The functional microdomain in transmembrane helices 2 and 7 regulates expression, activation, and coupling pathways of the gonadotropin-releasing hormone receptor

Structural microdomains of G protein-coupled receptors (GPCRs) consist of s patially related side chains that mediate discrete functions. The conserved helix 2/helix 7 microdomain was identified because the gonadotropin-releas ing hormone (GnRH) receptor appears to have interchanged the Asp(2.50) and Asn(7.49) residues which are conserved in transmembrane helices 2 and 7 of rhodopsin-like GPCRs. me now demonstrate that different side chains of this microdomain contribute specifically to receptor expression, heterotrimeric G protein-, and small G protein-mediated signaling. An Asn residue is requ ired in position 2.50(87) for expression of the GnRH receptor at the cell s urface, most likely through an interaction with the conserved Asn(1.50(53)) residue, which we also find is required for receptor expression. Most GPCR s require an Asp side chain at either the helix 2 or helix 7 locus of the m icrodomain for coupling to heterotrimeric G proteins, but the GnRH. recepto r has transferred the requirement for an acidic residue from helix 2 to 7. However, the presence of Asp at the helix 7 locus precludes small G protein -dependent coupling to phospholipase D. These results implicate specific co mponents of the helix 2/helix 7 microdomain in receptor expression and in d etermining the ability of the receptor to adopt distinct activated conforma tions that are optimal for interaction with heterotrimeric and small G prot eins.