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Regulation of glucagon receptor binding lack of effect of magnesium and preferential role for gdp

, : Regulation of glucagon receptor binding lack of effect of magnesium and preferential role for gdp. Journal of Biological Chemistry 260(13): 7829-7835

The effects of Mg2+ and guanine nucleotides on glucagon binding to its receptor were studied using [125I-Tyr10] monoiodoglucagon. Contrary to findings with .beta.-adrenergic receptors, high affinity binding of the stimulatory hormone was not dependent on Mg2+ and low affinity binding could be obtained on nucleotide addition regardless of presence of Mg2+. GDP, guanyl-5'-yl thiophosphate (GDP.beta.S), GTP, and guanyl-5'-yl imidodiphosphate (GMP-P (NH)p) were all able to induce low affinity hormone binding. Since the Ns component of adenylyl cyclase, with which the receptor interacts, is inactive in stimulating the catalytic component C of adenylyl cyclase in the absence of Mg2+, both before and after GDP addition, it is suggested that Ns has at least 2 domains that change conformation independently of each other: a r domain, that interacts with the receptor and confers to it high affinity binding, and a c domain, that interacts with the catalyst C and stimulates it. Ns is r+ c- when stabilizing the receptor in its conformation with high affinity for hormone, and r-c- when under the influence of GDP which results in the receptor adopting the conformation that exhibits low affinity for the hormone. Comparison of potencies of the 4 nucleotides to induce low affinity binding showed that GDP and GDP.beta.S were equipotent and 10 times more potent than GTP and 100 times more potent than GMP-P(NH)P. Under the conditions used it was impossible to substantiate that the effects of GTP or GMP-P(NH)P were not due to formation of GDP from GTP or presence of GDP-like material in GMP-P(NH)P. Contrary to widely held opinions, GDP and GDP-like compounds, and not GTP or its analogs, are responsible for the lowering of the affinity of adenylyl cyclase stimulating receptors for their hormones or agonists. The c+ conformation of the c domain of Ns co-exists with the r+ and not the r- conformation of its r domain.

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