Identification of the 5-hydroxytryptamine2C receptor as a 60-kDa N-glycosylated protein in choroid plexus and hippocampus

The rat 5-hydroxytryptamine2C (5-HT2C) receptor was identified as N-glycosylated polypeptide of 60-kDa apparent molecular mass using antibodies against its putative third and fourth (C-terminal) cytoplasmic domain. To show that the polypeptides detected on western blots and by immunoprecipitation represent the 5-HT2C receptor, binding studies of the 5-HT2C ligand [3H]-mesulergine to immunoprecipitates from extracts of pig choroid plexus were performed. We demonstrate the presence of a signal sequence that was cleaved off during membrane insertion resulting in a 38-kDa polypeptide. During further maturation, the receptor was N-glycosylated at two sites via a 48-kDa intermediate. This intermediate was far more abundant in choroid plexus than in hippocampus and may represent an intracellular receptor reserve. After transfection of 5-HT2C cDNAs into cultured cells, polypeptides were observed that differed from the ones found in vivo due to abnormal N-glycosylation and possibly other alterations depending on the system used. Thus the 5-HT2C receptor expressed in cell lines may also differ in function from the receptor in its native tissue.