PKA

delta-CREB is a spliced variant of cAMP response element binding protein (CREB). CREBtide (KRREILSRRPSYR), a synthetic peptide based on the phosphorylation sequence in delta-CREB. delta-CREB and CREBtide are tested as substrates of cAMP-dependent protein kinase (cAK). The apparent K _m of CREBtide phosphorylation by cAK is 3.9 μM, which is 10-fold lower than that of Kemptide (K _m =39 μM), the synthetic peptide substrate most often employed for cAK measurement. The V _max values are 12.4 mumol/(min.mg) for CREBtide and 9.8 mumol/(min.mg) for Kemptide. The apparent K _m of CREBtide phosphorylation by cGMP-dependent protein kinase (cGK) is 2.9 μM and the V _max value is 3.2 mumol/(min.mg). Both delta-CREB and CREBtide are phosphorylated at a much slower rate by cGK as compared with cAK, implying that the high cAK/cGK specificity exhibits by delta-CREB is retained by the peptide.