Multiple mechanisms underlying desensitization of the liver adenylyl cyclase system: Structure and cAMP regulation of liver adenylyl cyclases.
Item
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Title
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Multiple mechanisms underlying desensitization of the liver adenylyl cyclase system: Structure and cAMP regulation of liver adenylyl cyclases.
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Identifier
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AAI9224848
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identifier
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9224848
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Creator
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Premont, Richard Thomas.
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Contributor
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Adviser: Ravi Iyengar
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Date
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1992
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Language
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English
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Publisher
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City University of New York.
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Subject
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Health Sciences, Pharmacology | Biology, Molecular | Biology, Cell
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Abstract
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The hormone-sensitive adenylyl cyclase signalling pathway is a multi-component system on the cell surface. Hormone receptors activate the stimulatory guanine nucleotide binding regulatory protein G{dollar}\sb{lcub}\rm s{rcub}{dollar}, which in turn activates the enzyme adenylyl cyclase to convert ATP to cAMP. An important feature of this system is the desensitization of response following prolonged or repeated stimulation. Thus, the activity of adenylyl cyclase reflects previous states of activity as well as current stimulation.;I have utilized chick hepatocytes in primary culture to study in detail desensitization of the adenylyl cyclase system in response to glucagon treatment. Homologous desensitization results from regulation of glucagon receptors, which are first uncoupled from G{dollar}\sb{lcub}\rm s{rcub}{dollar} and later removed from the cell surface. Heterologous desensitization involves both loss of G{dollar}\sb{lcub}\rm s{rcub}{dollar} activity and the reduction in the ability of adenylyl cyclase to respond to G{dollar}\sb{lcub}\rm s{rcub}{dollar} stimulation. The reduction in the ability of adenylyl cyclase to respond to G{dollar}\sb{lcub}\rm s{rcub}{dollar} stimulation can be mimicked by treatment of hepatocytes with a cAMP analog, as well as by treatment of membranes with purified protein kinase A and ATP. Since G{dollar}\sb{lcub}\rm s{rcub}{dollar} is not phosphorylated by protein kinase A, this effect appears due to the direct phosphorylation of adenylyl cyclase by protein kinase A. A similar regulation is also observed in membranes from the mouse S49 lymphoma cell line.;I have further explored the mechanism of regulation of adenylyl cyclase in heterologous desensitization by phosphorylation by the cAMP-dependent protein kinase by deducing the primary structure of the liver form of the enzyme. I have cloned two members of a novel subfamily of adenylyl cyclases from a rat liver cDNA library, and the murine homolog of one of these adenylyl cyclases from the S49 lymphoma (UNC variant) cell line. Both of these adenylyl cyclase isoforms are widely expressed in rat tissues. Both adenylyl cyclases share a consensus site for protein kinase A phosphorylation, which may be responsible for regulation of the enzymes during heterologous desensitization. This new subfamily of adenylyl cyclases, unlike four previously cloned enzymes, appears to have the properties expected of the adenylyl cyclase activity found in most tissues.
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Type
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dissertation
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Source
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PQT Legacy CUNY.xlsx
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degree
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Ph.D.
