AbstractA non-synonymous single nucleotide polymorphism of the human serotonin 5-HT2Creceptor (5-HT2CR) gene that converts a cysteine to a serine at amino acid codon 23 (Cys23Ser) appears to impact 5-HT2CR pharmacology at a cellular and systems level. We hypothesized that the Cys23Ser alters 5-HT2CR intracellular signaling via changes in subcellular localizationin vitro. Using cell lines stably expressing the wild-type Cys23 or the Ser23 variant, we show that 5-HT evokes intracellular calcium release with decreased potency and peak response in the Ser23 versus the Cys23 cell lines. Biochemical analyses demonstrated lower Ser23 5-HT2CR plasma membrane localization versus the Cys23 5-HT2CR. Subcellular localization studies demonstrated O-linked glycosylation of the Ser23 variant, but not the wild-type Cys23, may be a post-translational mechanism which alters its localization within the Golgi apparatus. Further, both the Cys23 and Ser23 5-HT2CR are present in the recycling pathway with the Ser23 variant having decreased colocalization with the early endosome versus the Cys23 allele. Agonism of the 5-HT2CR causes the Ser23 variant to exit the recycling pathway with no effect on the Cys23 allele. Taken together, the Ser23 variant exhibits a distinct pharmacological and subcellular localization profile versus the wild-type Cys23 allele, which could impact aspects of receptor pharmacology in individuals expressing the Cys23Ser SNP.