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Hoffmann, R. A wiki for the life sciences where authorship matters. Nature Genetics (2008)

Dual regulation of beta-melanotropin receptor function and adenylate cyclase by calcium and guanosine nucleotides in the M2R melanoma cell line.

Binding of beta-melanotropin (beta-MSH) and subsequent activation of adenylate cyclase in the M2R mouse melanoma cell line is strongly dependent on the concentration of extracellular free calcium. This effect can be demonstrated both in the intact cell and in a plasma membrane preparation derived therefrom, using an EGTA buffer system. In contrast, stimulation of adenylate cyclase by prostaglandin E1, forskolin, or guanosine 5'-O-(2-thiotriphosphate) is calcium insensitive. It is shown that calcium increases the binding affinity of beta-MSH for its receptor by a factor of 20 (from 400 nM to 20 nM) without affecting maximal hormone binding. At supersaturating concentrations of beta-MSH (greater than 200 nM) binding gradually becomes calcium independent. Hormone-receptor complexes formed in the presence of calcium dissociated rapidly (less than or equal to 2 min) and reversibly upon the elimination of calcium by excess EGTA. Among nine divalent metal cations tested, calcium was found to be the most effective in facilitating hormone binding. Whereas calcium promotes beta-MSH binding, GTP and its stable analogs lead to a reduction in both maximal binding (65%) and affinity (2-fold). These effects are calcium independent, suggesting that the reciprocal control of beta-MSH binding by calcium and guanosine nucleotides is mediated by two separate and independent mechanisms.[1]


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