane domain. All these mutant receptors were successfully expressed in Cos 7A cells. The Glu410----Gln and Asp556----Asn mutants maintained normal affinities for hormone binding and cAMP production, but the Asp383----Asn mutant showed significantly lower affinities. Although Asp383 of the LH receptor is conserved in all G-protein coupled receptors cloned to date except the substance P receptor, which has Glu in the place of the Asp residue, this is the first observation of the critical role of the Asp in hormone binding and subsequent stimulation of cAMP production.