The dimerization properties of the ghrelin receptor (GRLN-R) and its non-signalling, naturally occurring, truncated splice variant (GHS-R1b) have been investigated in human embryonic kidney 293 cells heterologously expressing these proteins. Using the techniques of bioluminescence resonance energy transfer and co-immunoprecipitation, we detected the formation of GRLN-R homodimers and GRLN-R/GHS-R1b heterodimers, but ghrelin-induced conformational changes were only detected in the GRLN-R homodimers. When the expression of GHS-R1b exceeded that of GRLN-R, there was a decrease in the cell surface expression of GRLN-R with a consequent decrease in constitutive activation of phosphatidylinositol-specific phospholipase C (PI-PLC). Furthermore, there was no change in ghrelin affinity, and the efficacy of cell signalling as measured by stimulation of PI-PLC and extracellular signal-regulated kinase 1/2 was unchanged. Cellular localization studies suggest that GRLN-R is normally distributed between the plasma membrane and cytosolic fractions, but in the presence of GHS-R1b, GRLN-R is localized to the nucleus. Therefore, we propose that the decrease in GRLN-R constitutive signalling was due to translocation of GRLN-R to the nucleus due to the formation of GRLN-R/GHS-R1b heterodimers. Therefore, GHS-R1b appears to act as a dominant-negative mutant of the full-length GRLN-R.
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