A role of Histidine151 in the lamprey gonadotropin-releasing hormone receptor-1 (lGnRHR-1): Functional insight of diverse amino acid residues in the position of Tyr of the DRY motif in GnRHR from an ancestral type II receptor.

The highly conserved DRY motif located at the end of the third transmembrane of G-protein-coupled receptors has been described as a key motif for several aspects of GPCR functions. However, in the case of the vertebrate gonadotropin-releasing hormone receptor (GnRHR), the amino acid in the third position in the DRY motif is variable. In the lamprey, a most basal vertebrate, the third amino acid of the "DRY" in lamprey (lGnRHR-1) is His, while it is most often His/Gln in the type II GnRHR. To investigate the functional significance of the substitution of DRY to DRH in the GnRHR-1, second messenger signaling, ligand binding and internalization of the wild-type and mutant lGnRH receptors were characterized with site-directed mutagenesis. Treatment of the DRE(151) and DRS(151) mutant receptors with lamprey GnRH-I significantly reduced inositol phosphate compared to wild-type (DRH(151)) and DRY(151) receptors. The LogIC(50) of wild-type receptor (-9.554+/-0.049) was similar to the LogIC(50) of DRE(151), DRS(151) and DRX(151) mutants, yet these same mutants were shown to significantly reduce cell-surface expression. However, the DRY(151) mutant compared to the wild-type receptor increased cell-surface expression, suggesting that the reduction of IP production was due to the level of the cell-surface expression of the mutant receptors. The rate of internalization of DRX(151) (35.60%) was reduced compared to wild-type and other mutant receptors. These results suggest that His(151) of the lamprey GnRH receptor-1 may play a critical role in the retention of a certain level of cell-surface expression for subsequent cellular second messenger events.