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. 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 and MC5R (By similarity). Interacts with MC4R. 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. Predominantly expressed in the brain, mainly in the pons and cerebellum but also in regions involved in energy homeostasis, such as the hypothalamus and brainstem. Obesity. Mice are normal at birth, with normal weight gain and post-weaning food intake during early life, although young males trend toward greater weight and food intake with advancing age. Mice of both genders gradually become extremely obese on a diet of regular chow ad libitum. Belongs to the MRAP family.