Key amino acids alter activity and trafficking of a well-conserved olfactory receptor

In this study, we elucidate factors that regulate the trafficking and activity of a well-conserved olfactory receptor (OR), olfactory receptor 558 (Olfr558), and its human ortholog olfactory receptor 51E1 (OR51E1). Results indicate that butyrate activates Olfr558/OR51E1 leading to the production of cAMP, and evokes Ca²⁺ influx. We also find olfactory G protein (Golf) increases cAMP production induced by Olfr558/OR51E1 activation but does not affect trafficking. Given the 93% sequence identity between OR51E1 and Olfr558, it is surprising to note that OR51E1 has significantly more surface expression yet similar total protein expression. We find that replacing the Olfr558 N-terminus with that of OR51E1 significantly increases trafficking; in contrast, there is no change in surface expression conferred by the OR51E1 TM2, TM3, or TM4 domains. A previous analysis of human OR51E1 single nucleotide polymorphisms (SNPs) identified an A156T mutant primarily found in South Asia as the most abundant (albeit still rare). We find that the OR51E1 A156T mutant has reduced surface expression and cAMP production without a change in total protein expression. In sum, this study of a well-conserved olfactory receptor identifies both protein regions and specific amino acid residues that play key roles in protein trafficking and also elucidates common effects of Golf on the regulation of both the human and murine OR.