The recently deorphanized niacin receptor subtypes NIACR1 (GPR109A) and NIACR2 (GPR109B) play an
essential role in the regulation of metabolic processes and immune reactions. Both receptors belong to
the G-protein-coupled receptor (GPCR) family, whose members have traditionally been treated as monomeric
entities, but now appear to exist and function as both homodimers and heterodimers. In this study,
a close physical interaction is shown between the highly homologous niacin receptor subtypes, NIACR1
and NIACR2, using bioluminescence resonance energy transfer (BRET2
) in living cells. The extent of homoand
hetero-dimerization of the niacin receptors did not vary after activation of the receptors with selective
agonists, indicating that the dimerization state of NIACR1 and NIACR2 is not regulated by ligand
binding. Moreover, detection of niacin receptor dimers in both plasma membrane- and endoplasmic
reticulum-enriched fractions suggests that they are formed early in the biosynthetic pathway. Taken
together, these results demonstrate that niacin receptor dimerization is a constitutive process occurring
early during biosynthesis.