Modulator of melanocortin receptors (MC1R, MC2R, MC3R, MC4R and MC5R). Acts by increasing ligand-sensitivity of melanocortin receptors and enhancing generation of cAMP by the receptors. Required both for MC2R trafficking to the cell surface of adrenal cells and for signaling in response to corticotropin (ACTH). May be involved in the intracellular trafficking pathways in adipocyte cells (By similarity). Homodimer and heterodimer. Forms antiparallel homodimers and heterodimers with MRAP2. Interacts with MC1R, MC2R, MC3R, MC4R and MC5R (By similarity). 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. Upon insulin stimulation, it is redistributed into spotty structures throughout the cytoplasm (By similarity). Largely restricted to fat tissues. Predominantly expressed in mouse epididymal (white adipose tissue) and interscapular (brown adipose tissue) fat pads. Expression is weak or absent in lung, spleen, intestine, kidney, heart and skeletal muscle. Expressed during adipocyte differentiation. Expression appears 2 days following induction of adipose conversion, reaching a peak after 6 days. Belongs to the MRAP family.