Seven Dictyostelium discoideum phosphodiesterases degrade three pools of cAMP and cGMP
Résumé
The Dictyostelium genome uncovers seven cyclic nucleotide phosphodiesterases, of which six have been characterized previously and the seventh is characterized here. Three enzymes belong to the ubiquitous class I common in all eukaryotes, while four enzymes belong to the rare class II PDEs that are present in bacteria and lower eukaryotes. Since all Dictyostelium PDEs are now characterized we have calculated the contribution of each enzyme in the degradation of the three important pools of cyclic nucleotides: 1) extracellular cAMP that induces chemotaxis during aggregation and differentiation in slugs, 2) intracellular cAMP that mediates development, and 3) intracellular cGMP that mediates chemotaxis. It appears that each cyclic nucleotide pool is degraded by a combination of a enzymes that have different affinities, allowing a broad range of substrate concentrations to be degraded with first order kinetics. Extracellular cAMP is degraded predominantly by the class II high affinity enzyme DdPDE1 and its close homolog DdPDE7, and in the multicellular stage also by the low affinity transmembrane class I enzyme DdPDE4. Intracellular cAMP is degraded by the DdPDE2, a class I enzyme regulated by histidin kinase/phosphorelay, and by the cAMP/cGMP-stimulated class II DdPDE6. Finally, basal intracellular cGMP is degraded predominantly by the high affinity class I DdPDE3, while the elevated cGMP levels that arise after receptor stimulation are degraded predominantly by a cGMP-stimulated cGMP-specific class II DdPDE5. The analysis shows that the combination of enzymes is tuned to keep the concentration and lifetime of substrate within a functional range.
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