Like cAMP, cGMP is an important second messenger that modulates a wide variety of physiological processes, and cGMP-dependent protein kinase is a major mediator of cGMP action. The long-term objective of this investigation is to decipher the mechanisms of physiological regulation of cGMP-dependent protein kinase. Specificity of cGMP action will be studied by examining the cGMP/cAMP selectivity for activation of the enzyme and by studying the phosphorylation and role of two of its putative cellular substrates. cGMP-dependent protein kinase is believed to be the mediator of cGMP- induced relaxation of smooth muscle caused by agonists such as atrial natriuretic peptide and nitric oxide. Drugs that mimic these effects include nitrovasodilators (e.g., nitroglycerin), methylxanthines (e.g., caffeine), and perhaps beta-agonists through """"""""cross-activation"""""""" of cGMP- dependent protein kinase by cAMP. These agents are commonly used for relief of chest pain, asthma, male impotence, and high blood pressure. The kinase may also be involved in neural functions such as memory. The human type Ibeta cGMP-dependent protein kinase will be overexpressed in SF9 cells using baculovirus infection in order to do structure/function studies. cGMP/cAMP selectivity of the enzyme will be studied using site- directed mutagenesis based on modeled structures of the cGMP-binding sites. The first substrate to be studied will be the kinase itself (autophosphorylation). Attempts will be made to separate the phospho- and dephospho-forms of the enzyme chromatographically to use for these studies. The role of autophosphorylation in autoinhibition of catalysis will be examined. Functional effects of autophosphorylation will also be studied using phosphoprotein phosphatases. 32p incorporation into the enzyme will be determined in intact smooth muscle cells following treatment with agents that elevate cGMP or cAMP. The elements of the autoinhibitory domain will be defined by partial proteolysis and mutagenesis. The second substrate to be studied will be a cGMP binding phosphodiesterase. Based on sequences surrounding the phosphorylation site of this enzyme, synthetic peptides will be used to identify elements that contribute to its potency and specificity for phosphorylation by protein kinases. Whether phosphorylation of this phosphodiesterase activates the enzyme in intact cells will be studied by measuring changes in cGMP in response to cGMP analogs that activate cGMp-dependent protein kinase.
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