ADDITION OF CATFISH GONADOTROPIN-RELEASING-HORMONE (GNRH) RECEPTOR INTRACELLULAR CARBOXYL-TERMINAL TAIL TO RAT GNRH RECEPTOR ALTERS RECEPTOR EXPRESSION AND REGULATION

Mammalian GnRH receptor (GnRHR) is unique among G protein-coupled seve n-transmembrane segment receptors due to the absence of an intracellul ar C-terminal tail frequently important for internalization and/or des ensitization of other G protein-coupled receptors. The recent cloning of nonmammalian (i.e. catfish, goldfish, frog, and chicken) GnRHRs sho ws that these contain an intracellular C terminus. Addition of the 51- amino acid intracellular C terminus from catfish GnRHR (cfGnRHR) to ra t GnRHR (rGnRHR) did not affect rGnRHR binding affinity but elevated r eceptor expression by about 5-fold. Truncation of the added C terminus impaired the elevated receptor-binding sites by 3- to 8-fold, dependi ng on the truncation site. In addition, introducing the C terminus to rGnRHR altered the pattern of receptor regulation from biphasic down-r egulation and recovery to monophasic down-regulation, The extent of do wnregulation was also enhanced. The alteration in receptor regulation due to the addition of a C terminus was reversed by truncation of the added C terminus. Furthermore, addition of the cfGnRHR C terminus to r GnRHR significantly augmented the inositol phospholipid (IP) response of transfected cells to Buserelin, but this did not result from the el evation of receptor-binding sites. Addition of the C terminus did not affect Buserelin-stimulated cAMP and PRL release. GH(3) cells transfec ted with wild-type cfGnRHR did not show measurable Buserelin binding o r significant stimulation of IP, cAMP, or PRL in response to Buserelin (10(-13)-10(-9) M). GH(3) cells transfected with C terminus-truncated cfGnRHR showed no IP response to Buserelin (10(-13)-10(-7) M). These results suggest that addition of the cfGnRHR intracellular C terminus to rGnRHR has a significant impact on rGnRHR expression and regulation and efficiency of differential receptor coupling to G proteins.