A new nonhydrolyzable reactive cAMP analog, (Sp)-adenosine-3',5'-cyclic-S-(4-bromo-2,3-dioxobutyl)monophosphorothioate irreversibly inactivates human platelet cGMP-inhibited cAMP phosphodiesterase

New Insights from the Structure-Function Analysis of the Catalytic Region of Human Platelet Phosphodiesterase 3A

Human phosphodiesterase 3A (PDE3A) degrades cAMP, the major inhibitor of platelet function, thus potentiating platelet function. Of the 11 human PDEs, only PDE3A and 3B have 44-amino acid inserts in the catalytic domain. Their function is not clear. Incubating Sp-adenosine-3',5'-cyclic-S-(4-bromo-2,3-di-oxobutyl) monophosphorothioate (Sp-cAMPS-BDB) with PDE3A irreversibly inactivates the enzyme. High pressure liquid chromatography (HPLC) analysis of a tryptic digest yielded an octapeptide within the insert of PDE3A ((K)T(806)YNVTDDK(813)), suggesting that a substrate-binding site exists within the insert. Because Sp-cAMPS-BDB reacts with nucleophilic residues, mutants Y807A, D811A, and D812A were produced. Sp-cAMPS-BDB inactivates D811A and D812A but not Y807A. A docking model showed that Tyr(807) is 3.3 angstroms from the reactive carbon, whereas Asp(811) and Asp(812) are >15 angstroms away from Sp-cAMPS-BDB. Y807A has an altered K(m) but no change in k(cat). Activity of wild type but not Y807A is inhibited by an anti-insert antibody. These data suggest that Tyr(807) is modified by Sp-cAMPS-BDB and involved in substrate binding. Because the homologous amino acid in PDE3B is Cys(792), we prepared the mutant Y807C and found that its K(m) and k(cat) were similar to the wild type. Moreover, Sp-cAMPS-BDB irreversibly inactivates Y807C with similar kinetics to wild type, suggesting that the tyrosine may, like the cysteine, serve as a H donor. Kinetic analyses of nine additional insert mutants reveal that H782A, T810A, Y814A, and C816S exhibit an altered k(cat) but not K(m), indicating that catalysis is modulated. We document a new functional role for the insert in which substrate binding may produce a conformational change. This change would allow the substrate to bind to Tyr(807) and other amino acids in the insert to interact with residues important for catalysis in the active site cleft.

Crosstalk between Rap1 and Rac regulates secretion of sAPPalpha

Cyclic AMP (cAMP) is produced by activation of Gs protein-coupled receptors and regulates many physiological processes through activation of protein kinase A (PKA). However, a large body of evidence indicates that cAMP also regulates specific cellular functions through PKA-independent pathways. Here, we show that a small GTPase of the Rho family, Rac, is regulated by cAMP in a PKA-independent manner. We also show that Rac activation results from activation of Rap1 through the cAMP guanine nucleotide-exchange factor (GEF) Epac1. Activation of the Gs-coupled serotonin 5-HT(4) receptor initiates this signalling cascade in various cell types. Furthermore, we demonstrate that crosstalk between the Ras and Rho GTPase families is involved in cAMP-dependent processing of amyloid precursor protein (APP), a key protein in Alzheimer's disease. Indeed, Epac1 regulates secretion of the non-amyloidogenic soluble form of APP (sAPPalpha) through Rap1 and Rac. Our data identify an unsuspected connection between two families of small GTPases and imply that Rac can function downstream of cAMP/Epac1/Rap1 in a novel signal transduction secretory pathway.