Modulator of melanocortin receptor 4 (MC4R), a receptor involved in energy homeostasis. Plays a central role in the control of energy homeostasis and body weight regulation by increasing ligand-sensitivity of MC4R and MC4R-mediated generation of cAMP (By similarity). May also act as a negative regulator of MC2R: competes with MRAP for binding to MC2R and impairs the binding of corticotropin (ACTH) to MC2R. May also regulate activity of other melanocortin receptors (MC1R, MC3R and MC5R); however, additional evidence is required in vivo. Homodimer and heterodimer. Forms antiparallel homodimers and heterodimers with MRAP. Interacts with MC1R, MC2R, MC3R, MC4R and MC5R. The formation of antiparallel homo- and heterodimers suggest that N- and C-terminus can both localize in the cytoplasmic and extracellular parts, depending on the context. Expressed in the adrenal gland and brain. Not expressed in other tissues. Genetic variations in MRAP2 define the body mass index quantitative trait locus 18 (BMIQ18) [MIM:615457]. Variance in body mass index is a susceptibility factor for obesity. Disease susceptibility may be associated with variants affecting the gene represented in this entry. Belongs to the MRAP family.