Somatostatin (SST) receptors are well known to regulate exocrine secretions, neurotransmission and inhibit cell proliferation. SSTR subtypes exhibit heterodimerization with unique signaling characteristics. In the present study, using western blot and co-immunoprecipitation, FACS analysis, immunofluorescence, we investigated the differential interaction between MRAP1 and SSTRs and the role of MRAP1 on mouse SSTR2/SSTR3 and SSTR2/SSTR5 heterodimerization in HEK293 cells. The interaction binding sites of SSTR2/SSTR3 or SSTR2/SSTR5 with MRAP1 depend on SSTR3 and SSTR5 but not SSTR2. MRAP1 binding sites on SSTR3 are extensive, while the binding sites on SSTR5 are concentrated on TM6 and TM7. We also determined that MRAP1 enhanced agonist-mediated cAMP inhibition by inhibiting SSTR2/SSTR3 and SSTR2/SSTR5 heterodimerization. Up-regulation of ERK1/2 phosphorylation, p27Kip1 and increased cell growth inhibition when co-expressing SSTR2/SSTR3 or SSTR2/SSTR5 with MRAP1, suggested an effect of MRAP1 on anti-proliferative response of two SSTR heterodimers. Taken together, these results provide new insights for binding protein MRAP1 in the maintenance and regulation of mouse SSTR dimers which might be helpful to better understand the molecular mechanism involving SSTRs in tumor biology.

The seven trans-membrane spanning G protein couple receptors(GPCRs) constitute the largest family of FDA approved drug targets to treat numerous human diseases, including metabolic associated disorders. The Melanocortin Receptor Accessory Protein 2 (MRAP2), a small single transmembrane protein broadly expressed in multiple tissues has been defined as a vital endocrine pivot of five melanocortin receptors(MC1R-MC5R) and several other GPCRs in the regulation of central neuronal appetite and peripheral energy homeostasis. However, MRAP2 null mouse model exhibited late onset obesity syndrome without alteration on the eating behavior, a huge phenotypic departure from MC4R KO animals. In this study, we identified GLP1R and MCHR1, the two energy regulators as novel GPCR targets and characterized the specific regions of MRAP2 protein that required for the pharmacological modulation of the intracellular cAMP cascades. The simultaneous inhibitory effect of MRAP2 on the anorectic GLP1R and orectic MCHR1 signaling further elucidated the complex endocrine network of GPCR signaling which may explain the composite metabolic phenotypes of MRAP2 deficiency.