cAMP-dependent protein kinase, but not the cGMP-dependent enzyme, rapidly phosphorylates Δ-CREB, and a synthetic Δ-CREB peptide

Phosphorylation of the cAMP response element binding protein (CREB) by the catalytic subunit of cAMP-dependent protein kinase (cAK) has been implicated in the cAMP-dependent stimulation of gene transcription. Δ-CREB, a spliced variant of CREB, and CREBtide (KRREILSRRPSYR), a synthetic peptide based on the phosphorylation sequence in Δ-CREB, were tested as substrates of cAK. Phosphorylation of Δ-CREB (0.17 μM) was stoichiometric within 30 s when using a concentration of cAK which approximated the intracellular level (0.2 μM). The rate of phosphorylation of Δ-CREB was comparable to the rates of the best physiological substrates of cAK tested. The rate of CREBtide phosphorylation was at least as great as that of Δ-CREB, indicating that the peptide retained the determinants of Δ-CREB which were responsible for substrate efficacy. The apparent Km of CREBtide phosphorylation by cAK was 3.9 μM, which is 10-fold lower than that of kemptide (Km = 39 μM), the synthetic peptide substrate most often employed for cAK measurement. The Vmax values were 12.4 μmol/(min∙mg) for CREBtide and 9.8 μmol/(min∙mg) for kemptide. The apparent Km of CREBtide phosphorylation by cGMP-dependent protein kinase (cGK) was 2.9 μM and the Vmax value was 3.2 μmol/(min∙mg). Both Δ-CREB and CREBtide were phosphorylated at a much slower rate by cGK as compared with cAK, implying that the high cAK/cGK specificity exhibited by Δ-CREB was retained by the peptide. Taken together, the results indicated that Δ-CREB and CREBtide are among the best substrates tested for cAK and suggested that phosphorylation of CREB by this enzyme could occur in intact cells. However, cGK does not appear to be an adequate catalyst for this reaction.Key words: cAMP-dependent protein kinase, cGMP-dependent protein kinase, CREB.