Pituitary

Differential Impact of Intracellular Carboxyl Terminal Domains on Lipid Raft Localization of the Murine Gonadotropin-Releasing Hormone Receptor

Abstract
The mammalian Type I GNRH receptor (GNRHR) is unique among G protein-coupled receptors (GPCR) due to the absence of an intracellular C-terminus. Previously, we have found that the murine GNRHR is constitutively localized to low-density membrane microdomains termed lipid rafts. As such, association of the GNRHR with lipid rafts may reflect both a loss (C-terminus) and gain (raft association address) of structural characteristics. To address this, we fused either the full length C-terminus from the non-raft associated LH Receptor (LHCGR) (GNRHR-LF) or a truncated (t631) LHCGR C-terminus, (GNRHR-LT) to the GNRHR. These chimeric receptors are trafficked to the plasma membrane, bind ligand and display increased agonist induced receptor internalization but do not partition into lipid rafts. Thus, a heterologous C-terminus from a non-raft associated GPCR redirects localization of the GNRHR to non-raft domains. In contrast to the murine GNRHR, the catfish GNRHR (cfGNRHR) possesses an intracellular Cterminus. We find that the cfGNRHR is localized to lipid rafts and that the cfGNRHR C- terminus does not alter raft localization of the mammalian receptor. Consistent with placement in different lipid microenvironments within the plasma membrane, fluorescence recovery after photobleaching (FRAP) reveals different lateral diffusion phenotypes of the raft associated GNRHR and cfGNRHR vs. the non-raft associated GNRHR-LF fusion protein. We conclude that while an intracellular C-terminus is capable of redirecting the GNRHR to non-raft compartments, this is not a generalized feature of GPCR C-terminal tails. Thus, constitutive raft localization of the GNRHR is not simply due to the loss of an intracellular C-terminus.

Key words: Mechanisms of Hormone Action • Pituitary • Gonadotropin-releasing hormone receptor