The melanotrophic receptors belong to the family of G protein-coupled receptors with seven transmembrane domains. They form a unique subfamily of G protein-coupled receptors and are highly homologous to each other. One of the features of the melanotropic receptors is their relatively small size (297-360
amino acids). They appear to be more closely related to the cannabinoid receptors than to other receptors.
In primary cultures of melanoma cells, binding of a-MSH to its receptor stimulates the activation of adenylate cyclase and the phosphorylation of a 34-kDa molecule. This step is blocked by phorbolesters, suggesting that protein kinase C might be involved in the signal transduction of a-MSH.
In neurons, a-MSH stimulates the activation of adenylate cyclase and decreases the phosphorylation of a protein, called B-50, by inhibition of protein kinase C. Activation of the melanocortin receptors depends on extracellular calcium, indicating that these receptors belong to a unique class of calcium-requiring receptors.
Between 1992 and 1994, five distinct melanocortin receptors (MC1-MC5) were been identified, of which three (MC3-R, MC4-R and MC5-R) are expressed in the brain and selectively bind a-MSH.
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