Association of gonadotropin receptor precursors with the protein folding chaperone calnexin.

The lutropin/choriogonadotropin receptor (LHR) and follitropin receptor (FSHR) are members of the superfamily of G protein-coupled receptors. The carboxyl half of each receptor is composed of the classical seven membrane spanning regions connected by intracellular and extracellular loops. In addition, each receptor contains a large extracellular domain. Despite the complexity of the structure of G protein-coupled receptors, little is known about how these receptors assume their correct conformations during biosynthesis. Although the role of chaperone proteins in the folding of other proteins has been well documented, their role in the folding of G protein-coupled receptors has been an enigma. To better understand the folding of the LH and FSH receptors, we examined their association with the general chaperone proteins calnexin, binding protein (BiP), and the 94-kDa glucose-regulated protein (GRP94). Clonal 293 cell lines expressing comparably high levels of each receptor were solubilized, and the extracts were incubated with the appropriate antibody bound to Protein A-sepharose beads. Experiments were performed using two approaches: 1) coimmunoprecipitation of receptor/chaperone complexes with one of the antireceptor antibodies, then SDS-PAGE and Western blotting using either anticalnexin or anti-KDEL (which recognizes BiP and GRP94) antibodies; or 2) coimmunoprecipitation of receptor/chaperone complexes with anticalnexin or anti-KDEL, then Western blotting with one of the antireceptor antibodies. Using these protocols, we found that the immature forms of both the rLHR and rFSHR are associated with calnexin, but little or no association was observed for either receptor with BiP or GRP94. These experiments show that the precursor forms of the wild-type LHR and FSHR can associate with calnexin, raising the possibility that this chaperone protein may facilitate in the folding of the gonadotropin receptors.