Resolution and Properties of Two High Affinity Cyclic Adenosine 3′:5′-monophosphate-Binding Proteins from Wheat Germ

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Abstract
A high affinity cAMP-binding protein (cABP II) was purified to homogeneity from wheat germ. The apparent molecular weight of cABP II, as determined from gel exclusion chromatography, is 5.2 × 105 (at low ionic strength) and 2.8 × 105 (at high ionic strength). One polypeptide subunit (molecular weight, 80,000) was resolved by polyacrylamide gel electrophoresis of cABP II under subunit dissociating conditions. The purification protocol employed resolves cABP II from a distinct, less acidic cAMP-binding protein (cABP I). The Kd values for cAMP are about 10−6 molar and 10−7 molar for cABP II and cABP I, respectively. The cAMP-binding sites of cABP I and cABP II have a marked adenine-analog specificity, binding adenine, adenosine, adenine-derived nucleosides and nucleotides and a variety of adenine derivatives having cytokinin activity. While cABP II is phosphorylated in reactions catalyzed by endogenous protein kinases, there is no evidence for modulation of these cABP II-protein kinase interactions by cAMP.